Flowers of the Coast

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When waves break, beach material, coarse and fine, is churned up. There is often some order and arrangement in this movement. If the waves are approaching the shore at right-angles, the pebbles and small stones move up and down the beach. The waves break and send up the beach sheets of water called the swash or send, which carry material upwards. Some of the water of the swash percolates into the beach, some returns to the sea as the backwash. This is nearly always less powerful than the swash, but in its deeper parts can move a good deal of material. If, however, the waves approach the beach obliquely, so also may they advance up it, and stones and sand are not merely carried upwards, but also sideways. When the swash dies out, the backwash returns directly down the slope, and any material moved by it travels in the same direction. Thus on open coasts on to which the waves come obliquely, there is a great deal of lateral displacement of beach material. This process is called beach-drifting, and is of the utmost importance. Its effect is often seen where groynes or breakwaters are built athwart the beach to hold material travelling along it by this process. The beach on one side of a groyne is usually higher than on the other, although often after a storm from a different quarter the high and low sides may temporarily change places.

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The waves also have a sorting effect, and drive the stones to higher parts of the beach. This process can often be seen in action while waves are breaking on a beach of mixed material. There is still another important factor. Around our coasts there is usually a noticeable difference in the level of the water at low and high tide. On the open coast the range seldom exceeds twenty feet, but in bays and gulfs it may be more. The highest rises and the lowest falls occur about the times of new and full moon. At the half-moons the difference between high and low water is small. Suppose it is now the moon’s first quarter, and the weather generally fair. The waves at both the morning and evening tide will reach to much about the same level. But in succeeding days the high waters will rise higher and the low waters will fall lower with each succeeding tide until the time of full moon.

(#litres_trial_promo) What effect will this have on the beach? If marked pebbles have been scattered near the water line at the time of the moon’s first quarter they will be seen not only to have moved along the beach if the waves are oblique, but to have been pushed up it by waves at each successive tide, and have gathered near the top, usually in an existing ridge. During the subsequent fall of tide-level after full moon, the pebbles are left stranded, and they may only just be reached again at the next period of springs at the time of new moon.

Thus, if nothing else happened, the pebbles might remain at the beach top for ever. But two other factors are likely to affect them. First, the swing of the tides from neaps to springs is only part of a larger swing that shows itself in particularly high tides near the equinoxes, and sometimes at other times of the year. Secondly, if a severe storm attacks a coast, especially at a period of big tides, shingle may be either swept far above its normal level or dragged in large quantities down the beach. An exceptional storm may overtop the highest beaches. The effect of ordinary storms is plain along any shingle beach, since the seaward face is frequently marked by minor ridges parallel to its length. These are either the heights reached by the last high tides, or the limits of recent storms.

Quite apart from these up-and-down and lateral movements of some beach material, vast quantities of finer stuff are moved alongshore by a different process. A bather on a sandy shelving beach in ordinary weather and in water three or four feet deep notices the lifting effect of the waves: the sand is at the same time somewhat disturbed about his feet. If he allows himself to float off such a beach, he notices that the tidal current carries him one way or another along it. The sand stirred up by the waves may, when a tidal current is running with any speed, also be carried sideways for a short distance. Picture this process during a tidal cycle, and during rough and stormy conditions, and it is at once apparent what vast quantities of fine material can be carried along a beach.

Current-action, however, only takes place under water and below the zone of wave-break. It operates on the higher parts of the beach only at or near high tide; on the lower parts, on an open coast the current may run one way at or near low water, and the opposite way at about high water. Two things at least follow—first, in the deeper water, material may move in different ways at different stages of the tide, and whether there is a balance of movement will depend upon the relative strengths of the flood and ebb currents. Secondly, on the parts of the beach covered only at high water, the movement of material is likely to be in one direction only—that of the current at the time of high water. The resultant process is called long-shore drifting. With beach-drifting it is of the utmost importance in the study of shoreline phenomena.

A wave breaks when it enters water the depth of which is approximately half its wave-length. Thus on a shallow coast, big waves break farther out than do small ones. When breaking offshore, the waves—just as on a beach—drive material up in front of them, so that sometimes ridges of sand and shingle are built some distance from the original shore. If a shingle ridge of this sort attains a fair degree of stability, it becomes an outer beach along which beach-drifting can take place. Hence the ridge may lengthen and become what is called an offshore bar (see here (#litres_trial_promo)). If the process continues a lagoon-like expanse may be enclosed between it and the old shoreline. Offshore bars are seldom unbroken for long distances, since there are often gaps through which the tide enters and leaves the lagoon, in which marsh development is favoured.

If the supply of shingle is great, and if the lateral transport along a coast is marked, the shingle can accumulate in great forelands like Orfordness, Dungeness, the Crumbles, and the shingle ridges off the Culbin Sands and other parts of the Moray Firth shore. At an early stage a ridge is built. After a time the new shingle coming along the coast shallows the sea floor off the first ridge, so that the waves build another in front of it. This may go on until a whole series of such ridges is formed. It is often noticed that at one part of a shingle foreland the ridges run out to sea in such a way that it is clear they are suffering erosion, whereas at another part new shingle is accreting and being built up into ridges. A study of any big shingle foreland will illustrate this process, but there are few more striking examples than the shingle formation known as the Bar, near Nairn. Fig. 1 (#litres_trial_promo) shows that it is composed of a number of individual ridges, the north-eastern ends of which are being eroded, whereas growth is continuous at the other end. In short, the whole structure is slowly shifting along the coast.

Along the south-west facing side of Dungeness there are many ridges running directly out to sea, and obviously at one time they continued for some distance. Erosion has cut them, and the material thus provided has travelled round the point of Dungeness and gradually helped to build the numerous ridges forming Denge Beach. Erosion is constantly taking place on the one side, accretion on the other.

FIG. 1.—The Bar (from Steers, Geogr. Journal, 1937).

The shingle that composes the banks and beaches comes partly from the erosion of cliffs, partly from boulder clay and other materials on the sea floor, and largely from glacial and gravel deposits, from which it has been swept by rivers in past times. Along our east and south coast it is mainly composed of flint which originally came from the Chalk. In west coast and Scottish beaches, the percentage of local rocks is far higher, and flint may be quite absent.

In its lateral travel alongshore, shingle often builds ridges or embankments, running across the mouths of rivers and inlets. Nearly all rivers are to some extent obstructed by a bar composed of shingle or sand, or both. When a bar is growing across a river mouth, the unattached end extends very much in the same manner as does a tip heap. At first the bar may be wholly below water; it gradually grows up to the surface. But whether above or below water, the free end tends to be turned inwards as a result of wave action. Many bars of this sort have on their landward side laterals or recurved ends (see here (#litres_trial_promo)). Some bars grow forward, later turn inwards, and, after a time, grow forward again. It is easy to give general reasons for this—e.g. wave-attack in a storm—but it is extremely difficult to be precise. If the bar is obstructing a river, its form will depend in part on the power of the river to keep its mouth clear. Small streams like those at Chideock and other places on the Dorset coast are completely dammed. In others, e.g. the Exe and Teign, the river maintains a mouth; the Exe Bar is particularly interesting since it is double. At Orford Ness the shingle has not only formed a bar, but has grown into a great foreland and deflected the river for eleven miles. Some rivers like the Spey usually keep their mouths through shingle beaches in nearly the same place, whereas others, like the neighbouring Findhorn, by no means free from violent floods, are deflected.

Scolt Head Island and other features of the Norfolk coast and the Bar off Nairn on the Moray Firth are good examples of offshore bars, and, they, too, lengthen in the same way as ordinary beaches. Since they are offshore, they can send back long lateral ridges.

In bays and other inlets, shingle beaches are usually washed up at the head, to form what are called bay-head beaches. Normal beach-drifting for any distance along an indented coast is impossible. If, however, the bays are rather wider and more open, there is a certain amount of lateral travel of shingle in them, and it gathers at their leeward ends. The distribution of shingle along the several bays between Pwllheli and Penrhyndeudraeth is most instructive. Sometimes a ridge forms across the mid-part of a bay, such as Cemlyn Bay in Anglesey.

DUNES

Shingle ridges of various kinds have been discussed at some length because they form the “skeleton” combining the “flesh” of dunes and salt-marsh. By no means all dunes are built on shingle ridges, and many ridges have no dunes. But if a shingle ridge is being formed in a locality where an expanse of sand is bared at low tide, the wind blowing over the sand will carry much of it on to the ridge and deposit some of it in its interior and some on its surface. This may cause dune growth. But (see Chapter 7 (#litres_trial_promo)) the real dune-builders are the sea couch-grass (Agropyron junceiforme), marram-grass (Ammophila arenaria), and sea lyme-grass (Elymus arenarius) that take root in the shingle, send up shoots, and so begin to trap the sand. Marram-grass is remarkable in this way, thriving best where the sand supply is most prolific (Pl. III (#litres_trial_promo)). If another shingle ridge is built in front of the old one, the same process will begin on that, and the dunes on the inner ridge will be, at any rate partially, deprived of their sand supply. The dunes may remain, but are often partly blown away, whereas those on the newer ridge increase. Under favourable circumstances they grow and become permanent features. In England dunes seldom exceed fifty or sixty feet in height if built up from sea-level.

The evolution of coastal dunes depends largely on the available sand supply, the vegetation, the prevalent winds, and the nature of the foundation on which they rest. With a constant wind and sand supply they may attain large sizes, but the vegetation rarely forms a close covering, and there is always the chance of minor hollows being enlarged. This is especially the case in older dunes, somewhat cut off from their sand supply. Often these are riddled with blow-outs (Pl. XXI (#litres_trial_promo)) so that mere fragments of the original sand-dune chain remain. On some dunes the wind may enlarge a hollow, but new sand may sometimes replace that blown away.

Groups of coastal dunes are often arranged in lines, despite their superficial irregularity, and the lines correspond with the trends of old shingle ridges. This is shown at Blakeney Point, Scolt Head Island, and Morfa Harlech. But on Morfa Harlech and even more on Morfa Dyffryn this relatively simple arrangement is often confused by later movement. In both places there are numerous blow-outs, and the inner dunes have locally been set in motion again, so that they are advancing over the low ground on their landward side. On the other hand, at Culbin the major dunes covered groups of ridges and showed no relationship to their trends. The same may be true at Forvie, but unfortunately no detail is known of the surface upon which the dunes rest. Sometimes a line of coastal dunes advances downwind more quickly in its middle and higher parts, than near its lower ends. This is because it is easier for the plants to hold the ends than it is for them to restrain the sand on the higher parts once it has begun to blow again. A dune-line that has advanced in this particular way, when well-developed, is called a parabolic dune. The best examples in Great Britain are at Maviston (really a part of the Culbin Sands) where the dunes have advanced over and buried well-grown forest trees (Pl. VI (#litres_trial_promo)).

Every possible transition can be found around our dune shores from the tiny heap of sand just beginning to gather round a tuft of grass, or a small obstruction on a beach, through the elementary stage of a foredune based on a shingle ridge, to the more evolved forms on the older ridges which have not been deprived of their sand supply. If the sand supply is great and it blows along or oblique to the shingle ridges, rather than at right angles to them, there is no particular reason for the dunes to have any correlation in trend with the ridges, and they may grow to considerable heights. When the dunes have no relation to the ridges it is possible that there has been a change of direction of the prevalent winds. The old shingle ridges at Culbin are part of a raised beach system; the dunes are the result of conditions extending back only two or three centuries.

When dunes are deprived of a sand supply they take on a dead appearance. The wind may regain mastery, and most of the sand may be blown away, leaving hummocks behind, known as remanié dunes. If the sand blown away begins to accumulate, as well it may, quite near at hand, the observer may be confused by the contiguity of old and new dunes; the new ones in this case may sometimes be to landward of the old. What is more, the new dunes will almost certainly have a covering of fresh-looking marram-grass, although the sand is derived from the old ones! It is essential to keep in mind this liability to rapid change in dunes; a return to a familiar place after an interval of five or ten years may easily result in the visitor being temporarily lost!

Sometimes the sand blown by strong winds is spread out as a layer over considerable areas. To some extent this happens in any dunes; the sandy pasture inside Morfa Harlech is an instance. In the Western Isles of Scotland there is a special development known as the machair (see here (#litres_trial_promo)). True, it is often associated with prominent lines of dune, but the machair proper usually lies landward of dunes and shingle bars, and forms, as in South Uist and Tiree where it is particularly well developed, a low sandy plain between sea and peat and hills. It is primarily shell-sand and provides a most fertile sward. In high summer the numberless plants and flowers, the strong scent, the vivid colours, the blue sea and white beach, and the wide open views and great expanse of sky make the machair the centre of a most beautiful landscape.

SALT-MARSHES

Salt-marshes and their growth are described in detail in Chapter 5 (#litres_trial_promo). They show better than any other feature the intimate relation between ecology and physiography in coastal evolution. They are built up of the mud, silt, and fine sand carried by the tidal (and other) currents, and deposited in quiet places. Their nature will depend much on the material of which they are composed; there is a great difference (see here (#litres_trial_promo)) between the firm mud marshes of Norfolk and the sandy wastes of Morecambe Bay. Marsh growth begins in certain favoured places on the sea floor, especially on that part of it which is bared at low tide. Ideal conditions obtain on the coast of North Norfolk where formations like Blakeney Point, Wells Headland, Scolt Head Island, and the small high-tide island at Thornham provide shelter. Marsh growth takes place outwards from the original coast and inwards from the protecting ridge. Moreover, where structures like Blakeney Point throw off a number of laterals, or recurved ends, the best possible conditions for marsh formation often exist between neighbouring laterals, especially if they are so built as to make a narrow entrance to a relatively broad expanse inside (e.g. the Marram marshes at Blakeney Point). Into these quiet backwaters tidal water pours, and stands quietly for a period before the ebb (Pl. XIa (#litres_trial_promo)). Sedimentation takes place readily, and since the water has to drain out through the narrow mouths, material deposited round the margins is undisturbed. Hence, a considerable quantity of mud soon gathers. The same thing happens in less enclosed places, but the speed of accumulation is generally less.

(#litres_trial_promo) Between Wells and Blakeney there are extensive marshes, only here and there fringed on their seaward side by a ridge. The larger waves break well out, so that in the quiet inshore water sedimentation can proceed.

The same quiet conditions often prevail in parts of estuaries and other embayments. There is considerable marsh growth in Hamford Water, around Canvey Island and Sheppey in the Thames Estuary, in Chichester Harbour, in Southampton Water and Poole Harbour, on the upper parts of Plymouth Sound and Milford Haven, and in the Bristol Channel. Many other places round our coasts show similar growth. The principles of accumulation are similar everywhere and need not be analysed in further detail.

Reference has been made to beach-drifting, to the movement of dunes, and to tidal and other currents. Let us look generally at Great Britain in relation to wind-systems. The prevalent winds (i.e. those blowing most frequently) in any part of this country are from a westerly direction, usually somewhat south-westerly in England and Wales. On our western shores the dominant winds (i.e. those having greatest power or effect) also blow from the same general direction. On the east coast, however, the dominant winds come from the quarter between north and east. How any wind will affect a particular stretch of beach must depend greatly on local conditions. To take an example: south-westerly winds will have great effect in Mount’s Bay, but not just east of the Lizard Peninsula. Allowing, however, for detail of this kind, the westerlies are responsible in the main for eastward directed beach-drift along the Channel, for that up the Bristol Channel, and for that along the coasts of Cardigan Bay. Another factor is also important—the relation of wind-direction to the amount of open water off a particular coast. On the Cumberland coast, the direction of beach-drift is north and south from approximately St. Bee’s Head. This is in general conformity with the amount of open water off these two parts of the coast. However, the relationship is better seen on the east side of England. Along the Norfolk coast, excluding minor exceptions, the travel of beach material is on the whole westwards from Sheringham along the north coast and south-east and south from that same place along the east coast of the county. The dominant winds and waves approaching the Cromer-Sheringham coast are from the quarter between north and east: these, working in with the extent of open water offshore and with the general trend of the coast, are mainly responsible for the outward drift from that locality. The southward drift of beach material continues, apart from a few minor interruptions, as far as the Thames.

On the whole (except in the inner parts of the Firths of Tay and Forth) beach material travels southwards from north-eastern Aberdeenshire all down the east coast of Great Britain. It is, however, along the more open coast south of Flamborough Head that this is most noticeable. Along the south shore of the Moray Firth and the coast as far as Banff and even Rosehearty the general movement of beach material is to the west, and southwards from Wick it is also directed towards Dornoch Firth and Inverness.

On an indented coast of hard rocks it is difficult to generalise. Each separate bay usually has its own beach, and whatever solid stuff travels round the enclosing headlands does so below water level and cannot easily be traced. The individual coves of Cornwall, Devon, Pembrokeshire, the north coast of Scotland, and elsewhere may have their beaches temporarily removed by storms, but they will gather again in normal times. It is probably true to say that each bay has its own shingle and sand economy. On relatively deep water coasts, such as that of the west of Scotland, it is impossible to generalise about the travel of sand and silt.

The main contrasts we have made between the different parts of the coasts of Great Britain may perhaps be related to an even more general factor. Apart from the Lancashire coast, and excluding local occurrences of boulder clay, it is approximately true to say that a line joining the mouth of the Exe to that of the Tees separates a region of softer rocks and simpler structure to the south and east from a more complicated region of harder rocks to the west and north. The former is associated with long lines of open beach and sweeping curves along which beach and long-shore drifting are well exemplified. The latter is often a coast broken by inlets and hard and rocky lines of cliff, along which lateral movement is irregular.

CHAPTER 3 (#ulink_7b2d52fe-ad40-5778-9a7a-dc769b515a48) SOME ECOLOGICAL CONSIDERATIONS

IT IS HARDLY possible to understand how the vegetation is distributed round the coast-line without having some slight acquaintance with the principles of plant ecology. In this chapter we shall therefore consider quite briefly what ecology is about and also take the opportunity to explain some of the terms which are commonly used by ecologists. No attempt will be made to go more deeply into the subject than is necessary to follow the method used in the later chapters, which describe in detail the characteristic vegetation to be found in various typical habitats along the shore. For a fuller account of the subject the reader is referred to Professor A. G. Tansley’s fine book, The British Islands and their Vegetation. In the following short account the examples have been chosen as far as possible from seaside vegetation in the hope that the main characteristics of coastal habitats in general will become apparent.

Plant ecology is concerned with the study of plants in their natural habitats and their relations with their surroundings. It is thus primarily a field study and can be worked out only in the place where the plants are actually growing. The present popularity of both plant and animal ecology is to a certain extent a reaction from some of the more specialised lines of inquiry in biology, which have to be carried out indoors in laboratories.

One of the most fundamental differences between plants and animals is that the former are fixed in the soil, and cannot therefore move about when they are growing. They are thus, of necessity, gregarious and have to lead a communal life. Plants are, in fact, usually found in well-marked communities, whose composition depends on the nature of the habitat and a number of other factors, some of which are discussed later in this chapter. The word plant community is a general one which is used to describe any collection of plants growing together which can be said to possess a definite individuality. If there is much bare ground between the individual plants, which is available for colonisation by other species, the community is said to be open. The plants found growing on the front (seaward) range of sand-dunes in an area of blown sand form a typical open community (Pl. VII (#litres_trial_promo)). Other obvious examples to be found amongst coastal vegetation are the communities inhabiting exposed sea-cliffs (Pl. II (#litres_trial_promo)), and the mobile mud along the edges of salt-marshes (Pl. XIII (#litres_trial_promo)). When the vegetation is more or less continuous, and competition for the available space becomes an important factor, the community is said to be closed. An open community generally represents an early stage in the colonisation of an area, but it may also be found in a habitat where the conditions are so harsh that plants have great difficulty in existing at all.

Although the individual members of an open community depend largely on the nature of the habitat, the amounts and nature of the species present will depend increasingly on their inter-relations in the available space. Usually one or more dominant species, which are mainly responsible for the general appearance of the community, can be recognised. They are frequently the tallest-growing plants present and may thus exercise a profound influence upon the other inhabitants of the community, particularly by competing successfully for the available space or by causing shade. As examples from coastal vegetation, we may mention rice-grass (Spartina townsendii), which is the main dominant species in the communities formed on the soft mud of salt-marshes along the south coast (Pl. XIV (#litres_trial_promo)), and the sea-rush (Juncus maritimus), which frequently dominates a zone along the upper edges of salt-marshes elsewhere. The other species associated with these dominants are known as subordinate species. If these are found in nearly every example of a community, they are called constant species. Any other plants which turn up from time to time in the community, but are not really characteristic, are known as casuals.

Plant communities may be of very different sizes and importance, and it is customary to divide them into various classes. The largest unit of vegetation is called a plant formation and usually refers to a broad type of vegetation which remains roughly the same over a whole continent or even throughout the world. The character of a formation depends on the nature of the habitat and it reflects this in the distinctive life-forms of its principal species. Thus the Salt-marsh Formation contains a highly characteristic population of halophytes, whose specialised life-forms reflect the most important feature of the habitat, that of its periodical immersion by sea-water. Similarly the Sand-dune Formation contains another very characteristic population of plants, many of which are xerophytes and specially adapted to grow in the semi-arid conditions of blown sand. (Some ecologists restrict the use of this term to the ultimate climax vegetation which can be developed in a habitat under given climatic conditions, and would not therefore refer to either of these essentially transitional types as formations.)

The term plant association has in the past been used to refer to so many different units of vegetation that, to avoid confusion, it has not been employed in this book. It is now generally accepted that it should be used to describe a relatively large unit, usually a geographical sub-division of a formation which is characterised by a particular dominant species. As an example, we could say that the Oak-Beech Association is the typical form in the British Isles of the main European Deciduous Forest Formation. In the same way, the Marram-grass Association is typical of the Sand-dune Formation in this country, though associations with other plants as dominants may be found in similar habitats in other parts of the world.

From the point of view of our discussion of coastal vegetation, however, the most important unit to define is the plant consociation. This is a smaller affair than either of those so far mentioned, although it was frequently called an association in the old days. It consists of a community with (usually) a single dominant species. Salt-marshes generally show well-marked examples, since the vegetation often occurs in distinct zones. Thus the lowest strip is often dominated by annual glasswort (Salicornia stricta) (Pl. XIII (#litres_trial_promo)), and other typical zones are dominated by such plants as sea-aster (Aster tripolium) sea manna-grass (Puccinellia maritima) (Pl. XIb (#litres_trial_promo)), sea-lavender (Limonium vulgare) (Pl. 5 (#litres_trial_promo)), etc. Plant consociations are often named after the Latin name of their dominant species by adding the suffix etum to the stem of the Latin name of the genus. Thus the consociations referred to above are usually called the Salicornietum, Asteretum, Puccinellietum and Limonietum respectively. Should there be any possibility of confusion over the identity of the dominant species, the specific name is usually added in the genitive case. For example, consociations dominated by two separate rushes are found in salt-marshes in different areas, and the word Juncetum maritimae is therefore used for that dominated by the sea-rush, to distinguish it from that dominated by the mud-rush, which is called Juncetum gerardii.

The smallest unit with which we need concern ourselves is the plant society. This is a purely local community, which may sometimes be noticed within a consociation, dominated by a species which would be considered a subordinate one if the consociation were viewed as a whole. Societies generally owe their origin to some small local differences in the habitat. Thus the sea-purslane (Halimione (Obione) portulacoides) often forms a distinct society along the sides of the creeks which cut through the Puccinellietum or Asteretum in a salt-marsh, because the soil there is better drained (Pl. XV (#litres_trial_promo)). Another type of society is a layer society, which can be observed when the vegetation is composed of plants of very different heights. This is most obvious in a forest, but an important society of mosses and lichens can often be seen below the main herbaceous layer on the older sand-dunes, and there is frequently a layer of shade-loving plants in the Juncetum maritimae in a salt-marsh.

In explaining the various units of vegetation which are recognised by ecologists we have tacitly assumed that they remain stable and possess a constant composition and structure. This is, however, by no means the case; nearly all vegetation is continually changing, although the rate at which this is proceeding varies greatly. Some communities appear to be remarkably stable, but others are mere passing phases, which soon give place to others. We ought therefore to look upon all these units as representing positions of relative equilibrium into which plants group themselves for a time. Generally speaking, the changes which are in progress all tend towards a position of greater stability. All progressive change of this kind is known as succession.

There are two main types of change which can bring about a succession of vegetation. To the first type belong all those which are caused by purely physical factors which alter the habitat in some way, making it less suitable for the first occupants and more suitable for others. A long-term example of this kind of change would be a gradual alteration in the climate; there is plenty of geological evidence of the effect of such climatic changes in past eras upon the vegetation of the British Isles. It is often possible, however, to see much more rapid changes in progress. For example, the sand on the sea-shore always contains enough salt to make it somewhat alkaline, but as soon as it has been raised above the level of the highest tides in the form of a sand-dune, the salt will rapidly be washed out by the rain. If there is only a small amount of calcium carbonate (another substance causing alkalinity) in the sand, this will also in time be washed out from the surface layers, and plants which prefer more acid conditions can then become established. In this way, the first colonists, which prefer neutral or slightly limy soils, will be gradually replaced by others and eventually “dune-heath,” with heather as the dominant species, may sometimes be produced. Another example is provided in some dune areas, where water tends to accumulate between the ranges of the older dunes, producing a totally different type of habitat within the main area of blown sand. Here a community consisting almost entirely of marsh plants frequently appears. Yet another example of the effect of a physical change can often be seen in salt-marshes, where the tide has been artificially excluded from the upper levels by the construction of some sort of barrier. Here the vegetation is rapidly changed by the appearance of numbers of non-halophytes as soon as the rain has washed out the residual salt from the soil.

The second type of change which may bring about a succession of vegetation is one produced by the plants themselves. When any bare ground is colonised, there is nearly always at first a fairly rapid series of changes in the composition of the plant communities. The first colonists or pioneers will almost invariably give way to others later, and these in turn may afterwards be replaced by still others until a relatively stable equilibrium is reached between the habitat and its vegetation. This type of development can probably be observed taking place along the coast better than anywhere else in the country. The usual way in which plants alter a habitat is by adding humus to it. Humus is the dark organic material produced by the partial decay of plant remains, and as the first colonists die off this material begins to accumulate in the surface layers. In course of time the physical properties of the soil are modified by the addition of this humus and, in particular, its water-holding power is steadily increased. As a result of this, it becomes possible for a wider selection of plants to gain a footing. As a rule, the new occupants are of greater size and stronger growth, so that the earlier colonists are eventually swamped by them. Later on, these in turn may be choked out by other even stronger plants. Thus each successive community, by modifying the soil, tends to make the habitat more suitable for the growth of new species, but in so doing lays the way open for its own ultimate destruction. For example, many of the early colonists in the mobile sand of young sand-dunes are unable to exist in the thick sward of grasses and other plants which cover the surface of the older dunes, and the marram-grass itself is eventually stifled when the surface of the sand becomes completely fixed. The early colonists of sand-dunes, however, not only add humus to the sand but also modify the habitat by anchoring the surface of the sand. Only a limited number of pioneer plants can exist in the shifting sand between the clumps of marram-grass on the young dunes, but they all make their contribution towards fixing the surface of the sand. As a result of their efforts, it gradually becomes possible for a greater variety of plants to become established, and eventually the characteristic close sward of fixed dunes is produced.

In many cases a modification of the habitat may be produced by the combined efforts of plants and physical factors. The colonisation of the bare soft mud on the edge of a salt-marsh is a good example of this. The pioneer plants, such as glasswort or rice-grass, are instrumental in stabilising the mud and also add humus to it. In addition, they aid the natural physical process in which mud is deposited by causing a distinct slackening of the tide as it ebbs and flows over them, and in this way the level of the habitat is gradually raised and stabilised so that other plants can become established.

Generally speaking, all succession is directed towards developing the most complex vegetation which the climate will permit, no matter what the nature of the original habitat may have been. The ultimate vegetation produced in this way is called the climax formation or the climatic climax. The communities making up this formation will be more or less stable and will not be seriously threatened by new invaders. In most of England and the southern part of Scotland, if the vegetation were left completely undisturbed, oak or beech forest would eventually be developed. In the north of Scotland and most of the central portion also, if we exclude the tops of the higher mountains, the climatic climax would, however, be pine forest, an association of the Northern Coniferous Forest Formation. In comparatively recent times, most of the British Isles was forested in this way, but the large-scale felling of our woodlands during the Middle Ages and later has almost obliterated the natural forests. Nowadays, as a result of intensive agricultural operations, the climax formation is rarely reached in the course of natural succession. Where suitable areas exist, which are not cultivated or grazed, the absence of suitable seed-parents in the immediate neighbourhood precludes the development of natural woodland. Ecologists recognise, however, a number of relatively stable subclimaxes in the vegetation of this country, which are developed under the conditions which are normally present.

Any natural succession of communities which replace each other in a particular habitat is called a sere. Thus those which succeed each other in a salt-marsh all belong to the halosere, salt being the master-factor controlling each of them, and those developing on blown sand to the psammosere (Greek: psammos=sand). The sea-coast provides practically the only habitats in this country where one can see a more or less complete series of communities starting with bare ground and finishing with a type of vegetation which remains comparatively stable under the particular conditions. Elsewhere, succession can be most easily observed in an area which has previously carried some fairly stable type of vegetation, but which has subsequently been modified in some way or other. This is well illustrated when a wood is felled or a heath is burnt and is known as secondary succession. Good examples of this type of development can also be seen along the coast, as for instance when the surface vegetation on a sand-dune is broken through and the strong winds produce a “blow-out” (see here (#litres_trial_promo)), which is then recolonised in much the same way as the fresh sand on the newest dunes (Pl. XXI (#litres_trial_promo)). As another example, the seaward edge of a salt-marsh sometimes becomes eroded as a result of a sudden change of current or for some other reason. The original vegetation is thus destroyed, but in course of time the mud on which it originally grew may be colonised once more to form what is called “secondary marsh,” usually at a different level from the original one.

When we come to look into the reasons why particular plants grow where they do, we find that there are a large number of factors to take into account. Most of these are closely inter-related in the effects they produce, but it is worth while to discuss briefly some of those which are especially important in determining coastal vegetation.

The climate of the country is obviously of the greatest importance, for it controls such factors as the duration and intensity of the sunlight, the range of temperature, the rainfall, the humidity of the atmosphere and the strength of the winds. Climatic factors show their effect most clearly when vegetation is studied on a broad geographical basis, but even in a relatively small area like that of the British Isles the effects of small differences in climate are quite noticeable. Thus the average rainfall and the humidity of the air is much greater on the west coast than on the east, and this probably accounts for many small differences in the distribution of plants along the two coasts. It is certainly responsible for the much richer moss flora found on the western sand-dunes compared with those on the east coast, and may partly account for the occurrence of certain typical “Atlantic” species along our western and south-western coasts. In the same way, the mean temperature in the North is distinctly lower than that in the South, and this is one of the factors responsible for the absence or rarity of a number of plants in Scotland and northern England, which are comparatively common in the South, and also for the fact that certain characteristic north European plants are only found in the North.

Wind is obviously a very important factor in all coastal habitats. Its most pronounced effect is that it increases the loss of water vapour from the leaves of plants by constantly bringing dry air into contact with them. As a result, the growth of many seaside plants is considerably retarded and they are often found in a very stunted form. To combat this, many coastal plants adopt a mat or rosette habit for much of the year. Exposed parts of the coast are generally destitute of trees, and such few trees as do occur near the coast are usually found tucked away in sheltered valleys, or combes as they are called in the West Country. Trees and hedges in coastal areas often assume very distorted forms, which show clearly the direction of the prevailing winds (Pl. VIII (#litres_trial_promo)). This is due to the fact that only the shoots on the leeward side can develop normally, those continually exposed to the prevailing winds being dried off and killed. In this way they appear to have been blown over by the strength of the wind, whereas actually their peculiar shapes are due to the unequal development of the shoots on their two sides. The effect of wind in retarding growth is most marked on the east and north-east coasts, which are exposed to the driest winds, although it is very noticeable on any of our coasts.

Another group of factors to be considered depend upon the general topography of the habitat and may be called physiographical factors. The angle at which the ground slopes, the aspect or direction of the slope and the height of the land above sea-level, are examples of these. The familiar coastal processes of erosion, silting and the blowing of sand, which are discussed in Chapter 2 (#ub4febd31-fe98-519c-b9a8-5681d623f1d0), also come into this class. In addition, the prevalence of strong winds along the coast, whose effects have just been described, is clearly due to a combination of climate and topography. It is hardly necessary to give illustrations of the result on the vegetation produced by all these factors; the relation of the highly specialised community of plants which are found on mobile sand with their habitat, for instance, is sufficiently obvious. Some of them, however, become particularly important when we consider cliff-vegetation. Thus the angle at which the cliffs slope largely controls the amount of soil available for supporting plants in the rock crevices, and will indeed determine the stability of the surface of the cliff itself, if it is composed of soft material. The height above the sea will also determine the amount of spray to which the habitat is exposed, and most cliffs show some zoning of the vegetation which can be correlated with this factor. The direction towards which a cliff faces is also important in determining the amount it will be exposed to the prevailing winds and thus, indirectly, the amount of spray it is likely to receive, and will also control the duration of the periods of shade. There is often a marked difference in the vegetation of cliffs with different aspects, in particular those on the opposite sides of small islands.

Another group of factors, which in some ways show the most pronounced effects on the composition of the vegetation, are those related to the physical and chemical properties of the soil. These are called edaphic factors (Greek: edaphos=the ground). On the coast the commonest physical characteristic of most habitats is that of a poor water-supply. Sand-dunes, shingle beaches, and most cliffs are all subject to periodical drought conditions, which are aggravated by the drying winds. We shall see in the next chapter that the leaves of many seaside plants are equipped with devices of various kinds to check excessive loss of water, and that their root-systems are often very extensive. The amount of air contained in the soil is also related to its physical state, and it is noticeable that a number of plants, such as marram-grass on dunes, sea purslane in salt-marshes, and the shrubby seablite (Suaeda fruticosa) on shingle grow luxuriantly only when their roots are well aerated.

The chemical nature of the soil is also of great importance. Salt is obviously the master-factor in determining the highly specialised vegetation of salt-marshes, and the presence of halophytes in other coastal habitats, such as shingle beaches and exposed cliffs, shows that there also salt spray is deposited in sufficient amount to be an important factor. The ultimate vegetation developed on sand-dunes also varies greatly with the amount of calcium carbonate initially present in the sand. We have already seen that, if this is small, it will be washed out of the surface-layers in time, and that typical plants of acidic soils like heather and heath may eventually appear, as the supply of humus increases. Many west coast dunes, however, have been formed from sand which contains so much calcium carbonate in the form of broken shells that the relatively slow leaching action of the rain has produced little effect on it. As a result, the final vegetation on these dunes remains fundamentally calcicole (lime-loving), and is remarkably similar in composition to the grassland commonly found on chalk and limestone. In the same way, chalk and limestone cliffs may be expected to show some different plants from those which are found on acidic rocks.

Finally, we must say a word about the effects on the habitat caused by living organisms. These are called biotic factors (Greek: bios=life), and include the activities of man and his animals, the effects of rabbits, birds and insects and those produced by the plants themselves. The effects of previous generations of plants in altering the physical and chemical properties of the soil have already been briefly discussed. As far as man is concerned, his activities are less in evidence along the coast than in most parts of the country, since coastal areas do not lend themselves well to agricultural development. Nevertheless, in a thickly populated area like ours, there is no region where the hand of man has not played some part in modifying the vegetation. For instance, large areas of many salt-marshes are used for the grazing of cattle, which has the effect of restricting some plants but not others. Many old salt-marsh areas, too, have been completely transformed by drainage operations or the construction of sea-walls to exclude the tides, and the laying out of golf-courses has altered the vegetation in sand-dune areas in a number of places. Moreover, in certain districts marram-grass has actually been planted to stabilise shifting sand-dunes, and elsewhere rice-grass has been employed in a similar way for reclaiming salt-marshes, so that it is often impossible to distinguish between natural and partly artificial vegetation. Nor should it be forgotten that the large-scale felling of the native forests all over the country in the past has had the indirect effect of preventing the natural development of the climax vegetation in many suitably undisturbed areas along the coast.

Rabbits are frequently responsible for considerable modification of the vegetation, and are often extremely common in coastal areas. In particular, the grassland on the tops of cliffs is often infested with them, and the older sand-dunes provide a veritable rabbit’s paradise. In all probability, the somewhat stunted vegetation which is so characteristic of such areas results as much from its being continually nibbled by rabbits as from its exposure to strong winds. Some plants, however, are more attractive to rabbits than others, so the actual composition of the vegetation may be considerably altered. Even salt-marshes are not exempt from the attentions of rabbits; in some districts, for instance, it is unusual to see more than a quite small proportion of the sea-aster plants reaching the flowering stage. Birds also sometimes have a marked effect on the vegetation, particularly when large colonies gather on small islands for breeding purposes. Needless to say much excreta is deposited on the cliff-ledges and cliff-tops near their nesting sites, and the increase in the amount of nitrogen and phosphates in the soil produced in this way has the effect of altering the composition of the vegetation considerably.

The above brief summary can do no more than suggest the kind of factors which must be looked for if we are to make any attempt to understand why coastal vegetation is distributed as it is, and why particular species occur in some places and not in others. Our knowledge of these matters is still extremely incomplete, and it is well to realise that much valuable information can still be easily collected by amateur botanists who are prepared to make a fairly detailed survey of the vegetation in a particular habitat and to keep their eyes open for the factors which have been responsible for its composition.

CHAPTER 4 (#ulink_78a092f1-a1df-52e1-bfe9-6eb9386f80ef) FORM AND HABIT OF COASTAL PLANTS

THE MAJORITY of the plants we find growing round the coast have to contend with unusually harsh conditions, and many of them are specially adapted to enable them to survive in their inhospitable habitats. In this chapter we shall consider some of the characteristic growth-forms they adopt.

Undoubtedly the main problem for most of these plants is to obtain adequate supplies of water, particularly in the early stages of their growth. This applies both to those growing in such obviously dry habitats as sand-dunes, shingle beaches or rocky cliffs, and to those growing in saline ground, such as salt-marshes or brackish swamps, although the reason for the difficulty is quite different in the two cases. The whole question of water-supply is sufficiently fundamental to merit discussion in some detail.

To deal first with the dry habitats; the whole trouble here is that they do not retain sufficient quantities of water in their surface layers, since the “soil” they provide is largely made up of coarse particles. The water-holding power of a soil depends in the first instance on the size of its particles. If these are large, water can percolate easily through them, and will also evaporate more quickly because of the large air-spaces between them. Thus the greater the number of small particles, the longer the soil will take to become dry after rain. Furthermore, it is a well-known fact that water tends to stick on to the outside of all relatively small particles on account of the force known as “surface-tension,” and since the total surface-area of a given weight of small particles is clearly greater than that of the same weight of coarse particles, the finer the soil the greater its powers of retaining water. But in addition to a lack of small particles, there is usually a shortage of humus in all the habitats in question. This important material, consisting of dead plant-remains in the process of decay, has already been briefly referred to (see here (#ulink_fe6021ce-152f-51d7-916b-9d35e0de0895)). Without discussing the varied forms in which this organic matter can occur, the amount present in a soil can be roughly guessed from its colour. Thus dark-coloured “peaty” soils contain the greatest amount and sands the least. All farmers are familiar with the fact that adequate quantities of this material are necessary in all “light” (i.e. coarse) soils, if they are not to suffer from frequent drought conditions. Humus possesses great powers of absorbing water, chiefly because much of it is usually in the form of very small particles of what are called “colloidal” size (i.e. they are so small that they easily pass through a filter-paper, and take a long time to settle when they are suspended in water). Quite apart from this, it is a valuable source of plant food, partly on account of the nitrogen it contains, but principally because it absorbs valuable salts and prevents them from being washed away.

The plants growing in saline habitats also have trouble with their water-supply, though of a very different kind. Here there is often an abundance of water, but it is, of course, salt water. As a result, the plants may suffer from what has been called a “physiological drought.” This means that, despite an abundance of water in the soil, they are unable to make use of it on account of the high concentration of salt it contains. Many salt-marsh plants, therefore, live under conditions of partial drought rather similar to those encountered in other coastal habitats. As evidence of this, it can often be noticed that they are greatly benefited by the dilution of their soil-water, when a spell of wet weather occurs in the summer. Indeed many, though not all, halophytes can grow luxuriantly in ordinary garden soil.

Another characteristic of coastal habitats is that they are all to a greater or lesser extent exposed to strong winds. The most important result of this, as has already been pointed out, is to increase the rate of evaporation of water at the leaves (transpiration). This causes plants to draw further on their slender water-supplies, and if these are inadequate, wilting may take place. Thus wind aggravates the results of the water-shortage.

In order to understand the various ways in which maritime plants deal with this fundamental problem of water-supply, it is necessary to say a word about two processes, common to all plants, which are specially important in this connection. These are osmosis and transpiration.

OSMOSIS

All plants obtain the water and soluble salts required for their growth through the agency of cells situated near the ends of their roots, which are known as the root-hairs. These cells are filled with sap, which contains small quantities of soluble salts and much larger amounts of soluble organic substances, such as sugars, in solution.

FIG. 2.—Experiment to demonstrate osmosis.

When the root-hairs are in close contact with the soil-water, a suction pressure is developed through the walls of these cells, called the osmotic pressure. As a result of this, water passes into the cell and temporarily dilutes the sap. This is a familiar chemical phenomenon and can easily be demonstrated in a number of ways. If any solution is enclosed in what is called a “semipermeable membrane” (i.e. one which will allow water, but not dissolved substances, to pass freely through it), the osmotic pressure of the solution will cause water to be sucked into the solution through the membrane.

A simple way in which this can be demonstrated is to tie a piece of pig’s bladder very firmly over the end of a small funnel (preferably a “thistle funnel”). If the funnel is now filled with a strong solution of (say) cane sugar and is immersed in a vessel containing water, the water will soon start to pass through the membrane into the sugar solution and rise up the stem of the funnel (Fig. 2 (#ulink_516121d3-1b3e-59a9-a63d-1761574bd631)). If a long piece of glass tubing is attached to the stem, it will be noticed, after a day or two, that a column of water several feet high has risen up the tube. It is important to realise that the height of this column is not a measure of the pressure exerted by the initial sugar solution, since this solution is naturally becoming steadily more dilute as the water enters it. In point of fact the osmotic pressure of a 15 per cent solution of sugar is about 10 atmospheres, or sufficient to support a column of water of well over 300 feet in height!

The walls of the root-hair cells function in exactly the same way as semipermeable membranes, though they allow considerable amounts of the substances dissolved in the root-water to pass through them also. Measurements of the osmotic pressures exerted by the cell-sap of many different plants have been carried out. These have been found to vary considerably, but a value of about 10 atmospheres for normal plants (mesophytes) can be taken as an average figure. As a result of this large suction pressure, it is obvious that a root-hair which is freely supplied with water will soon become distended with diluted sap and develop a corresponding balancing pressure. This is called the “turgor pressure.” An equilibrium between this and the osmotic pressure, resulting in the cessation of the flow of water into the cell, would soon be reached were it not for the fact that water is continually passing from the root-hairs into the root and stem of the plant. It is this movement which maintains a flow of water through the whole plant, the excess water being eliminated largely through the leaves. The process by which water is conducted through a plant is extremely complex, and there is nothing to be gained by attempting to discuss it here. The important thing to understand is that, provided water is in contact with the root-hairs, a steady flow into the root can be maintained. On the other hand, if insufficient water is available in the soil, the plant may not be able to obtain an adequate supply to build up its turgor pressure, with the result that the whole plant becomes limp and is said to “wilt.” Obviously this danger is greater if water is eliminated too rapidly by the leaves, and plants growing in dry habitats are often provided with devices to prevent excessive transpiration.

Measurements of the osmotic pressures exerted by both halophytes and xerophytes have been shown to be, in general, much larger than those characteristic of normal plants. We have mentioned 10 atmospheres as a typical value for mesophytes, whereas 40 atmospheres would appear to be an average value for plants in the former classes. Indeed, some desert plants have been shown to exert pressures running up to the enormous figure of 100 atmospheres and more. Obviously this greatly increased power of suction must be of much assistance to plants growing in arid soils in enabling them to obtain what little water there is. In the case of halophytes, a high osmotic pressure is virtually essential if they are to overcome the considerable pressure of the salt water in which they have to grow. Ordinary sea-water has an osmotic pressure of about 20 atmospheres, but in a salt-marsh the concentration of salt may become very much higher during a spell of dry weather in those areas which are not submerged by every tide. If halophytes were incapable of exerting a greater osmotic pressure than that of the salt water in a marsh, osmosis would take place in the wrong direction and water would be sucked out of the plant into the soil-water. Thus the plant would not only fail to obtain its water-supply, but would lose much of the water it already contained. This effect can easily be demonstrated by putting any normal plant into salt water, when it will be seen to wilt in a very short time.

A good deal of work has been done on the measurement of the osmotic pressures developed by halophytes when growing in salt solutions of varying concentrations. The results show clearly that they fluctuate considerably and are able to alter rapidly to adjust themselves to changes in the concentration of salt in the soil-water. This adaptability accounts for the wide tolerance shown by many halophytes growing in different parts of salt-marshes. The whole problem of the mechanism by which water is absorbed by plants is very complex, and the above account is much simplified in order to explain the main principles.

TRANSPIRATION

Passing now to the other end of the plant, we must say something about the process by which the surplus water is disposed of at the leaves. This is known as transpiration. On the surface of any leaf a number of minute pore-like openings are to be found which are called the “stomata.” Each stoma usually takes the form of a slit between two elongated cells known as “guard cells,” lying side by side (Fig. 3(c) (#litres_trial_promo).) The opening or closing of the pore is controlled by the swelling or contraction of this pair of cells. Thus, when the turgor pressure of the plant is high, the cell-walls expand and the slit is opened to aid the elimination of water. When the water-supply is less abundant, the turgor pressure falls and the cells contract so that they lie with their walls in contact with each other, thus closing the slit. It should be emphasised that the stomata are not only concerned with the elimination of water-vapour but are also the organs through which the plant absorbs carbon dioxide and gives out oxygen in the carbon assimilation process (photosynthesis). They are in fact the openings through which the exchange of all gases takes place, although to some extent the whole surface of the leaf and even the stem functions in this capacity. When the external covering or “cuticle” of the leaf is thick, however, the process is largely confined to the stomata. Usually these occur more thickly on the under-surface of the leaf, as being better protected from the drying influence of the sun. Only in water-plants with floating leaves are they confined to the top surface. Although the number of stomata found on the leaves of different plants varies greatly, there do not appear to be any fewer on those belonging to halophytes or xerophytes than on the leaves of normal plants. A moderately large leaf with an average density of stomata may possess several millions of such openings.

Although there is much which is obscure about the transpiration process, it has two important effects. Firstly, it maintains a constant flow of water from root to leaf through the wood of the plant, bringing with it also small quantities of dissolved salts which are essential for the plant’s growth. Secondly, it tends to reduce the temperature of the leaf when it is exposed to the heat of the sun. It is a well-known fact that when a liquid is changed into vapour, energy (latent heat) has to be expended. This heat is derived from the air immediately in contact with the surface of the leaf and in this way the leaf itself is cooled. In hot climates and in dry habitats this result may be important. The chief danger with xerophytes, and to a lesser extent with halophytes, is that the loss of water by transpiration may be so rapid that it cannot be replaced from the scanty supply of water available at their roots. Many plants belonging to both these classes are therefore equipped with devices to check excessive transpiration, and some of these will now be described.

TRANSPIRATION-CHECKS

OR DEVICES FOR REDUCING TRANSPIRATION

The development of a thick cuticle or outer skin on the leaves is the simplest and most frequently adopted method for the reduction of transpiration. The leathery feel of the leaves produced by many seaside plants is a characteristic which can hardly be overlooked, though the development of thick cuticles is by no means confined to coastal plants. In some cases this thickening is supplemented by the secretion of wax on the leaf surface, as in the case of the sea-holly (Eryngium maritimum) (Pl. 1 (#litres_trial_promo)). These protective layers have the effect of confining the evaporation of water entirely to the stomata, for in their absence a considerable amount of water is lost through the rest of the surface. Fig. 3 (#litres_trial_promo) shows diagrammatically a transverse section round a stoma of a leaf with a thin cuticle (a) and a similar section from a leaf with a thick cuticle (b). It will often be noticed that the thickness of the cuticle varies considerably amongst individuals of the same species, according to the habitat in which they are growing. The leaves of the scarlet pimpernel (Anagallis arvensis) (Pl. 2b (#litres_trial_promo)), for instance, become thick and leathery when it is growing on bare sand amongst dunes, although under normal conditions in garden soil they are soft and slender.

FIG. 3.—Types of stomata: a. Transverse section of leaf with a thin cuticle: b. Transverse section of leaf with a thick cuticle, showing a sunken stoma; c. Surface view of a stoma.

In many plants the stomata are protected by being placed in grooves or hollows sunk well below the surface of the leaf (Fig. 3(b) (#litres_trial_promo)). In the dune grasses, marram-grass (Ammophila arenaria) and sea lyme-grass (Elymus arenarius), the stomata are mostly confined to the bottom and sides of the deep grooves in their leaves. This protection is much improved by the tendency of the leaves to roll up into a narrow tube in dry weather, which has the effect of maintaining a layer of air, largely saturated with water-vapour, between the stomata and the outside air, and thus reducing evaporation. The manner in which air is enclosed when the leaf rolls up is clearly shown in Fig. 4 (#litres_trial_promo), and the corrugated inner (i.e. upper) surface is due to the deep grooves along which the stomata are scattered. The outer (i.e. under) surface is furnished with a thick cuticle and is devoid of stomata. This habit of rolling the leaf under dry conditions is shared by many plants, and is a good example of the way they can adjust themselves to variations in their water-supply. When water is plentiful, the blade opens out and becomes flat, thus exposing a greater surface for transpiration. The fresh appearance of marram-grass on open sand-dunes after abundant rain is quite distinct from its parched look after a long period of dry weather, and on closer inspection will be found to be due to the unfolding of its leaves.

FIG. 4.—Transverse section of marram-grass leaf when rolled (from Fritsch & Salisbury, 1946).

Another common way in which the stomata are protected is by the growth of hairs on the surface of the leaf. These are often associated with sunken stomata and are very effective in maintaining a damp atmosphere round the opening, since moisture tends to condense on them. The stiff hairs protecting the furrows on the upper surface of the marram-grass leaf will be noticed in Fig. 4 (#litres_trial_promo). Many seaside plants have hairy leaves, and some are covered with a thick down. The yellow horned poppy (Glaucium flavum) (Pl. IX (#litres_trial_promo)), the sea stock (Matthiola sinuata) and the buck’s horn plantain (Plantago coronopus) (Pl. XXXVI (#litres_trial_promo)) are good examples of coastal plants with hairy leaves, while the leaves of sea-wormwood (Artemisia maritima) (Pl. XXXI (#litres_trial_promo)) and the tree-mallow (Lavatera arborea) are markedly downy. The characteristic silvery foliage of the sea-buckthorn (Hippophae rhamnoides) (Pl. XX (#litres_trial_promo)) and sea-purslane (Halimione (Obione) portulacoides) (Pl. 16 (#litres_trial_promo)) is also due to scale-like (peltate) hairs covering the surface of the leaves. These hairs are usually dead when the leaf is mature, and contain only air. Apart from aiding the retention of moist air near the surface, they reflect much of the sun’s heat. Some leaves possess simple unbranched hairs, but those on many others are branched and occur in very different forms. Some typical covering hairs from the leaves of coastal plants are shown in Fig. 5 (#litres_trial_promo). Like the thickness of the cuticle, the degree of hairiness shown by individuals of the same species often varies with availability of the water-supply in the habitat. Thus the sand-dune form of silverweed (Potentilla anserina) commonly shows a thick felting of silvery hairs on the upper surface of its leaves, as well as on the lower.

FIG. 5.—Typical covering-hairs on various leaves: a. Plantago coronopus; b. Cynodon dactylon; c. Erophila verna; d. Matthiola sinuata; e. Hippophae rhamnoides.

Still another way in which relatively damp air is maintained over the surface of the leaves is by the plant adopting a dense mat habit, so that the transpiring surfaces of the leaves are kept in close contact with each other. Alpine plants often mass their foliage in this way, but amongst coastal plants thrift (Armeria maritima) provides one of the best examples, since its habit varies considerably with the place in which it is growing. Thus the close rosette form is typical when it is growing on rocky cliffs and other dry habitats, or when it is heavily grazed, whilst with a better water-supply it assumes a much more open habit (Fig. 6 (#litres_trial_promo)). Many sand-dune plants spend most of the year in the form of a rosette, only sending up a vertical stem during the flowering season. In this way, only the upper surface of the leaf is exposed to the wind, the under surface being kept closely pressed against the surface of the sand, where it is fully protected from both sun and wind and consequently remains cool and moist.

Transpiration is discouraged in a large number of widely differing plants by a reduction in the actual surface of the leaves. Many conifers furnish examples of this; pines have needle-shaped leaves, and cypresses have scale-like leaves, which are closely pressed to the stem over part of their surface. Among coastal plants, tamarisk (Tamarix gallica) (Pl. 8 (#litres_trial_promo)), now a well-established alien in Britain, has numerous little scale-like leaves, and in the glassworts (Salicornia) the rudimentary leaves are only just visible as tiny scales which are firmly attached to the joints of the succulent stems (Fig. 9(b) (#litres_trial_promo)). In some plants the same result is achieved by the leaves taking the form of spines. Gorse (Ulex spp.) is the best-known example in this country, but in desert regions the majority of the xerophytic plants show this modification, the Cacti being a familiar case. Our native xerophytes more frequently develop spiny margins to their leaves, thistles furnishing the obvious example. The most striking seaside plant to show this development is the sea-holly (Pl. 1 (#litres_trial_promo)), though the leaves of the prickly saltwort (Salsola kali) (Pl. I (#litres_trial_promo)) also terminate in stout spines. The tendency to form woody tissue in the form of spines appears to be closely related to a shortage in the water-supply. A number of plants which produce spines when growing in dry habitats do not possess any when moisture is abundant.

Occasionally the function of the leaf is taken over by specially modified branches known as “cladodes.” The only coastal plant exhibiting this modification is the wild asparagus (Asparagus prostratus), a rare plant found on sandy shores in a few localities only in this country. If the familiar feathery foliage of the garden asparagus is examined, it will be seen to consist of tufts of short leaf-like branches arising from the axils of minute scaly leaves (Fig. 7 (#litres_trial_promo)). It is difficult to see exactly what advantage a plant can gain from the substitution of a leaf-like stem for an ordinary leaf—possibly the tissue of the cladode is more resistant to shrinkage when the plant is suffering from a shortage of water.

FIG. 6.—Different forms adopted by thrift: a. Rosette form under grazing or in dry ground; b. More diffuse habit when protected from grazing and with a good water-supply (from Tansley after Yapp, 1917).

FIG. 7.—Part of a branch of asparagus, showing cladodes and scale-leaves (s).

Quite apart from these permanent alterations in leaf-form, the shape and size of the leaves of many common plants vary greatly with the conditions under which they grow. For example, the first leaves of the red-fruited dandelion (Taraxacum laevigatum), when growing in a moist hollow among sand-dunes, are often quite entire (i.e. with smooth edges); later in the season, when the sand has become dry, it produces the more familiar deeply divided leaves with a much smaller surface-area (Fig. 8 (#litres_trial_promo)). Most of the common inland plants found on sand-dunes possess smaller leaves than when they grow on more hospitable ground. Nor must we forget that the semi-prostrate form so frequently adopted by dune-plants is still another method by which excessive transpiration can be reduced, since every extra inch in height exposes the plant more to the desiccating action of the strong winds.

FIG. 8.—Different leaf-forms of the red-fruited dandelion: a. Young leaf from a plant growing with abundant moisture. b. Leaf from a plant growing on dry sand.

It will be clear from what has been said that many plants are capable of modifying their normal form when growing in dry habitats. Of the various transpiration-checks which have been described, undoubtedly the development of a thick cuticle is the most frequent one employed by coastal plants. It possesses an added importance for plants inhabiting open sand-dunes in that it also protects them from possible injury caused by the sand being blown against them. Anyone who has done any botanising on exposed sand-dunes during a high wind will know how violent this bombardment can be!

It is important to point out, before we leave this subject, that it is only during periods of water-shortage that these mechanisms for reducing transpiration become important. Recent research has shown that xerophytes transpire during wet spells at least as much as, and often more than, ordinary plants. In those cases where the transpiration-rate becomes unusually high, it may be related to the necessity for rapid growth and carbon assimilation during the infrequent wet periods. What really characterises a xerophyte is that it can, if need be, decrease its transpiration-rate to a minimum when living under drought conditions. In addition, the actual protoplasm (living matter) seems able to withstand desiccation to an unusual extent.

SUCCULENCE

Some xerophytes employ quite a different method to provide against water shortage, though it is often found in combination with the leaf-modifications already described. It will be noticed that many plants growing in dry places have a fleshy or succulent appearance. This is due to the development of large colourless cells, known collectively as “aqueous tissue,” which are employed for storing water. This is usually confined to the leaves as in the stonecrops (Sedum spp.) (Pl. 14 (#litres_trial_promo)) or the sea-spurge (Euphorbia paralias) (Pl. XIX (#litres_trial_promo)), but sometimes the whole stem is succulent as in the glassworts (Salicornia spp.) (Fig. 9(b) (#litres_trial_promo)) or the familiar Cacti, the leaves in these cases being reduced to mere scales or spines. As a rule these cells occupy the centre of the leaf or stem, and the green cells which are used for photosynthesis occur nearer the edges. In dry weather, as water is gradually lost by transpiration or by its passage into the green cells, the water-holding cells shrink; when the water-supply improves, they expand once more. They function, in fact, as water-storage cisterns for use by the plant in times of drought. Desert succulents, which often possess extremely thick cuticles to reduce transpiration, can exist for prolonged periods without an external supply of water, during which they gradually shrivel until they can replace their internal water supplies when the rain comes.

It is rather surprising that halophytes should form the largest class of plants exhibiting succulence in this country. It has already been pointed out that the chief characteristics of this group are that they can exert sufficiently large osmotic pressures to withdraw water from a soil which is saturated with sea-water, and that the protoplasm forming their cells is not injured by exposure to salt solutions. Under normal conditions, therefore, they should not encounter much trouble with their water-supplies, and it is difficult to understand why they should develop aqueous tissue so extensively. It is possible that they draw on their internal reserves of water when the concentration of salt in the soil alters too rapidly for them to accommodate their osmotic pressure to it, but this can hardly account for such a widespread characteristic. Furthermore, it has been shown that halophytes do not, in fact, withstand drought like succulent xerophytes, but actually wither quite rapidly.

The most likely explanation is that the similarity in appearance of succulent xerophytes and halophytes is largely accidental. There is considerable evidence to suggest that the latter become succulent as a result of some chemical effect associated with salt, probably with the chlorine rather than the sodium part of it (salt is a simple compound between these two elements). All plants when growing in a saline soil absorb some salt, for the cell-walls of the root-hairs never function as “perfect” semipermeable membranes, but allow a certain amount of the dissolved substances to pass through them. This is, of course, true of all types of plant, for otherwise they would be unable to obtain the small quantities of other mineral salts essential to their growth. Many halophytes, however, absorb very large amounts of common salt; the ash of some of them, like the glassworts, was formerly used on a large scale to provide soda for glass-making, and certain plants, as for example thrift, actually excrete surplus salt from the glands on their leaves. That the development of succulent leaves is closely connected with the absorption of salt is borne out by the behaviour of many non-halophytes when they grow in places exposed to sea-water. Many inland plants found on open beaches or on cliffs within reach of sea-spray possess much more fleshy leaves than they have in their normal habitats; bird’s-foot trefoil (Lotus corniculatus), kidney vetch (Anthyllis vulneraria) and the greater knapweed (Centaurea scabiosa) are species which often show this effect. Some years ago I analysed the ash of certain inland plants which had been exposed to sea-spray in this way and found 13.5 per cent of salt in that of the kidney vetch. Evidence such as this points strongly to the conclusion that some chemical action connected with salt is the primary cause of succulence in halophytes, and there seems little reason to associate it with the problem of conserving water. The similarity between succulent xerophytes and halophytes is remarkable and we must leave it at that.

FIG. 9.—Comparison of root-systems of annual glasswort and rice-grass:-a. Small plant of annual glasswort; b. Two joints from stem of annual glasss wort enlarged to show leaf-scales; c. Base of a rice-grass plant showing vertical anchoring roots and horizontal feeding roots and stolons (Fig. c from Tansley after Oliver, 1926).

ROOT-SYSTEMS

The roots of coastal plants are very characteristic and present many points of interest. The majority of true halophytes (i.e. plants which normally grow where the soil-water is saline) possess very deep roots, generally markedly woody. Most of these plants are perennials and the chief value of their long roots in a salt-marsh is to enable them to secure a firm anchorage in relatively unstable mud. It also allows them to derive their main water supplies from regions where the concentration of salt is less variable than it is in the surface layers. Annual glasswort (Salicornia stricta) (Pl. XIII (#litres_trial_promo)) possesses only quite short roots, and as a result is liable to become dislodged from the unstable mud if there is a strong tidal flow (Fig. 9(a) (#litres_trial_promo)). This is in marked contrast to rice-grass (Spartina townsendii) (Pl. XIV (#litres_trial_promo)), which occupies much the same position as a pioneer colonist in many of the south coast salt-marshes. This plant develops a most extensive root-system and becomes so firmly anchored in the mud that it can easily resist the strong currents produced by the ebb and flow of the tides (Fig. 9(c) (#litres_trial_promo)). Its remarkable powers of spreading over soft mud and stabilising the surface are described more fully on page 71. Plants growing on sea-cliffs also develop very long roots, which serve the dual purpose of anchoring them firmly against uprooting by the violent winds encountered in these exposed places and of enabling them to tap deep-seated supplies of water. Well-established plants of thrift or samphire (Crithmum maritimum) (Pl. 10 (#litres_trial_promo)) frequently possess roots several feet long, which penetrate deeply into the crevices between the rocks.

Extensive root-systems are also a characteristic feature of xerophytes in all parts of the world. In sand-dunes and shingle this enables the plants to utilise the moisture which is always present some way below the surface (see here (#litres_trial_promo)). In addition, the elaborate root-systems developed by many dune plants perform the important function of binding blown sand. All pioneer colonists on sand-dunes, and to a lesser extent those on mobile shingle, have to contend with the possibility of being periodically swamped by loose sand or shingle. Most of them have, in varying degrees, the ability to form fresh shoots easily when they are submerged in this way, and to grow up through this covering. Marram-grass easily outstrips all other plants in the vigour with which it can do this. When once established in loose sand, it soon produces a mass of underground runners from which new shoots continually spring. Where these new shoots occur, fresh adventitious roots are produced under them; as the stems and leaves become buried by sand, further shoots and leaves are produced at a higher level on the stem (Fig. 10 (#litres_trial_promo)). The thick tufts of leaves and young shoots are very stiff and offer a considerable obstruction to the wind, causing it to drop some of its load of sand round them. Provided the blown sand does not accumulate too rapidly over the plants, marram-grass can continue to grow upwards through many feet of sand. In this way dunes rising to considerable heights can be produced, their size depending on the supply of available sand and the strength of the prevailing winds. The whole interior of the dune remains closely penetrated by a mass of rhizomes (underground stems) and fine roots which bind it together. As these rhizomes and roots are constantly being renewed at higher levels, the lower ones gradually die off, but their dead remains persist in the lower regions of a dune for a long time and continue to exercise a stabilising effect on it (Pl. XXI (#litres_trial_promo)).

FIG. 10—Leaf production at different levels shown by marram-grass (from) Firistch & salisbury, 1946).

To a lesser extent the sea couch-grass (Agropyron junceiforme) can produce a similar result. This plant has much the same habit as ordinary couch-grass or twitch (Agropyron repens), which is an all too familiar agricultural weed. Like marram-grass, it produces a mass of rhizomes from which new shoots spring up at frequent intervals, but its runners tend to spread more rapidly in a horizontal direction than vertically when covered by loose sand. As a result, the dunes it produces are comparatively low compared with those formed by marram-grass. None the less, its powers of binding sand are considerable, and single plants have been shown to cause dunes as much as 20 feet across during a few years’ growth. The sea lyme-grass (Elymus arenarius) has a similar root-system and occasionally forms low dunes of the same type on certain parts of the coast.

Many other pioneer plants on shifting sand can accumulate small amounts of sand round them to form miniature dunes, provided they are sufficiently virile to shoot up again when they become buried. For example, the sea-sandwort (Honckenya (Arenaria) peploides) is a low-growing plant not more than a few inches high, but it possesses surprisingly extensive creeping roots and readily produces fresh shoots when it is covered. Even those common pioneers of sandy beaches, the sea-rocket (Cakile maritima) (Pl. VII (#litres_trial_promo)) and the prickly saltwort (Pl. I (#litres_trial_promo)), which are only annuals, can collect tiny dunes round their long trailing branched stems. Their dead remains usually persist for a considerable time in the winter and continue to hold the sand, although the principal agent here is the stem rather than the roots.