Disease in Plants

Stripping.—Now suppose that, instead of a mere slit with the knife-point, a strip of bark is removed down to the wood. Exactly the same processes of corking and lip-like callus formation at the edges of the wound occur, but of course the occlusion of the bared wood-surface by the meeting of the lips occupies a longer time. Moreover, the living cells of the medullary rays exposed by the wound on the wood-surface also grow out under the released pressure, and form protruding callus pads on their own account. In course of time the wood is again completely covered by the coming together over its face of these various strips of callus, but two important points of difference are found, as contrasted with the simpler healing of the slit-wound. In the first place the exposed wood dries and turns brown, or it may even begin to decay if moisture and putrefactive organisms act on it while exposed to the air; and, in the second place, the normal annual layer of wood—or layers, as the case may be—formed by the cambium only extends over that part of the stem where the cambium is still intact, and is entirely wanting over the exposed area. Thus, if it takes two years for the cambium to extend across the wound, a layer of wood will be formed all round the intact part of the stem, from lip to lip of the cut tissues during the first year; then a second annual layer outside this will be formed during the second year, but extending further over the edges of the wound, and nearly complete, because the cambium has now crept further across the wounded surface to meet the opposite lip of cambium; and during the third year, when the cambium has once more become continuous over the face of the wound, the annual wood layer will be complete. But, of course, this last layer covers in the edges of the two previously developed incomplete wood-layers as well as the exposed and brown, dry, or rotten dead face of the wood. It also covers up the trapped-in brown cork and any débris that accumulated in the wound, and this "blemish," though buried deeper and deeper in the wood during succeeding annual deposits of wood-layers, always remains to remind us of the existence of the wound, the date of which can be fixed at any future time by counting the annual rings developed subsequently to its formation. Obviously, also, the deficiency of wood at this place makes itself visible on the outside by a depression.

Cuttings.—When a cutting of Pelargonium, Willow, or other plant is made, we have a typical knife-wound, the behaviour of which is very instructive in illustration of plant-surgery, and may be most easily seen by keeping it in damp air instead of plunging it into sand or soil.

All the living cells actually cut or bruised turn brown and die as before; those beneath—e.g. the living pith, medullary rays, cambium, phloem, and cortex, grow out under the released pressure and form a callus, the outermost layer of which becomes cork, while those below, abundantly supplied with food-materials, proceed to spread, as if flowing over the surface of the cut wood, and rapidly occlude the wound. Meanwhile new roots are formed adventitiously from the cambium just above the plane of section, and push out through the cortex into the damp air, and if the cutting had been in soil it would now be capable of independent existence. It is important to keep cuttings upright, as the roots only spring from the lower end. Such cuttings can be obtained not only from stems, but also from roots and even leaves.

Callus-formation is not confined to the basal end of a cutting; it has nothing to do with position, but is a reaction to the wound stimuli, independent of light, gravitation, etc. As time goes on, however, the internal organisation of the erect cutting usually reacts on the callus at either end, and roots only rise from the lower one, while shoot-buds may form in the upper one, though it is possible to bring about the formation of buds from the lower end also.

Branch stumps.—A more complex example is furnished by a branch cut off short some distance—say a foot—from the base, where it springs from the trunk. As before, the immediate effect of the section is the formation of a callus from the cambium, phloem and cortex, which begins to rise as a circular occluding rim round the wood. The transpiration current in the trunk, however, is not deflected into the 12 inches or so of amputated branch, because there are no leaves to draw the water up it, and so the stump dries up and the cortex and cambium die back to the base, leaving the dead wood covered with shrivelled cortical tissues only. This dead stump gradually rots under the action of wet, fungi, and bacteria, and since the pith and heart-wood afford a ready passage of the rot-organisms and their products into the heart of the trunk, we find in a few years a mere stump of touch-wood and decayed bark, which falls out at the insertion like a decayed tooth, leaving a rotten hole in the side of the trunk.

If, however, instead of allowing the basal part of the amputated branch to protrude as a stump, we cut it off close to the stem, and shave the section flush with the normal surface of the latter, the callus formed by the cambium, etc., rapidly grows over the surface, and soon forms a layer of cambium continuous with that of the rest of the stem. The wound heals, in fact, much as if it were a strip-wound, and beyond a slight prominence for a year or two no signs are visible from the outside after the occlusion. Of course these matters depend on the relative thickness of branch and stem, and if much wood is exposed the dangers of rot and a resulting hollow in the stem are increased. It is interesting to note how much thicker the callus lips are at the sides of the wound than above and below, owing to differences in the distribution of the nutrient materials.

Stool-stumps.—When a tree is felled, the stump may, if the section is close to the ground and kept moist, begin to form a thick rim-like callus round the wood, in which adventitious buds soon make their appearance, and grow out into so-called Stool-shoots. The products of assimilation of these, and the stores accumulated in the stump, often suffice to feed the callus sufficiently to enable it to grow over and completely occlude the wound, if the wood surface is not too large, or so long exposed that rotting processes have meanwhile set in.

Ringing.—If the strip of cortical tissues and cambium is removed all round the stem, exposing the wood in a form of a ring, complications may ensue owing to the following circumstances. A well-marked callus appears at the upper edge of the wound, because, the transpiration current up the young wood not being stopped, plenty of water and salts from the soil can reach the leaves; but the nutritive materials supplied by the latter are accumulated at the upper lip of the wound owing to the stoppage there of their descent in the phloem, cortex, etc. No such callus-lip appears at the lower margin of the wound owing to want of these supplies. Consequently the occlusion and healing of the ring-wound only takes place from above downwards, and if the ring of cortical tissues removed is a broad one, the healing may be a long process, or may even be indefinitely delayed, a thicker and thicker callus projecting over from above. For similar reasons no annual wood layers are formed below, but only above the wound, and thus the branch or tree may die. The latter contingency is the more likely the further up the tree the ringing takes place, owing to the risk of drying up which threatens the exposed wood, and to the consequent interruption of the transpiration current, and the likelihood that lateral shoots below the wound may divert the water to their own leaves. If the ringing occurs low down on a stem, and the environment remains damp, the upper thick callus may put out new roots; the part above the wound then behaves like a cutting. If the ringing is done on a young and vigorous branch of an old tree, the lower lip may receive supplies from the leaves of branches below the wound, or from shoots which spring from adventitious buds close to it, and the wound may heal over normally. Such healing may be rendered more certain by keeping the wounded surface moist—e.g. by means of damp moss, and so encouraging the formation of callus-bridges from the medullary rays.

If on ringing a tree or a branch the young wood is removed as well as the cambium and cortical layers, the death of the parts above the wound is almost certain, owing to the stoppage of the transpiration current: the exceptions to this rule depend simply on the existence of other channels of communication, such as internal phloems, very thick sap-wood, and so forth.

Bruises.—If a branch or woody stem is struck sharply, with a hammer, for instance, the bruised cortex, phloem and cambium are killed by the blow, and the general effect is as if these tissues had been removed at that spot by the knife, but with the following complications. The bruised cortical tissues rapidly dry as they perish, and may adhere to the wood below. Consequently the still sound parts bordering on the wound are not released from pressure, but, on the contrary, have to advance towards each other over the surface of the wood under still greater pressures, in part due to the tightening of the whole cortex as the dead parts dry and contract, and in part due to the above-mentioned adherence of the latter to the wood. It results from this that such wounds heal very slowly and badly, and when the killed patch at last ruptures, wound-fungi, insects, and other injurious agencies may get in and do irreparable damage, as has been found to occur in cases where such wounds have been made in striking trees to shake down insects, fruit, etc.

Notes to Chapter XXI

The essential facts regarding wounds and healing by occlusion are given in Marshall Ward, Timber and some of its Diseases, 1889, chapters viii. and ix., and in Laslett, Timber and Timber Trees, 1894, chapters iv. and v. More detailed treatment will be found in Frank, Krankh. d. Pflanzen, B. 1. cap. 2, where the special literature is collected. The reader may also consult Hartig, Diseases of Trees, Engl. ed. 1894, pp. 225-269.

CHAPTER XXII.

NATURAL WOUNDS

Burrows and excavations. Bark-boring—Wood-boring—Wood fungi—Leaf-miners—Pith flecks—Erosions. Skeleton leaves—Irregular erosions—Shot holes. Frost cracks—Strangulations—Spiral grooving.

Natural wounds are produced in a variety of ways during the life of the plant, and, generally speaking, are easily healed over by the normal process if the area destroyed is not too large, and the parts remaining uninjured are sufficiently provided with foliage, or with supplies of food-materials stored up in the roots, rhizomes, medullary rays, etc., to feed a vigorous callus.

The nature of such wounds and the mode of healing are explained by what we know of artificial wounds, and it only remains to point out that the principal danger of ordinary wounds is not so much the direct traumatic action, because the simpler organisation of the plant does not involve matters connected with shock, loss of blood, etc., as in animals; the danger consists, rather, in their affording access to other injurious agents, especially fungi, and the treatment of wounds frequently resolves itself into cutting or pruning in order to get clean surfaces which can heal readily.

Wounds on leaves imply loss of foliar surface—i.e. of chlorophyll action—and the remarks on page 193 apply.

Burrows may be taken as comprising all kinds of tunnel-like excavations in the various organs of plants, including those cases where insects burrow into hollow stems of grasses, etc., as indicated by the perforations they make in the outer tissues.

Bark-boring is done by many species of beetles, especially Scolytidae, which excavate characteristically formed branching passages tangentially in the inner bark of Conifers and other trees. Some of them also bore down to the surface of the sap wood (e.g. Tomicus bidentatus) or even burrow right into the latter (e.g. T. lineatum). It commonly happens that the external apertures show up clearly, owing to the brown dust and excrement, sometimes accompanied by turpentine, which exude from them. Many of these Bark beetles only attack trees which are already injured by fire, lightning, etc.; possibly they cannot bore through a cortex which swamps them with sap, as a vigorous one might do.

Wood-boring is also done by many of the bark-beetles as well as by Longicorns, e.g. Saperda in Poplars and Willows, the young shoots of which often show characteristic swellings with lateral holes indicating the points of exit. From the external apertures comminuted wood, like saw-dust, is frequently ejected in quantity and betrays the presence of the insects. Certain wood-wasps (Sirex) and the larvae of moths (Cossus) also make large perforations in the wood of Willows and other trees, often destroying it completely. In the case of these larger borers, whose tunnels may be as broad as the little finger, the foul smell as well as abundant "saw-dust" betray the evil.

Excavations in wood are by no means caused only by insects: several of the larger Hymenomycetes—Stereum, Thelephora, Polyporus, etc.—tunnel the timber in characteristic ways and often after a fashion very suggestive of insects. They usually obtain access through fractures.

Tunnels in leaves are invariably due to the activity of miners belonging to the smaller moths and beetles—e.g. Tinea, Orchestes, etc.—the larvae of which eat out the mesophyll but leave the covering epidermis or cuticle untouched, and since the insect bores forwards only, in an irregular track, and leaves its excrement in the winding passage, the effect is very characteristic.

Whitish leaf tunnels in Peas are excavated by Phytomyza.

Characteristic foxy-red tunnels are mined in the leaves of Apples by Lyonetia, Coleophora, etc.

Falling of fruit, of Apples, Plums, Apricots, etc., before they are ripe, is frequently due to insects, of which the various species of Grapholitha or Carpocapsa are conspicuous: the fallen fruits show a small hole leading by a labyrinth of passages to the "core" or "stone," and in which the grub and its excrement are visible. The cutting off of the vascular bundles and disturbance of the water supply only partly explain the premature fall.

Pith-flecks are minute brown specks or patches found in the wood-layers of many trees, and consist of dead parenchymatous thick-walled cells, reminding one of the structure of pith. They are explained as due to the borings of minute insects, Diptera or Beetles, the larvae of which pierce the cortex and phloem and bore their way into the cambium. The latter then occludes the tunnels by filling them up with cells, and continuing its wood-forming activity gradually buries them deeper and deeper in the wood. Such pith-flecks are common in Willow, Birch, Alder, Sorbus, etc. It is possible that they may be due to other causes also in other trees.

Erosions or irregular wounds on leaves are caused by large numbers of grubs and caterpillars and other insects, such as earwigs, as well as slugs, snails, and other animals; but it must by no means be assumed that all marginal leaf wounds, for instance, are caused by animals, since many fungi which rot the tissues, as explained below (p. 208), also cause such erosions, the putrescent parts falling out—e.g. the Potato disease.

Skeleton leaves frequently result from the ravages of caterpillars, which leave the coarser ribs and veins untouched, but much finer skeletons with the minute veins almost intact may be found on plants infested with certain insects—e.g. Selandria on Cherries. Skeletonised patches on Cherry leaves, often pink or brown-pink, are eaten out by this grub.

Shot-holes are perforations in leaves presenting the appearance, from their more or less rounded shape, of gunshot wounds. They may be due to insects which bore through the young leaves while still folded in the bud—e.g. Willow Beetle—or which gnaw out the tissue—e.g. the Beech Miner. Similar but usually more torn and irregular holes are eaten out by many caterpillars—e.g. the Cabbage Moth.

Shot-holes on Peas may be the work of Thrips.

Leaf perforations are commonly caused by severe hail-storms, the hail-stones beating right through the thin mesophyll. Certain chemicals used for spraying have also been known to cause shot-holes by killing the tissue beneath the standing drops.

There is, however, a class of shot-holes in thin leaves which are due to the action of minute fungi, the mycelium of which so rots the tissues in a more or less circular area round the point of infection, that, in wet weather, the decomposing mass falls out and leaves a round hole—e.g. certain Chytridiaceae, Peronosporeae, Gloeosporium, Exoascus, etc. If dry weather supervenes these holes frequently dry at the edges, and the leaves appear as if eaten out.

Shot-holes in Cherry, Walnut, Tobacco, and Plum leaves are due to Phyllosticta, in Cherry leaves also to Clasterosporium, and in Potato leaves to Haltica.

Frost-cracks.—The trunks of trees exposed to the north-east, and occasionally with other aspects, are apt to show longitudinal ridges which realise on a larger scale the features of healed wounds scored with a knife. These wounds are due to the outer layers of wood losing water from their cell-walls as it congeals to ice in their lumina, more rapidly than do the warmer internal parts of the trunk; as this drying of the wood causes its shrinkage, especially in the tangential direction, the effect of a sudden frost and north-east wind is to rend the wood, which splits longitudinally with a loud report, as may often be heard in severe winters. Since the cortex and bark are ruptured at the same time the total effect resembles that of a deep knife-cut, and the same healing processes result on a larger scale when the wood swells and closes up the wound again in spring. But this recently-closed lesion is evidently a plane of weakness, and if a similarly severe winter follows the wound reopens and again heals, and so on, until after a succession of years a prominent Frost-ridge results, which may finally heal completely if milder winters ensue or the tree be eventually protected.

Strangulations.—We are now in a position to understand the so-called strangulations which result when woody climbers, telegraph wires, etc., kill or injure trees by tightly winding round them. If strong wire is twisted horizontally round a stem, the growth in thickness of the latter causes the trapping of the cortex and cambium, etc., between the wire and the wood, and a ringing process is set up in consequence of the death of the compressed tissues. A callus then forms above the wound, as in the case of true ringing by means of a cut, and eventually bulges over the upper side of the wire: in the course of years this overgrowth may completely cover in the wire, and, pressing on to the lower lip of the wound, may at length fuse with the cambium below. Hereafter the thickening rings of wood are continuous over the buried wire. The process is obstructed by all the impediments referred to in dealing with ringing, and of course the stem thickens more above than below the wire. If the sapwood is thin, and the bark is so thick as to put great obstacles in the way of the junction of the upper and lower cambiums, death may result—the tree is permanently ringed. (See p. 201.)

Spiral grooves are frequently met with where Wood-bine or other woody climbers have twined round a young stem or branch, the upper lip of the groove always protruding more than the lower. If a kink or a crossing of two plants or branches of the twiner results in a complete horizontal ring, the results are as in the above cases of ringing and strangulation. Naturally grooved walking sticks are often seen.

Buried letters, etc.—These processes of healing by occlusion enable us to understand how letters of the alphabet, cut into the wood of trees, come to be buried deep in the timber as successive annual rings cover them in more and more. Chains, nails, rope, etc., have frequently been found thus buried in wood.

Notes to Chapter XXII

In addition to the notes to the last chapter, the reader may be referred to Fisher in Vol. IV. of Schlich's Manual of Forestry, Chap. VI., for an account of Hess' excellent work on Boring Beetles, etc.

The authority on Wood-fungi is Hartig, see especially his Zersetzungs-erscheinungen des Holzes, the principal results of which are condensed in his Diseases of Trees already referred to. As regards "Pith-flecks," the reader should consult Frank, Krankh. der Pflanzen, B. I., p. 212: the subject needs further investigation.

CHAPTER XXIII.

EXCRESCENCES

Herbaceous excrescences, or galls—Erineum—Intumescences—Corky warts, etc.—Pustules—Frost-blisters—Galls and Cecidia—Root nodules.

Excrescences, or out-growths of more or less abnormal character from the general surface of diseased organs, are very common symptoms, and widely recognised. They are due to hypertrophy of the tissues while the cells are young and capable of growth, and may be induced by a variety of causes, among which the stimulus of insect-punctures and of the presence of insect eggs are best known; but that of fungi, though less widely recognised, plays an equally important part, and, as we shall see, galls and other excrescences may be due to widely different agents.

Galls or Cecidia are protuberances of the most varied shapes, colours, and sizes found on herbaceous parts attacked by insects, fungi, etc. In the simplest cases the insects only pierce and suck the young cellular tissue—e.g. Phytoptus, Aphides, etc.—but in others the stimulus to hypertrophy starts by the puncture of the embryonic tissue of a leaf, root, etc., by the ovipositor of the female insect, which then lays an egg—e.g. Cynips, Cecidomyia, etc.—the presence of which appears to intensify the irritating action, or such only occurs when the young larva escapes.

Our knowledge of the primary cause of gall-formation amounts to very little. Generally speaking, only embryonic or very young cellular tissue reacts, and galls on adult leaves and branches have usually been initiated long before. The same gall-insect may induce totally different galls on different plants, or even on different parts of the same plant, and different insects call forth different galls on any one plant. These facts point clearly to the co-operation of both plant and insect in the gall-formation, and the best hypothesis yet to hand is to the effect that a gall is a hypertrophy of cells, the normal nutrition, growth, and division of which have been disturbed owing to the action of some poison or other irritant derived from the insect, or fungus, or other organism. Attempts have been made to reproduce galls by injecting the juices of similar galls into the tissue, but as yet without success, and this may point to the co-operation of mechanical irritation during the hypertrophy in normal gall-formation.

Galls, in the broad sense, are not always preceded by a wound, however. Insects on the outside of young tissues may cause such irritations that the parts in contact with the animal are arrested in their growth, while those further away grow more rapidly—e.g. where Mites, etc., cause puckers and leaf-rolling. In true galls the hypertrophy may consist merely in the enlargement of cells already present, and no new cell-divisions and, still less, changes in the nature of the tissues result—e.g. some pocket galls on Viburnum, Pyrus, etc., and the hairy outgrowths of the epidermis known as Erineum. In other cases there is not only hypertrophy of existing cells, but new cell-divisions are instituted: these cell-divisions may be confined to the direction perpendicular to the epidermis, and the tissues grow only in the direction of the surface, producing puckerings—e.g. the Aphis galls on Ribes, Phytoptus galls of Salvia, leaf galls on Tilia, Acer, Alnus, etc., and the curious galls on Plums due to Cecidomyia Pruni, and which must not be confounded with the "pocket plums" and similar galls due to Exoasci.