Disease in Plants

The wilting and shrivelling of Clover is sometimes due to Sclerotinia, the mycelium of which pervades the roots and stock, on which the sclerotia may be found. Lucerne is similarly killed in Europe by the barren mycelium of Leptosphaeria, which may be found as a purple mat on the roots.

Dwarfing consists in partial atrophy of all the organs, and is a common result of starvation in poor, dry, shallow soils, as may often be seen in the case of weeds on walls or in stony places. Dwarfs which are thus developed in consequence of perennial drought are not, however, necessarily diseased, in the more specific sense of the word; their organs are reduced in size proportionally throughout in adaptation to the conditions, and simply carry out their functions on a smaller scale.

Dwarfing is frequently a consequence of the lack of food materials, or of some particular ingredient in the soil, and in such cases is a diseased condition of some danger; similar results may ensue in soils containing the necessary chemical elements, but in unavailable forms.

Dwarfing may also be brought about by repeated maiming, nipping off the buds, pruning, etc., as in the miniature trees of the Japanese; and the case of trees continually browsed down by cattle, or of moor plants perennially dwarfed by cutting winds, are further illustrations in the same category, as are also those of certain alpine and moraine plants, whose only chance of survival depends on their adapting themselves to the repeated prunings suffered by every young shoot which rises into the cutting winds, since there is no question of lack of food-materials in these cases.

The practice of the Japanese is to pinch out the growing tips of the shoots wherever they wish to prune back, and it is by the judicious use of this heading in, and suitable pot-culture, that the dwarfs are made, 6-20 inches high at from 30-80 years old.

Dwarfing is often brought about by grafting on a slow-growing stock, and this method is employed in practice, as are also heading in, pruning of roots, and confinement in pots.

Dwarfing may also be due to poor or shrivelled—partially atrophied—seeds or such as have had their endosperms or embryos injured by insects or fungi, and although it is possible to nurse such dwarfs into normal and vigorous plants with good culture, they do not usually recover under natural conditions in competition with more vigorous plants.

Distortions or Malformations may be defined as abnormalities in the form of organs which concern all, or nearly all the parts, and do not refer merely to swellings or excrescences on them or excavations, etc., in them.

Fasciation.—Shoots of Asparagus, Pine, Ash, and many other plants are occasionally expanded into broad ribbon-like structures often studded with more than the normal number of buds or leaves, etc., such as would be found on the usual cylindrical shoots. Such fasciations are due to several buds fusing laterally under compression when young and the whole mass growing up in common, or, in a few cases, to the unilateral overgrowth of one side of the terminal bud. Fasciations appear to depend on excessive nutrition in rich soils. They may spread out above in a fan-like manner, exaggerating the abnormality, or they may revert to the original form. Some cases are more or less fixed by heredity—e.g. Celosia. Fasciated stems are frequently curved like a crozier, owing to one edge growing more rapidly than the other.

Cauliflowers are really cultivated monstrosities. Fasciated Dandelions, Crepis, monstrous Chrysanthemums, peloric Linaria, five-leaved Clovers, spiral Teazels, etc., may all, if grown with care, be kept more or less constant in the monstrous state. That is to say, the particular kinds of variation here manifested can be maintained in proportion as the external conditions controlling the variation are maintained. Such conditions are chiefly rich supplies of food-stuffs, plenty of water and air, suitable temperature and lighting, etc. Mutilations, favouring the development of abnormal buds may also induce fasciations.

Torsions or spiral twistings of stems also frequently arise among plants grown in rich soils, and are often combined with fasciations—e.g. Asparagus, Dipsacus; and De Vries has shown that the peculiarity is not only transmissible by seed, but may be more or less fixed by appropriate culture.

Contortions of stems are often due to the unequal growth on different sides of the stems owing to the presence of fungi—e.g. Caeoma on Pines, Aecidium on Nettles, also Puccinia on petioles of Mallow, Cystopus on inflorescences of Capsella, etc.

Distortions of roots may be brought about in various ways by the hindrances afforded by stones.

Spiral roots occur occasionally in pot plants.

Flattened roots usually result from compression between rocks, the young root having penetrated into a crevice, and been compelled to adapt itself later. The distortions of stems by constricting climbers, wire, etc., have been described, and fruits—e.g. Gourds—are easily distorted by means of string tied round them when young.

Distortions of leaves are very common, and are sometimes teratological—i.e. due to no known cause—e.g. the pitcher-like or hood-like cucullate leaves of the Lime, Cabbage, Pelargonium, etc., and of fused pairs in Crassula. Also coherent, bifurcate, crested, displaced and twisted leaves occasionally met with, and in some cases fixed by cultivation, may be placed in this category.

Puckers must be distinguished from pustules, since they consist in local upraisings of the whole tissue, not swellings—e.g. the yellowish green pockets on Walnut leaves, due to Phyllereum.

Puckered leaves in which the area of mesophyll between the venation is increased by rising up in an arched or dome-like manner are sometimes brought about by excessive moisture in a confined space.

Leaf-curl is a similar deformation caused by fungi, such as Exoascus on Peaches.

Wrinkling or puckering of leaves is also a common symptom of the work of Aphides—e.g. Hops.

Characteristic curling and puckering, with yellow and orange tints, of the terminal leaves of Apples, Pears, etc., are due to insects of the genera Aphis, Psylla, etc.

Small red and yellow spots with puckerings and curlings of the young leaves of Pears, the spots turning darker later on, are due to Phytoptus.

Leaf-rolling.—The leaves of Beeches, Poplars, Limes, and many other plants, instead of opening out flat, are often rolled in from the margins, or from the apex, by various species of Phytoptus, Cecidomyia, or other insects, which puncture or irritate the epidermis in the young stages and so arrest its expansion in proportion to the other tissues. According as the lower or upper surface is attacked the rolling is from the morphologically upper surface downwards, or vice versa. Very often the mesophyll is somewhat thickened where rolled and Erineum-like hairs may be developed—e.g. Lime. Many caterpillars also roll leaves, drawing the margins inward to form shelters—e.g. Tortrix viridana, the Oak leaf-roller. Certain beetles—Rhynchitis—also roll up several leaves to form a shelter in which the eggs are laid.

Webs are formed among the mutilated leaves of Apples by the caterpillars of Hyponomeuta.

It must be borne in mind that instances can be found of teratological change of every organ in the plant—e.g. stamens transformed into carpels or into petals; anthers partly polliniferous and partly ovuliferous; ovules producing pollen in their interior, and so on, being simply a few startling examples of what may happen. Such abnormalities are frequently regarded as evidence of internal causes of disease, and this may be true in given cases; in a number of cases investigated, however, it has been shown that external agents of very definite nature bring about just such deformations as those sometimes cited as examples of teratology due to internal causes, and the question is at least an open one whether many other cases will not also fall into this category. The study of galls has shown that insects can induce the formation of not only very extraordinary outgrowths of tissues and organs already in existence, but even of new formations and of tissue elements not found elsewhere in the plant or even in its allies; and Solms' investigations on Ustilago Treubii show that fungi can do the same, and even compel new tissues, which the stimulating effects of the hyphae have driven the plant to develop, to take part in raising and distributing the spores of the fungus—i.e. to assume functions for the benefit of the parasite. Molliard has given instances of mites whose irritating presence in flowers causes them to undergo teratological deformations, and Peyritsch has shown that the presence of mites in flowers induces transformations of petals into sepals, stamens into petals. Similarly De Bary, Molliard, Magnus, Mangin, and Giard have given numerous cases of the transformation of floral organs one into another under the irritating action of fungi, of which the transformation of normally unisexual (female) flowers into hermaphrodite ones, by the production of stamens not otherwise found there, are among the most remarkable.

These and similar examples suffice to awaken doubts as to whether any teratological change really arises "spontaneously," especially when we learn how slight a mechanical irritation of the growing point may induce changes in the flower; e.g. Sachs showed that a sunflower head is profoundly altered by pricking the centre of the torus, and Molliard got double flowers by mechanical irritation.

Notes to Chapter XXVII

For the details and classification of the multitude of facts, the student is referred to Masters' Vegetable Teratology, Ray Society, 1869, and the pages of the Gardeners' Chronicle since that date.

Concerning torsions, etc., the student should read De Vries, "On Biastrepsis in its Relation to Cultivation," Ann. of Bot., Vol. XIII., 1899, p. 395, and "Hybridising of Monstrosities," Hybrid Conference Report, Roy. Hort. Soc., 1900, Vol. XXIV., p. 69.

The reader will find an excellent account of the abnormalities in flowers due to the action of parasitic insects and fungi in Molliard, "Cécidies Florales," Ann. des Sc. Nat., Ser. VIII., Bot., T. 1, 1895, p. 67.

CHAPTER XXVIII.

PROLIFERATIONS

Proliferations—Vivipary—Prolepsis—Lammas shoots—Dormant buds—Epicormic shoots—Adventitious buds—Apospory and apogamy.

Proliferation consists in the unexpected and abnormal on-growing or budding out of parts—stems, tubers, flowers, fruits, etc.—which in the ordinary course of events would have ceased to grow further or to bear buds or leaf-tufts directly. Thus we do not expect a Strawberry—the swollen floral axis—to bear a tuft of leaves terminally above the achenes, but it occasionally does so, and similarly Pears may be found with a terminal tuft of leaves, Roses with the centre growing out as a shoot, Plantains (Plantago) with panicles in place of simple spikes, and so on.

We regard such cases as teratological, because they are exceptional for the particular species, and as pathological because they appear to be connected with over-feeding in soils with excessive supplies of available food-materials; but it should be noted that conditions quite comparable to proliferation are normal in the inflorescences of Pine-apples, some Myrtaceae, Conifers, etc., and that many instances of proliferations come under the head of injurious actions of fungi, insects, and other agents.

Proliferation of tubers is sometimes seen in Potatoes still attached to the parent plant in wet weather following a drought. The eyes grow out into thin stolons, or forthwith into new tubers sessile on the old tuber. Similarly in store we sometimes find the eyes transformed directly into new tubers, and cases occur where the growth of the eye is directed backwards into the softening tuber, and a small potato is formed inside the parent one.

Threading is also occasionally met with in the "sets" when ripened too rapidly in hot dry soils.

Vivipary is a particular case of proliferation, in a certain sense, where the seeds appear to germinate in situ, and we have small plants springing from the flowers, reminding us of wheat which has sprouted in the shocks in damp weather. In reality, however, the grains are here replaced by bulbils which sprout before they separate from the inflorescence. In varieties of Poa, Polygonum, Allium, Gagea, etc., this phenomenon is constant in plants growing in damp situations.

Prolepsis.—It frequently happens that branches or whole plants are suddenly defoliated in summer,—e.g. by caterpillars or other insects—at a time when considerable stores of reserves had already been accumulated during the period of active assimilation. In such cases the axillary buds, which would normally have passed into a dormant condition over the winter had the leaves lived till the autumn-fall, suddenly shoot out into proleptic shoots (also termed Lammas shoots), and reclothe the tree with foliage. The wood of the year in which this occurs may exhibit a double annual ring, and the vigour of the tree is likely to suffer in the following season and no fruit be matured.

Proleptic branches may also be due to the shooting out of accessory buds—i.e. extra buds found in or near the leaf-axils of many plants, such as Willow, Maples, Cercis, Robinia, Syringa, Aristolochia, etc.—which do not normally come to anything, or do so only if a surplus of food materials is provided.

Dormant buds, or preventitious buds, are such as receive no sufficient supply of water and food materials to enable them to open with the other buds in ordinary years, for in most trees only the upper buds on the branches develop into new shoots. The lower buds do not die, however, but merely keep pace with the growth in thickness of the parent branch, and may be elongated sufficiently each year to raise the minute tips level with the bark, their proper cambium only remaining alive but not thickening the bud.

When, by the breaking of the branch above the insertion of the dormant bud—or by pruning, defoliation by insects, etc.—the transpiration current and supplies of food materials are in any way deflected to the minute cambium and growing points of the dormant buds, they are stimulated to normal growth, and may grow out as epicormic shoots or "shoots from the old wood." In many cases such epicormic shoots are stimulated to grow out by suddenly exposing an old tree to more favourable conditions of root-action and assimilatory activity, owing to the felling of competing trees which previously hemmed it in from light and air, and restricted the spread and action of its roots in the soil. This is often seen in old Elms, Limes, etc.

It is by such means as the above that substitution branches are obtained when a leader is broken or cut away.

Adventitious buds are such as are newly formed from callus or other tissues in places not normally provided with buds, as is often seen on occluding wounds—e.g. stool shoots. They may also be developed on roots, a fact utilised in propagating Bouvardias, Horse-radish, etc., by means of root-cuttings, and the suckers of Plums and other fruit trees are shoots springing from adventitious buds on roots.

Adventitious buds are also common on leaves (e.g. Bryophyllum, Ferns, etc.), and are frequently induced on them by wounds—e.g. Gesneria, Gloxinia, etc. Even cut cotyledons may develop them, and pieces of leafless inflorescence (Hyacinth), hypocotyl (Anagallis), and in fact practically any wounded tissue with a store of reserve materials may be made to develop them: thus they have been found arising from the pith of Sea-kale, and are commonly developed from the cut bulb scales of Hyacinths.

Apospory and Apogamy are particular cases of the production of vegetative buds on the leaves in place of sporangia in Ferns (Apospory), and on prothallia in place of Archegonia (Apogamy), in the latter case induced by dry conditions and strong illumination.

Notes to Chapter XXVIII

In addition to the literature quoted in the notes to Chapter XXVII., the student should consult the works on Forest Botany for the scattered information regarding adventitious buds. A good account may be found in Büsgen, Bau und Leben unserer Waldbäume, Jena, 1897.

For Apospory and Apogamy, see Lang "On Apogamy and the Development of Sporangia upon Fern Prothalli," Phil. Trans., vol. 190, 1898, p. 187, where the literature is collected.

CHAPTER XXIX.

GRAFTS

Grafting—Comparison with cuttings—Effects of environment—Relations between scion and stock—Variation in grafts—Grafting and parasitism—Infection—Pollination—Grafts-hybrids—Predisposition of Natural grafts—Root-fusions.

Grafting is a process which consists in bringing the cambium of a shoot of one plant into direct union with that of another, and is practised in various ways, the commonest of which is as follows:

One plant—the stock—rooted in the ground, is cut off a short distance above the surface of the soil, and a shoot from the second plant—the scion—cut off obliquely with a sharp knife, is inserted into a cleft in the stock, so that the two cambiums (and sometimes the cortex and pith of each as well) are in close contact: the scion is then tied in position, the wounds covered with grafting wax, and the whole left until union of the tissues is completed. This union depends on the formation of callus at the cut surfaces, and the intimate union of the ingrowing cells from each callus.

The development of the callus follows the course described for wounds, cuttings, etc., and the union is exactly comparable to the union of the two lips of a healing callus over a wound (see p. 197).

Grafting was known and practised far back in the ages. Virgil was well acquainted with the process, and Theophrastus compared it with propagation by cuttings.

The scion differs from a cutting, however, in having no roots of its own: it is parasitic upon, or rather is in symbiosis with the stock, the root and tissues of which intervene between it and the soil. Consequently the selective absorption, size and number of vessels, and innumerable other physiological and anatomical peculiarities of the stock determine what and how much shall go up into the scion, while the latter supplies the former with organic materials and rules what and how much food, enzymes, and other secretions, etc., it shall receive to build up its substance. Surely, then, if such factors as the nature of the soil, the water and mineral supplies, the illumination, and the various climatic factors of altitude can cause variations on a plant direct, these and other factors are still more likely to be effective on stock and scion, and each must affect the other.

Nevertheless opinions have differed much as to whether any important effect is to be seen, and on no point more than on whether the scion can affect the stock, in spite of such examples as Cytisus Adami, Garreya on Aucuba, Sunflower on Jerusalem Artichoke, etc. Recent results, especially of experiments with herbaceous plants, show that not only can the stock affect the scion (and vice versa) directly, but the effect of the changes may be invisible on the grafted plant and only show itself in the progeny raised from the seed of the grafted plant. In other words, variation occurs in grafts either directly, as the results of the effects of the environment on the graft, or owing to the interaction of scion and stock, showing as changes in general nutrition in the tissues concerned, etc., owing to special reactions of the protoplasm of the uniting cells one on the other, and of the results of the further protoplasmic secretions, sortings, and so forth, on the cells developed as descendants of these in the further growth of the graft: or indirectly, in that some of these changes so alter the nature of the special protoplasm put aside for reproductive purposes, that the resulting embryo in the seed transmits the effects, and they show as variations in the seedling. If these results are confirmed they should meet all objections that have been urged against the transmission of acquired characters.

In fact there are analogies between grafting and parasitism which cannot be overlooked, and should not be underestimated, their commonest expression appearing in the alterations in stature, habit, period of ripening, and so forth. These analogies are easily apprehended when we compare parasites like the Mistletoe, Loranthus, or even such root-parasites as the Broom-rapes and the Rhinanthoideae with grafts; but they also exist in the case of many fungus-parasites, and we might almost as accurately speak of grafting some fungi on their hosts as of infecting the latter with them, especially when it is borne in mind that the effect of the scion on the stock is by no means always to the benefit of the latter, and that there are reasons for regarding the action of some such unions as that of a sort of slow poisoning of the stock by the scion. Why do we not here say that the stock has been infected by the scion?

The resemblances between pollination and the infection by fungus hyphae may also be insisted upon. If we take into account Darwin's remarkable experiments showing that in "illegitimate unions" the pollen exerts a sort of poisonous action on the stigmas or ovules, it is possible to arrange a series of cases starting with perfectly legitimate pollinations where the pollen tube feeds as it descends the style on materials provided by the cells, and proceeding to cases where the pollen is more and more merely just able to penetrate the ovary and reach the ovules, to the extreme cases where no union at all is possible.

Side by side with such series could be arranged analogous cases where fungus spores can enter and infect the cells of the host, and live symbiotically with or even in them, or can penetrate only with difficulty, or with poisonous effects, and finally cannot infect the plant at all.

Less obviously, but nevertheless existing, are gradations in grafting to be observed, where one and the same stock may be successfully combined with a scion which improves it—or which is improved by it—or the scion may unite but acts injuriously on it, or, finally, cannot be induced to unite.

But we may go further than this in these comparisons. Just as the results of pollination frequently induce far-reaching effects on distant tissues—e.g. the swelling of Orchid ovaries, and rapid fading of the floral organs—so also the effects of hyphae in the tissues may induce hypertrophies, deflection of nutrient materials, and the atrophy of distant parts—e.g. the curious phenomena observed in Euphorbia attacked by Uromyces—and some of the distant actions in grafts may be compared similarly.

Going still further, we may compare the effects of cross-breeding or of hybridisation, where the progeny show that changes have resulted from the mutual interactions and reactions of the commingled protoplasm, with Daniel's results, in which he obtains proof of such interactions of the commingled protoplasmic cell-contents of grafts in the seedling progeny; although there is no probability—we may even say possibility—in this latter case that the effects are due to nuclear fusions, but only that the germ-plasm of the seed-bearing plant has been affected by the changes in the cell-protoplasm which nourishes it when the reproductive cells are forming.

In the case of graft-hybrids the matter appears to be somewhat different, and we may well suppose, with Strasburger, that the commingling of characters observed in flowers, fruits, foliage, etc., on shoots borne after grafting are due to the occurrence of nuclear fusions during the union of the grafted tissues; though it is by no means impossible that what has really happened is profound alterations in the nuclear substance (germ-plasm) owing to its being nourished by cell-protoplasm (somato-plasm) which has been itself affected by the interchanges of substance between scion and stock, and therefore itself furnishes a different nutrient medium from the unaltered cytoplasm of either.