The bibliography here quoted will be found in Berkeley, "Vegetable Pathology,"Gardener's Chronicle, 1854, p. 4; Plowright,British Uredineæ and Ustilagineæ, 1889; Eriksson and Henning,Die Getreideroste, Stockholm, 1896; De Bary,Comparative Morphology and Biology of the Fungi, etc., 1887; Frank,Die Krankheiten der Pflanzen, 1895-96, and scattered in the works referred to in them and in the text.
The bibliography here quoted will be found in Berkeley, "Vegetable Pathology,"Gardener's Chronicle, 1854, p. 4; Plowright,British Uredineæ and Ustilagineæ, 1889; Eriksson and Henning,Die Getreideroste, Stockholm, 1896; De Bary,Comparative Morphology and Biology of the Fungi, etc., 1887; Frank,Die Krankheiten der Pflanzen, 1895-96, and scattered in the works referred to in them and in the text.
Variation—Disease—Comparison to a top. Health—Extinction of species—Natural demise. Examples of complex interactions in health—Interference, and tendencies to ill-health.
Variation—Disease—Comparison to a top. Health—Extinction of species—Natural demise. Examples of complex interactions in health—Interference, and tendencies to ill-health.
When we come to enquire into the causes of disease, it appears at first an obvious and easy plan to subdivide them into groups of factors which interfere with the normal physiology of the plant. Scientific experience shows, however, that the easy and the obvious are here, as elsewhere in nature, only apparent, for disease, like health, is an extremely complex phenomenon, involving many reactions and interactions between the plant and its environment. If we agree that a living plant in a state of health is not a fixed and unaltering thing, but is ever varying and undergoing adaptive changes as its life works out its labyrinthine course through the vicissitudes of the also ever-varying environment, then we cannotescape the conviction that a diseased plant, so long as it lives, is also varying in response to the environment. The principal difference between the two cases is, that whereas the normal healthy plant varies more or less regularly and rhythmically about a mean, the diseased one is tending to vary too suddenly or too far in some particular directions from the mean; the healthy plant may, for our present purposes, be roughly likened to a properly balanced top spinning regularly and well, whereas the diseased one is lurching here, or wobbling there, to the great danger of its stability. For we must recognise at the outset that disease is but variation in directions dangerous to the life of the plant. Health consists in variation also, but not in such dangerous grooves. That the passage from health to disease is gradual and ill-defined in many cases will readily be seen. In fact we cannot completely define disease. Mere abnormality of form, colour, size, etc., is not necessarily a sign of disease, in the usual sense of the word, otherwise the striking variations of our cultivated plants would suggest gloomy thoughts indeed, whereas we have reason to believe that many cultivated varieties are more healthy—in the sense of resisting dangerous exigencies of the environment—than the stocks they came from. Strictly speaking, no two buds on a fruit-tree are alike, and the shoots they produce vary in position, exposure, number, and vigour of leaves, and so forth. The minute variations here referred to arenot seen by the ordinary observer, but those who bud, graft and multiply by cuttings on a large scale know that such bud-variations are important, quite apart from more extensive "sports" which occasionally occur.
On the other hand, we have reason to believe that many species have died out gradually as the environment altered. These plants died because they did not vary sufficiently, or did not vary in the right directions; they became diseased with respect to the then prevailing conditions of normal physiology or health.
Disease, therefore, may be said to be variation of functions in directions, or to extents, which threaten the life of the plant, the normal in all cases being the state of the plant characteristic of the species.
Even now, however, we have not obtained a complete definition, because, since all plants die sooner or later, we have not excluded the natural demise of the individual or its parts, and no one would call the autumnal fall of leaves, or the withering of an annual after flowering, death from disease. Clearly then the idea of disease implies danger of premature death, and probably this is as near as we shall get to a satisfactory definition. Since this matter is of primary importance for our present theme, I will add the following instances for consideration.
A plant in perfect health and in the fullest exercise of all its functions, has its roots in a soil which is suitably warmed and aerated, containsthe right quantities of water which dissolve just the proper proportions of all the essential mineral salts, but nothing poisonous, while the soil itself has a texture such that the roots and root-hairs can extend and do their utmost in absorbing.
The leaves above are exposed to just the right intensity of light, in air which is not too dry, and is of suitable temperature and composition, containing no poisonous exhalations, etc.; and as the foliage is gently moved by the breeze, it manufactures carbohydrates at the optimum rate in the chlorophyll, and the so-called "elaborated sap" containing the dissolved organic food-supplies is prepared in the tissues in maximum quantities and of just the right degrees of concentration and quality for use in the buds, stem, roots, etc., for which it is destined as they draw on the supplies.
Between these assimilating organs, the leaves, and the absorbing roots, we have in the stem the wood, with its vessels adapted in quantity and calibre to convey the water containing dissolved salts from the absorbing roots to the leaves (to say nothing of other parts) and, separated from this wood by the cambium, we find the sieve-tubes and cortical tissues in suitable quantity conveying the "elaborated sap"—the solutions of organic food-materials from the leaves down to the roots, up to the buds, and elsewhere. Joining these cortical and wood tissues are adapted series of medullary rays which, apart from other connections, bring about the necessary interchanges of water and"elaborated sap" with the cambium, the formative tissue which has to be fed and served by them, and which by its growth supplies new vessels and sieve-tubes, etc., to carry the continually increasing quantities of water and food substances as the roots and leaves increase in number and area, and thus enables this ideally correlated system to go on working at maximum energy.
Now suppose the same plant with its roots in an unsuitable soil—too dry or too poor in mineral supplies, for instance—the transpiring leaves above cannot obtain sufficient water and salts to supply their needs, but we will suppose hypothetically that they still assimilate under the same ideal conditions as before. The supplies now coming to the cambium are diminished, since the want of water and minerals compels the leaves to put aside any excess of carbohydrates (e.g.as stored starch-grains), and the plastic materials which do pass to the cambium so deficient in water cannot be directly utilised, and a starvation period sets in. Consequently the cambium forms less wood, and this will contain fewer and smaller vessels, and so reduce the conducting passages: fewer sieve-tubes also are constructed, and the paths of the water current and food supplies narrowed, which of course reacts on the tissues everywhere. The reserve substances may slowly be dissolved and distributed, however, and considerable quantities be passed in course of time into the roots, which, as opportunity offers, gradually employ them in making new roots, and if the disturbance has not gone too far and theconditions do not become unfavourable, an increased root-supply may by its larger absorbing area gradually establish the former state of equilibrium of functions. But this at the expense of the plant, which is smaller, has fewer leaves and narrower water channels, etc., than a plant not thus checked, and it may take a long time to make up for the loss of time and stature thus incurred. Indeed if the plant is an annual no recovery at all may occur, the reserves passing into fruit and seeds instead of slowly supplying the roots as described.
If it be asked, can such a condition of affairs as that described really occur, we have only to think of a transplanted specimen with its roots maimed and put into unsuitable soil, or of plants in the open with feeding roots gnawed by an insect, etc., or of a tree hitherto in equilibrium with its fellows in a plantation suddenly set free by thinning and so forth.
Now take the case where the roots are maintaining their maximum functional activity, but the leaves—owing to want of light, too much moisture or too low a temperature of the air—are functionally depressed. Here we get a state of over-saturation with water set up, the tissues are turgid to bursting point, what supplies do traverse the sieve-tubes, cortex, etc., do so slowly and are excessively diluted, and the cambium again forms less wood, but the lumina of the vessels are larger and the lignification less complete. Growth in length is excessive, but more leaves are formed,though they are apt to be abnormally thin and may be small. Little or no reserves are stored anywhere, and the watery tissues contain dangerously diffusible substances which may render them an easy prey to parasitic fungi. Here again, however, if the disturbance of equilibrium has not gone too far, and if the season permits, the new leaves may come into full activity and the situation be saved by transpiration and assimilation gradually increasing and restoring the equilibrium. But, as before, the plant has suffered, and shows the effect in its weak shoots, retarded flowering, and other ways.
Such plight as is here described may actually be attained in greenhouses where over-watering is the fault, and even in the open it is not uncommon in rainy summers, or in plantations where dominant trees get the upper hand and partially shade more slowly growing species, or in fields where rank grass is allowed to overwhelm crops of lower stature.
Now it will be evident that either of these typical cases of temporary disturbance of functional equilibrium may be carried too far: in the first case the plant may wilt and wither, in the second it may rupture and rot, to take these eventualities only. And yet it is difficult to call these indispositions diseases: they are rather examples of extreme departures from the normal standard of health, just on the borderland between health and disease. A step further, as it were, and disease supervenes: certain tissues die from want of water,and a necrotic area is formed, or the cortex bursts and a wound is formed in another way, or some fungus gets a hold, and so on. These abnormal states are particularly apt to predispose the plant to disease—insects revel in such semi-wilted leaves and shoots crammed with reserves, and fungi in the water-logged leaves of the second case, while a cold dry wind is peculiarly fatal to such tissues.
The reader may consult Hartig,Diseases of Trees, Eng. ed., 1894, Introduction; Sorauer,Pflanzen Krankheiten, pp. 1-12, and Frank,Die Krankheiten der Pflanzen, B. 1, p. 5, for definitions of disease.
The reader may consult Hartig,Diseases of Trees, Eng. ed., 1894, Introduction; Sorauer,Pflanzen Krankheiten, pp. 1-12, and Frank,Die Krankheiten der Pflanzen, B. 1, p. 5, for definitions of disease.
A. External causes—I. Non-living environment: soil, atmosphere, temperature—II. Living environment: plants, animals—Complex interactions—Predisposing causes—No one factor works alone—Tangled problems of natural selection involved. B. So-called internal causes.
A. External causes—I. Non-living environment: soil, atmosphere, temperature—II. Living environment: plants, animals—Complex interactions—Predisposing causes—No one factor works alone—Tangled problems of natural selection involved. B. So-called internal causes.
It is customary to classify the causes of disease in plants into two principal groups—(1) those due to the action of the non-living environment—soil, atmosphere, physical conditions such as temperature, light, etc.; and (2) those brought about by the activities of living organisms—plants and animals of various species. Before passing to further subdivisions under these two heads, however, it is necessary to observe that no disease can be efficiently caused by an organism alone, since its powers for injury as a parasite, or otherwise, are affected by its non-living environment as well as by the host-plant. For instance, thespores of a parasitic fungus which would infect and rapidly destroy a potato plant in moist warm weather may be showered on to such a plant with impunity if the air remains dry and cool—or on to a cabbage under any circumstances as far as we know.
Again, probably no one factor of the non-living environment ever suffices to induce a disease, possibly because no such thing as only one change at a time ever occurs. For instance, it is difficult to say, when a soil becomes sodden with water, whether the excess of water and dissolved matters, the want of air displaced by the water, the lowering of the temperature, or the accumulation of foul products, etc., is the principal factor in causing the damage which results, and we have to determine by the balance of experimental evidence which is the dominant factor in all such cases.
The study of aetiology of disease is in fact only a particular case of that of aetiology in general. Plants at high altitudes in the Alps acquire very different characteristics from the same species in the plains. Is this due to the low temperature, the rarer atmosphere, the more intense illumination, the changes in moisture, etc., etc.? The question is more difficult than it appears at first sight, and we must remember that, complex as are the factors working on the host, they are equally complex in their actions on a parasite attacking the host, whence the resulting disease becomes indeed a tangled problem of natural selection.
Finally it remains to say a few words about a numerous class of cases where no external cause of disease can be discovered. It was formerly the custom to group such cases of "Internal Causes" by themselves, but apart from the fact that many of these mysterious diseases have subsequently been shown to be due to the action of external agencies, the whole question of internal causes resolves itself into one of relations between the plant and its surroundings, and it becomes evident that no inherited or internal disease can be regarded as explained until we know the external causes which have so modified the structure and working of the living cells as to make them abnormal in their reactions to other parts of the plant. "Internal causes" of disease, therefore, is a phrase expressing our ignorance, but somewhat more emphatically than usual. If this is clearly understood there seems no reason against its employment for the time being in the artificial scheme of classification we require. With regard to external causes due to the non-living environment, excess or deficiency of materials in the soil, water, or atmosphere plays an important part, and—since we may neglect purely aquatic plants—it is customary to speak of diseases due to unsuitable soils or to injurious atmospheric influences. For instance, any deficiency in the supplies of the necessary mineral salts (compounds of calcium, magnesium, potassium with sulphuric, nitric and phosphoric acids, etc.) leads to pathological changes, as also does the lack of the necessary traces of iron. But it is equallytrue that the presence of such ingredients in excess or in combinations unsuited to the plants also leads to disaster, as also does the presence of minerals or other compounds which poison the root-hairs—e.g.products of decomposition, soluble salts of copper and other poisons. That these matters are bound up with the whole question of manuring and of proper soil-analyses will be evident.
Another essential factor is the nature and quantity of organic materials in the soil, whether leaf-mould and decomposing vegetable remains, stable manures, or other animal matters, all of which affect different species very differently, and produce very different results in different soils. It is necessary to apprehend in this connection what has been stated above: that soil is not a mere dead structureless medium, and that the root-hairs of ordinary plants cannot deal with large quantities of putrefying organic matter: that a good soil must abound in useful bacteria and fungi to render such substances available—and in very various ways—and that it must be open and aerated, of proper temperature and suitably supplied with water, and so forth, or disaster will result. Here, again, then we are brought into close contact with all that is known of fermentation, nitrification, and the various biological changes going on in soil, and the application of such knowledge to the practice of manuring and tillage in all its forms.
In view of the above remarks, the danger of "over-feeding," in this sense, has a real meaningfor horticulturists, though it must not be forgotten that no substance is really a food until it is assimilable into the protoplasm: manures, etc., are food-materials, not food. The futility of mere chemical analyses to prove what a plant requires is now well known, and it is only on the basis of long and carefully conducted experiments that we can ever discover what a particular plant in a particular soil, situation, and climate requires for healthy development. Again, the quantity of water in soil may be too great or too small for given species, and this either on the average for the year, or during critical periods only; and it is obviously important whether the excess or deficiency is due to improper supplies of water, the depth or shallowness of the soil, its retentive powers, or the nature of the sub-soil and so on, again bringing the whole matter into connection with our understanding of the physical constitution and structure of soils, and the nature of soil-drainage.
For instance, a common way of killing ferns is to keep the roots and soil wet and the air and fronds dry, whereas the natural habitats provide for wet and shaded fronds and well-drained soil.
It may be noted here that in most cases where gardeners speak of plants being killed under the "drip" of trees—e.g.Beech, the injury is due, not to the effects of water but to the shade: the loss of light is so great that the shaded plants die of inanition because their leaves are not able to provide sufficient carbohydrates.
Closely bound up with this is the question of the gases in soils. Apart from the disastrous effects of poisons—e.g.coal gas escaping from pipes under pavements in towns, etc., diseased conditions often result from deficiency of oxygen at the root-hairs, due to imperfect aeration of soils, brought about by stagnant water, excess of animal matter, and so forth.
Unsuitable constitution of the atmosphere is also a fruitful source of disease, though its effects are commoner in closed stoves and greenhouses than in the open. Nevertheless the continual exhalation of sulphurous fumes, chlorine, and other poisonous gases in the neighbourhood of manufacturing centres or of large smoky towns, volcanoes, etc., play their part in injuring plants; and excessive moisture in the form of mist, rain, etc., is also important. All these matters bring us at once into the region of physiology, and only an intelligent appreciation of what is known about the action of the atmosphere on the soil and the plant will save the peasantry of a country from a hopeless mysticism but little removed from that of the Middle Ages, when blights and other evils were vaguely referred to the river-mists, thunder clouds, and easterly winds.
If we summarise the above as the material factors of the environment, we may classify another set of external non-living causes of disease as the non-material factors. Such are principally the following:
The space at the disposal of plants greatlyaffects their welfare. The crowding of roots in the soil and of foliage in the air, resulting in the loss of light to the leaves, involves deficiency of all the materials referred to above—minerals, organic materials, gases, and water—and no better illustration of the intense struggle for existence among these apparently passive and motionless beings, plants, can be given than an over-crowded seedbed or plantation. If left to themselves such over-stocked areas exhibit to the keen eye of the trained observer all the phases of starvation, weakness, wounding, rot, and, so to speak, brutal dominance of the stronger over the weaker which it is the object of cultivation to prevent. Here, then, we are brought face to face with the true significance of thinning and weeding out, pruning, and similar processes.
Unsuitable temperature is one of the commonest of all sources of disease, for every plant is adapted to certain ranges of temperature, and best adapted to a given optimum somewhere between the maximum and minimum temperature for each function. Consequently any serious departure from the mean may bring about physiological disturbances of the nature of disease, and this in very various ways, as exemplified by the results of frost, sun-scorching, drought, hail-storms, forest fires, and so forth.
As a predisposing factor to disease abnormal temperature effects play a great part. Many wound-fungi gain their entrance through frost-cracks, bruises due to hailstones, or into tissues chilled below the normal.
No less remarkable are the diseases primarily due to insufficient or improper exposure to light, which affects the chlorophyll-apparatus and the process of carbon-assimilation and through these the whole well-being of the plant. Every plant is adapted to certain ranges of light intensity, and most cultivators know how impossible it is to grow shade plants in fully exposed situations, and how easily plants which live in open sunny situations are "drawn" and killed by shade. It is equally important to have the right kind of light, as disastrous experiences with greenhouses glazed with glass which cut off certain rays of light have taught. Here, again, it is important to notice that the optimum intensity or quality of light may differ for different functions and organs of the plant, as is shown by many adaptations on the part of species growing in natural situations—e.g.bud protection, orientation of leaves, etc.—and it may be taken as a rule that etiolated plants are peculiarly susceptible to other diseases.
As regards other factors of the inorganic environment, disasters which come within the scope of our subject may be brought about by many agencies, the mechanical effects of snow and hail, wind, avalanches, etc., the effects of lightning, and so forth, being a few of them.
For other detailed classifications of the causes of disease the reader is referred to the works of Sorauer and of Frankreferred to in the last chapter. Also Kirchner,Pflanzen Krankheiten, Stuttgart, 1890.Of more historical importance are the older classifications of Berkeley,Gardeners' Chronicle, 1854, and Re,Gardeners' Chronicle, 1849-50. These latter are interesting as showing the very different views held by the earlier workers, and comparison of these with the modern views helps to mark the progress of physiology during the half century which has intervened.
For other detailed classifications of the causes of disease the reader is referred to the works of Sorauer and of Frankreferred to in the last chapter. Also Kirchner,Pflanzen Krankheiten, Stuttgart, 1890.
Of more historical importance are the older classifications of Berkeley,Gardeners' Chronicle, 1854, and Re,Gardeners' Chronicle, 1849-50. These latter are interesting as showing the very different views held by the earlier workers, and comparison of these with the modern views helps to mark the progress of physiology during the half century which has intervened.
Causes due to animals—Vertebrata—Wounds, etc.—Invertebrata—Insects, etc.—Plants as causes of disease—Phanerogams, weeds, etc.—Cryptogams, fungi—Epidemics, etc.
Causes due to animals—Vertebrata—Wounds, etc.—Invertebrata—Insects, etc.—Plants as causes of disease—Phanerogams, weeds, etc.—Cryptogams, fungi—Epidemics, etc.
Passing now to those causes of disease which are connected with the living environment, we may obviously divide them into two groups of agents, animals and plants.
Among animals, the various vertebrata, including man, are especially responsible for the larger kinds of wounds and wholesale destructive processes due to breakage, stripping of leaves and bark, cutting and biting, and so forth. Cattle, rabbits, rats and mice, squirrels and birds of various kinds stand out prominently as enemies to trees and other plants, to which they do immense injury in various ways by their horns, teeth, claws, and beaks; and the damage which an ignorantgardener or forester can do with his ill-guided footsteps, axe, spade, and knife can only be appreciated by one who knows the habits of plants.
It is among the invertebrata, however, especially insects and worms, that the most striking agents of disease in plants are to be found, for, with the exception of certain rodents—and we may logically include also human invasions—vertebrate animals do not often appear in such numbers as to bring about the epidemics and scourges only too commonly caused by insect pests.
Insects injure plants in very various ways. Some, such as locusts, simply devour all before them; others,e.g.caterpillars, destroy the leaves and bring about all the phenomena of defoliation. Others, again, eat the buds—e.g.Grapholitha; or the roots—e.g.wire-worms, and so maim the plant that its foliage and assimilation suffer, or its roots become too scanty to supply the transpiration current. Many aphides, etc., puncture the leaves, suck out the sap, and produce deformations and arrest of leaf-surface, as well as actual loss of substance, and when numerous such insects induce all the evils of defoliation. Others, such as the leaf-miners, tunnel into the leaves, with similar results on a smaller scale.
It must be remembered that a single complete defoliation of a herbaceous annual, or even of a tuberous plant like the potato, so incapacitates the assimilatory machinery of the plant, that no stores can be put aside for the seeds, tubers, etc., ofanother year, or at most so little that only feeble plants come up.
In the case of a tree the case is different, and since most large trees in full foliage have far more assimilatory surface than is actually necessary for immediate needs, a considerable tax can be paid to parasites or predatory insects before the stores suffer perceptibly. Still, it should be recognised that the injury tells in time, especially in seed years.
Many larvae of beetles, moths, etc., bore into the bark and as far as the cambium or even into the wood or pith of trees, the local damage inducing general injuries in proportion to the number of insects at work: moreover, the wounds afford points of entrance for fungi and other pests.
Galls and similar excrescences result from the hypertrophy of young living tissues pierced by the ovipositors of various insects, and irritated by the injected fluid and the presence of the eggs and larvae left behind. They may occur on the buds, leaves, stems, or roots, as shown by various species ofCynipson oak,Phylloxeraon vines, etc., in all cases the local damage being relatively small, but the general injury to assimilatory, absorptive, and other functions is great in proportion to the number of points attacked.
Many grubs—larvae of flies, beetles, etc.—bore into the sheaths or internodes of grasses, or the pith of twigs, or into buds, fruits, and other organs of plants, and do harm corresponding to the kind and amount of tissues injured.
Various species of so-called eelworms—Nematodes—also cause gall-like swellings on young roots, or they invade the grains of cereals.
Finally, various slugs and snails cause much injury by devouring young leaves and buds and diminishing the assimilatory area.
Plants as agents of disease or injury fall naturally into the two main categories of flowering plants (Phanerogams) and Cryptogams, among which the fungi are the especially important pests.
Beginning with weeds, we find a large class of injurious agents. Weeds damage the plants we value by crowding them out in the struggle for existence, as already stated, and when the weed-action is simply due to superfluous plants of the same species, we speak of overcrowding. But it must not be overlooked that the competition between crowded plants of the same species—where every individual is acting as a weed to the others—may be more dangerous than between plants and weeds belonging to other species and genera, because in the former case they are struggling for the same minerals and other necessary food-materials: a matter of importance in connection with the rotation of crops.
The question of allowing grass to grow at the foot of fruit trees, as in orchards, is a good case in point. Such grass may increase the damp and shade, thus favouring fungi at one season, and dry up the moisture of the soil to the injury of the fine superficial roots at another, as well as exhaust the soil, owing to the competition of theroots for salts and other materials. On the other hand, the checking of surface roots by competition with the grass has been claimed as advantageous. In this connection probably the whole question of the composition of the turf arises, as well as that of possible cropping for hay, and manuring.
As regards any particular weed, the cultivator should learn all he can respecting its duration, seeding capacity, method of dissemination, the depth and spread of its root-system, and any other particulars which enable him to judge when and how to attack it. It is only necessary to see the victory of such drought-resisting weeds asHieracium pilosella, Plantains,Hypochaeris, on lawns to realise how weeds may win in the struggle for existence with the finer grasses.
Many so-called weeds are, however, partially parasitic, with their roots on the roots of others—e.g.Rhinanthus,Thesium, etc., and much damage is done to meadow grasses and herbage by the exhaustive tax which these semi-parasites impose.
This is carried still further in the case of such root-parasites asOrobanche, where the host-plant is burdened with the whole support of the pest, because the latter, having no chlorophyll, is entirely dependent on the former for all its food.
Even ordinary climbing plants may injure others by shading them, either by scrambling over their branches—e.g.Bramble, or twisting their tendrils round the twigs—e.g.Bryony, or twining round them—e.g.Woodbine,Convolvulus, etc. The principal direct injury is in these cases owing tothe loss of light suffered by the shaded foliage, but the weed-action is often increased by the competition of their roots—e.g.briars; and in the case of woody climbers the gradually increased pressure of the woody-coils round the thickening stems compresses the cambium and cortex of the support and induces strictures and abnormalities which may be fatal in course of time.
Epiphytes, or plants which support themselves wholly on the trunks, branches, or leaves of other plants, also injure the latter more especially by shading their foliage—e.g.tropical Figs, Orchids, Aroids, etc.; and similar damage is done by our own Ivy, the main roots of which are in the soil, but the numerous adventitious roots of which cling to the bark.
When the climber or epiphyte is also parasitic, as in the case of the Dodder,Loranthus, Mistletoe, etc., the direct loss of substance stolen from the host by the parasite comes in to supplement any effect of shading that the latter may bring about if it is a leafy plant.
Of Cryptogams, apart from a few epiphytic ferns, and the intense weed-action of certain Equisetums, the rhizomes and roots of which are as troublesome as those of twitch and other phanerogamic weeds, it is especially the fungi which act as agents of disease, and which, as we now know, arepar excellencethe causes of epidemics.
The action of fungi may be local or general; and restricted, slow and insidious, or virulent and rapidly destructive.
Examples of local action are furnished bySchinzia, which forms gall-like swellings on the roots of rushes;Gymnosporangium, which induces excrescences on the stems of junipers, and numerous leaf-fungi (Puccinia,Æcidium,Septoria, etc.), which cause yellow, brown, or black spots on leaves, as well as byUstilago, which attacks the anthers or the ovary of various plants, and so forth. In such cases the injury done by a few centres of infection is very slight, but prolonged action may bring into play secondary effects such as the gradual destruction of the cambium round a branch, when, of course, the effect of ringing results; or if the fungus becomes epidemic and myriads of leaf-spots are formed, the destruction of foliar tissue, gradual taxing of the assimilatory cells, etc., may end in rapid defoliation, and renewed attacks soon exhaust the plants and lead to sterility and death, as often occurs with Uredineae—e.g.the coffee leaf-disease.
It is highly probable that such fungi are particularly exacting owing to their exhausting demands for compounds of potassium, phosphoric acid, and other bodies.
Examples of virulent and rampant general action are afforded by finger and toe in turnips, etc., where the roots are invaded byPlasmodiophora, which induces hypertrophy and rotting of the roots; and by the damping off of seedlings, where the fungusPythiumrapidly invades all parts of the seedlings and reduces them to a water-logged, putrefying mass; or the potato-disease, which is dueto the rapid spread ofPhytophthorain the leaves and throughout the plant, which it blackens and rots in a few days.
Many fungi not in themselves very virulent or aggressive do enormous harm owing to the secondary effects they induce. Some of the tree-killing hymenomycetes, such asAgaricus melleus, for instance, penetrate the wood of a pine at the collar, and the result of the large flow of resin which results is to so block up the water passages that the tree dies off above with all the symptoms of drought. Similarly, thePezizacausing the larch disease, having obtained access to the stem about a foot or so above the ground, will gradually kill the cambium further and further round the stem, and so girdle the tree as effectually as if we had cut out the new wood all round. In all such cases—and the same applies to the leaf-diseases referred to above—the fungus may be compared to an army which is not strong enough to invade the whole territory, but which, by striking at the lines of communication, cuts off the supplies of water, food, etc., and so brings the struggle to an end. Indeed we might compare the cases of fungi which attack the root and collar, and so strike at and cut off the water supply, to a compact army which at once cuts off the enemy from his narrow base; whereas the innumerable units which bring about an epidemic attack on the leaves, and so surround the enemy and cut off his food supplies all round, is rather like a much larger army which cannot get inbeyond the natural barriers of the tissues, and so puts acordonall round the territory and seizes the multitudes of food-stuffs at the frontiers. The end result is similar in both cases, but the methods of warfare differ.
Many fungi, however, though they make their presence noticeable by conspicuous signs, cannot be said to do much damage to the individual plant attacked. The extraordinary malformations induced by parasites likeExoascus, which live in the ends of twigs of trees and stimulate the buds to put out dense tufts of shoots, again densely branched—Witches' brooms—are a case in point. Also the curious distortions of nettle stems swollen and curved byÆcidium, of maize stems and leaves attacked byUstilago, and of the inflorescences ofCapsellabyCystopus, etc., are not individually very destructive; it is the cumulative effects of numerous attacks, or of large epidemics, which tell in the end.
Some very curious effects are due to fungi such asÆcidium elatinum, which, living in the cortex of firs, stimulate buds to put out shoots with erect habit, and with leaves which are radially disposed, annually cast, and differently shaped from the normal—characters quite foreign to the species of fir in its natural condition.
Equally strange are the shoots ofEuphorbiainfested with the æcidia ofUromyces, those of bilberries affected withCalyptospora, etc. In all these cases we must assume a condition of toleration, so to speak, on the part of the host, which adapts itself to the altered circumstances by markedadaptations in its tissue developments, mode of growth and so forth.
This toleration is perhaps most marked in the case of those cereals which, though infected by the minute mycelium ofUstilagowhile still a seedling, nevertheless go on growing as apparently healthy green plants indistinguishable from the rest, although the fine hyphae of the parasite are in the tissues and keeping pace with the growth of the shoots just behind the growing points. As the grains of the cereal begin to form and swell, however, the hyphae suddenly assume the part of a dominant aggressor, consume the endosperm of the enlarging seed, and replace the contents of the grain with the well-known black spores known as Smut.
The reader will find a summary of such fungi as are here concerned in Massee,A Text-Book of Plant Diseases, 1899, or Prillieux,Maladies des Plantes Agricoles.For further details the student should consult the works of Frank and Sorauer referred to in thenotes to Chapter IX., and Tubeuf,The Diseases of Plants, Engl. ed. 1897, pp. 104-539.For experiments on the effects of grass on orchard trees, seeReport of the Woburn Experimental Fruit Farm, 1900, p. 160.For the further study of weeds, the interesting bulletins of the Kansas State Agricultural College, 1895-1898, will show the reader what may be done in the matter of classifying them according to their biological peculiarities.In regard to insects, the reader will find the following list embraces the subject: Somerville,Farm and Garden Insects,1897; Theobald,Insect Life, 1896; Ormerod,Manual of Injurious Insects, 1890, andHandbook of Insects Injurious to Orchards, etc., 1898.The admirable series of publications of the U.S. Department of Agriculture under the editorship of Riley and Howard, and entitledInsect Life, 1888-1895, also abounds in information.Further, Taschenberg'sPraktische Insektenkunde, 1879-1880, and Judeich and Nietsche,Lehrbuch der Mitteleurop. Forst. Insektenkunde, 1889.For an elementary introduction to the study of fungus diseases, see Marshall Ward,Diseases of Plants, Soc. for Promoting Christian Knowledge, London.
The reader will find a summary of such fungi as are here concerned in Massee,A Text-Book of Plant Diseases, 1899, or Prillieux,Maladies des Plantes Agricoles.
For further details the student should consult the works of Frank and Sorauer referred to in thenotes to Chapter IX., and Tubeuf,The Diseases of Plants, Engl. ed. 1897, pp. 104-539.
For experiments on the effects of grass on orchard trees, seeReport of the Woburn Experimental Fruit Farm, 1900, p. 160.
For the further study of weeds, the interesting bulletins of the Kansas State Agricultural College, 1895-1898, will show the reader what may be done in the matter of classifying them according to their biological peculiarities.
In regard to insects, the reader will find the following list embraces the subject: Somerville,Farm and Garden Insects,1897; Theobald,Insect Life, 1896; Ormerod,Manual of Injurious Insects, 1890, andHandbook of Insects Injurious to Orchards, etc., 1898.
The admirable series of publications of the U.S. Department of Agriculture under the editorship of Riley and Howard, and entitledInsect Life, 1888-1895, also abounds in information.
Further, Taschenberg'sPraktische Insektenkunde, 1879-1880, and Judeich and Nietsche,Lehrbuch der Mitteleurop. Forst. Insektenkunde, 1889.
For an elementary introduction to the study of fungus diseases, see Marshall Ward,Diseases of Plants, Soc. for Promoting Christian Knowledge, London.
General and local disease—General death owing to cutting-off supplies, etc.—Disease of organs—Tissue-diseases, e.g. timber—Root-diseases—Leaf-diseases, etc.—Diseases of Respiratory, Assimilatory, and other organs—Physiological and Parasitic diseases—Pathology of the cell—Cuts—Cork—Callus—Irritation—Stimulation by protoplasm—Hypertrophy.
General and local disease—General death owing to cutting-off supplies, etc.—Disease of organs—Tissue-diseases, e.g. timber—Root-diseases—Leaf-diseases, etc.—Diseases of Respiratory, Assimilatory, and other organs—Physiological and Parasitic diseases—Pathology of the cell—Cuts—Cork—Callus—Irritation—Stimulation by protoplasm—Hypertrophy.
On going more deeply into the nature of those changes in plants which we term pathological or diseased, it seems evident that we must at the outset distinguish between various cases. A plant may be diseased as a whole because all or practically all its tissues are in a morbid or pathological condition, such as occurs when some fungus invades all the parts or organs—e.g.seedlings when completely infested byPythium, or a unicellular Alga when invaded by a minute parasite; or it may die throughout, because some organ with functions essential to its life isseriously affected—e.g.the roots are rotten and cannot absorb water with dissolved minerals and pass it up to the shoot, or all the leaves are infested with a parasite and cannot supply the rest of the plant with organic food materials, in consequence of which parts not directly affected by any malady become starved, dried-up, or poisoned or otherwise injured by the results or products of disease elsewhere.
In a large number of cases, however, the disease is purely local, and never extends into the rest of the organs or tissues—e.g.when an insect pierces a leaf at some minute point with its proboscis or its ovipositor, killing a few cells and irritating those around so that they grow and divide more rapidly than the rest of the leaf tissues and produce a swollen hump of tissue, or gall; or when a knife-cut wounds the cambium, which forthwith begins to cover up the dead cells with a similarly rapid growth of cells, the callus. Numerous minute spots due to fungi on leaves, cortex, etc., are further cases in point, the mycelium never extending far from the centre of infection.
Many attempts have been made to classify diseases on a basis which assumes the essential distinction of the above cases, and we read of diseases of the various organs—root-diseases, stem-diseases, leaf-diseases, and so forth; or of the various tissues—timber-diseases, diseases of the cambium, of the bark, of the parenchyma, and so on. Furthermore, attempts have been made to speak of general functional disease, ofdiseases of the respiratory organs, of the absorptive organs, and so forth, as opposed to local lesions.
Critical examination, however, shows that no such distinctions can be consistently maintained, partly because the organs and functions of plants are not so sharply marked off as they are in animals, the diseases of which have suggested the above classification, and partly because all disease originates in the cells and tissues, and it is a matter of detail only that in some cases—e.g.severe freezing or drought of seedlings, or when some ingredient is wanting in the soil—the diseased condition affects practically every cell alike from the first, while in others it spreads more or less rapidly from some one spot.
Even the distinction into physiological diseasesversusparasitic diseases cannot be maintained from the standpoint of the nature of the disease itself. All disease is physiological in so far as it consists in disturbance of normal physiological function, for pathology is merely abnormal physiology, no matter how it is brought about. This is not saying that no importance is to be attached to the mode in which disease is incurred or induced: it is merely insisting on the truth that the disease itself consists in the living cell-substance—the protoplasm—not working normally as it does in health, and this, whether want of water, minerals, or organic food be the cause, or whether the presence of some poison or mechanical irritant be the disturbing agent, as also whethersuch want or irritation be due to some defect in soil or air, or to the ravages of a fungus or an insect.
This being understood I need not dwell on the common fallacy of confounding the fungus, insect, soil or other agent with the disease itself, or of making the same blunder in confusing symptoms with maladies. In this sense, wheat rust is not a disease: it is a symptom which betrays the presence of a disease-inducing fungus, the Rust fungus. Similarly, chlorosis is not a disease: it is a symptom of imperfect chlorophyll action, and the best proof of the truth of both statements is that in both cases the fundamental disease-action is the starvation of the cell-protoplasm of carbohydrates and other essential food matters—in the one case because the fungus steals the carbohydrates as fast as the leaves can make them, in the second because the leaf is unable to make them.
The foundation of a knowledge of disease in plants therefore centres in the understanding of the pathology of living cells.
If a suitable mass of living cells is neatly cut with a sharp razor the first perceptible change is one of colour: the white "flesh" of a potato or an apple, for instance, turns brown as the air enters the cut cells, and the microscope shows that this browning affects cell-walls and contents alike. The cut cells also die forthwith; and the oxygen of the air combining with some of their constituents forms the brown colouring matter which soaks into the cell-walls. The uninjured cells below themgrow longer, pushing up the dead débris, and divide across by walls parallel to the plane of the wound, and so form series of tabular cells with thin walls, which also soon turn brown and die, the cell-walls meanwhile undergoing changes which convert them into cork. The living cells deeper down are now shut off from the outer world by a skin, of several layers, of cork-cells, which prevent the further free access of air or moisture. During the period of active cell-division which initiates the cork, the temperature of the growing cells rises: a sort of fever (wound-fever) is induced, evidently owing to the active respiration of the growing cells.
This healing by cork occurs in any tissue of living cells exposed by a cut—leaf-tissue, young stem or root, fruit, cambium, etc.; and the same applies to any other kind of cutting or tearing injury—such as a prick with a needle or the proboscis of an insect, a stripping, or even a bruise.
Such healing is prepared for and carried out very thoroughly in the case of falling leaves and cast branches, the plane of separation being covered by a cicatrix of cork.
If the cell-tissue under the wound is actually growing at the time, however, a further process is observed when the wound-cork has been formed. The uninjured cells below go on growing outwards more vigorously than ever, the pressure of the overlying tissues taken off by the cut having been removed, and, lifting up the cork-layer as they do so, they rapidlydivide into a juicy mass of thin-walled cells which is of a cushion-like nature and is termed aCallus. This callus is at first a homogeneous tissue of cells which are all alike capable of growing and dividing, but in course of time it undergoes changes in different parts which result in the formation of tracheids, vessels, fibres and other tissue-elements, and even organs, just as the embryonic tissues of the growing points, cambium, etc., of the healthy plant give origin to new growths. Such wound-wood, however, is apt to differ considerably in the arrangement, constitution and hardness of its parts as compared with normal wood, and its peculiar density and cross-graining are often conspicuous.
If instead of a simple tissue, the cut or other wound lays bare a complex mass such as wood, the resultant changes are essentially the same to start with. The living cells bordering the wound form cork, and then those deeper down grow out and form a callus. The exposure of the wood however, entails alterations in its non-living elements also. The lignified walls of tracheids, fibres, etc., turn brown to a considerable depth, and this browning seems to be—like all such discolorations in wounds—due to oxidation changes in the tannins and other bodies present: the process is probably similar to what occurs in humification and in the conversion of sap-wood into heart-wood in trees. Such wood is not merely dead, but it is also incapable of conveying water in the lumina of its elements, whichslowly fill with similarly dark-coloured, impervious masses of materials termed "wound-gum," the nature of which is obscure, but which slowly undergoes further changes into resin-like substances.
The exposure of wood by a wound results also in another mode of stopping up the vessels and so hindering the access of air, loss of water, etc., for the living cells of the medullary rays and wood-parenchyma grow into the lumina of the larger vessels through the pits, formingthyloses, again a phenomenon met with in heart-wood. In Conifers the stoppage of the lumina is increased by deposition of resin, which also soaks into the cell-walls and the wounded wood becomes semi-translucent owing to the infiltration.
Every living cell in an active condition is irritable, and one of the commonest physiological reactions of growing tissues is that of responding to the touch of a resistant body, as is vividly shown by the movements of the Sensitive plant,Dionaea, etc., and by those of tendrils, growing root tips, etc., on careful observation. We have reason for stating that if a minute insect, too feeble to pierce the cuticle, cling on to one side of the dome-shaped growing point of any shoot, the irritation of contact of its claws, hairs, etc., would at once cause the protoplasm of the delicate cells to respond by some abnormal behaviour; and, as matter of experiment, Darwin showed long ago that if a minute piece of glass or other hard body is kept in contact with one side of the tip of a root, the growth on the side in contact is interferedwith. Moreover we know from experiments on heliotropism, thermotropism, etc., that even intangible stimuli such as rays of light, etc., impinging unsymmetrically on these delicate cells cause alterations in their behaviour—e.g.arrest or acceleration of growth.
Perhaps the most remarkable class of stimulations, however, is that due to the presence of the entire protoplasmic body of one organism in the cell of another, each living its own life for the time being, but the protoplasm of the host cell showing clearly, by its abnormal behaviour, that the presence of the foreign protoplasm is affecting its physiology. A simple example is afforded by Zopfs'Pleotrachelus, the amoeboid protoplasmic body of which lives in the hypha ofPilobolus, causing it to swell up like an inflated bladder, in which the parasite then forms its sporangia. ThePleotrachelusdoes not kill thePilobolus, but that its protoplasm alters the metabolic physiology of the latter is shown by the hypertrophy of the cells, and by the curious fact that it stimulates thePilobolusto form its sexual conjugating cells, otherwise rare, an indication of very far-reaching interference with the life-actions of the host.
An equally remarkable example is that ofPlasmodiophora, the amoeboid naked protoplasm of which lives and creeps about in the protoplasm of a cell of the root of a turnip, to which it gains access through the root-hairs. It does not kill the cell, but stimulates its protoplasm to increased activity and growth and division, itself dividing alsoand passing new amoebae into each new daughter-cell of the host. Here the processes of stimulation, hypertrophy and further division are repeated, until hundreds or thousands of the turnip root-cells are infected. The externally visible result is the formation of distorted swellings on the root (Finger and Toe), most of the cells of which are abnormally large and filled with amoeboidPlasmodiophoraprotoplasm, which finally devours the turnip-protoplasm and itself passes over into spores. Here we have most convincing proof of the stimulation of protoplasm by other protoplasm in direct contact with it; and that the metabolism of the host-cells is profoundly altered is shown not only by the abnormal growth of the cells, but also by the starvation of the rest of the turnip plant as thePlasmodiophoragets the upper hand. We have here, in fact, a local intracellular parasitic disease, gradually invading large tracts of tissue and eventually inducing general disease resulting in death—a state of affairs reminding us of cancer in animals.
Irritation and hypertrophy of cells, however, may be induced by parasites which never bring their protoplasm into direct contact with that of the host. Many Chytridiaceae penetrate the cells of plants, and grow inside them as short tubes, vesicles, etc., the protoplasm of which is separated by their own cell-walls from that of the host-cell; nevertheless hypertrophy and abnormal cell-divisions and secretions are induced, and the effect even extends to neighbouring cells—e.g.Synchytrium—showingthat some influence is exerted through cells themselves not directly affected. This latter point need not surprise us now we know that the cells of plant-tissues are connected by fine protoplasmic strands passing through the separating cell-walls.
But the invading plant need not actually enter the cells, and may still stimulate them through both its own and their own cell-walls to abnormal growth. This is well shown by the intercellular mycelium ofExoacusandExobasidium, and the latter affords an excellent illustration of the far-reaching effects of hyphae on the cells (ofVaccinium) into which they do not penetrate. Not only are the cells stimulated to grow larger and divide oftener than normally, thus producing large gall-like swellings, but the chlorophyll disappears, the cell sap changes colour to red, the numerous compound crystals normally found in the tissues diminish in number and are different in shape, large quantities of starch are stored up, and even the vascular bundles are altered in character. All these changes indicate very profound alterations in the physiological working of the protoplasm of the cells of the host, and yet the fungus has done its work through both its own cell-walls and those of the host.
Even harmless endophytic algae in the intercellular spaces of plants may stimulate the cells in their immediate neighbourhood to increased growth,e.g.Anabaenain the roots of Cycads.