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 cannot escape 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 are not 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, contains the 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 the conditions 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.