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.
Notes to Chapter X.
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.
CHAPTER XI.
CAUSES 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.