CHAPTER XII.
CAUSES OF DISEASE. THE LIVING ENVIRONMENT.
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 ignorant gardener 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., of another 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.