The admirable series of publications of the U.S. Department of Agriculture under the editorship of Riley and Howard, and entitled Insect Life, 1888-1895, also abounds in information.
Further, Taschenberg's Praktische 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.
CHAPTER XIII.
NATURE OF DISEASE.
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 by Pythium, or a unicellular Alga when invaded by a minute parasite; or it may die throughout, because some organ with functions essential to its life is seriously 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.