Part 8 (1/2)

Examples of virulent and rampant general action are afforded by finger and toe in turnips, etc., where the roots are invaded by _Plasmodiophora_, which induces hypertrophy and rotting of the roots; and by the damping off of seedlings, where the fungus _Pythium_ rapidly invades all parts of the seedlings and reduces them to a water-logged, putrefying ma.s.s; or the potato-disease, which is due to the rapid spread of _Phytophthora_ in 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 as _Agaricus 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 pa.s.sages that the tree dies off above with all the symptoms of drought. Similarly, the _Peziza_ causing 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 in beyond the natural barriers of the tissues, and so puts a _cordon_ all round the territory and seizes the mult.i.tudes 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 like _Exoascus_, 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 by _aecidium_, of maize stems and leaves attacked by _Ustilago_, and of the inflorescences of _Capsella_ by _Cystopus_, etc., are not individually very destructive; it is the c.u.mulative effects of numerous attacks, or of large epidemics, which tell in the end.

Some very curious effects are due to fungi such as _aecidium 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 of _Euphorbia_ infested with the aecidia of _Uromyces_, those of bilberries affected with _Calyptospora_, etc. In all these cases we must a.s.sume a condition of toleration, so to speak, on the part of the host, which adapts itself to the altered circ.u.mstances by marked adaptations 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 of _Ustilago_ while 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 a.s.sume 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 s.m.u.t.

NOTES TO CHAPTER XII.

The reader will find a summary of such fungi as are here concerned in Ma.s.see, _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 the notes to Chapter IX., and Tubeuf, _The Diseases of Plants_, Engl. ed. 1897, pp. 104-539.

For experiments on the effects of gra.s.s on orchard trees, see _Report 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 cla.s.sifying 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, and _Handbook of Insects Injurious to Orchards, etc._, 1898.

The admirable series of publications of the U.S. Department of Agriculture under the editors.h.i.+p of Riley and Howard, and ent.i.tled _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, a.s.similatory, 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 pa.s.s 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 cla.s.sify diseases on a basis which a.s.sumes 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, of diseases of the respiratory organs, of the absorptive organs, and so forth, as opposed to local lesions.