Part 11 (2/2)

That certain thick-skinned Apples, Tomatoes, and Plums pack and store better than those with a more tender epidermis seems proved--that is to say, they suffer less from fungi which gain access through bruises and other wounds; but it cannot be said that any convincing proof is yet to hand explaining in detail why some races of wheat resist Rust, or why the roots of American Vines suffer less from _Phylloxera_ than others.

One of the most extraordinary cases known to me in this connection is the unconscious selection on the part of native Indian cultivators, perfectly ignorant of the principles involved, of spring and autumn forms of Rice, Wheat, Castor Oil, Sugar Cane, Cotton, and other crops.

”It has been estimated that Bengal alone possesses as many as 10,000 recognisable forms of rice.” Now there is not the slightest ground for doubt that these have been unconsciously bred from the semi-aquatic native species during the many centuries of Indian agriculture, and nevertheless they have, among other peculiar races, some hill-breeds which they cultivate on dry soils and without direct inundation. That is to say, they possess tropical and temperate races differing far more than our spring and summer wheats.

Something has been gained, then, if we can show that there is nothing absurd or hopeless in the search for disease-proof or resistant races, and I think this can be done. We must not forget that the ideal usually set before himself by a breeder of plants has. .h.i.therto been almost exclusively some standard of size, form, colouring, and so forth, of the flower, or of taste and texture of the fruit, tuber, etc., though experiments with _Cinchona_, with brewery yeasts, and other plants remind us that variations in other directions have been attended to also.

Now it is obvious that in breeding sour limes and sweet oranges the cultivator is selecting, and intensifying by selection, very different metabolic processes in the cell: he can test the results of these, and so the selection proceeds.

The question is, Could he select at the same time those variations in cell activity which express themselves in properties of the flower, fruit, foliage, etc., he desires, as well as such variations as aid the cells in repelling fungi, insects, or exigencies of the non-living environment?

That more or less disease-proof varieties could be selected if that object alone were kept in view can hardly be doubted; plenty of examples exist already which show that the necessary variations to work upon exist in just those secretions of protoplasm, etc., which we have seen are concerned in repelling or attracting parasites.

The Sweet Almond has lost the power of producing amygdalin and prussic acid in its cells; Cinchona plants vary immensely in the quant.i.ty of quinine formed, and in European hot-houses may even form none at all; some varieties of Maize have sugar and dextrine instead of starch in their endosperms, or coloured instead of clear sap in the aleurone layer, and recent researches prove that they can transmit these peculiarities to hybrid offspring; non-poisonous bacteria have frequently been got from poisonous species simply by cultivation under special conditions, and pigmented forms can be bred into non-pigmented races.

But we see that the difficulty of selection is increased in the case postulated above, because two ideals are to be worked up to, and they may conceivably be incompatible. Not necessarily so, however, for breeders have solved such problems before in obtaining early _and_ heavy cropping races of potatoes, wheat, etc., sweet _and_ large grapes, strawberries, etc., hardy _and_ brilliant flowers, and so forth.

There is, however, another aspect of this question of variability in organisms in this connection to be considered. Ever since cultivation began man has probably been cultivating not only the crops he desires, but also the pests which infest them, and if variation of his chosen plants occurs--and no one will deny that--surely variation of the fungi and insects which live on them also takes place. That this is so can be demonstrated, though, since it is not part of my theme to go into the question of peculiarities of species and races of parasites, the subject must here be pa.s.sed over with a few remarks only.

Recent researches have shown not only that fungi vary immensely in form and morphological characters according to the amount and kind of food-materials put at their disposal, thus bringing the whole question of polymorphism into the domain of experimental physiology, but that their capacities for infection, spore formation, etc., are also capable of variation and are dependent on the quality and quant.i.ty of food supplies, water, as well as on the temperature, illumination, and other factors of the environment. This is true of parasites as well as of saprophytes. _Botrytis_ forms conidia only in darkness and in moist air.

Klebahn found that a _Puccinia_ growing on _Digraphis_ infected _Polygonatum_ readily and completely, _Convallaria_ imperfectly, whereas if sown on _Majanthemum_ it only just infected the plant and then remained sterile, while it refused to infect _Paris_ at all. Magnus has shown that _Peronospora parasitica_ can only infect meristematic tissues, and that when it co-exists with _Cystopus_ on _Capsella_, as is usually the case, it enters the latter plant by infecting the gall-like pustules of hypertrophied tissue induced by that parasite. Numerous parasitic fungi can only penetrate particular parts of plants. For instance, the _Ustilago_ of wheat can only infect the young seedling, and grows for weeks as a barren mycelium, only becoming a dominant fungus in the endosperm. Numerous other examples could be given, but these suffice to show some of the ways in which the nature of the food substratum supplied by the host affects the fungus. It is obvious that if the nature of this food changes, the fungus is also affected, and no doubt this is the princ.i.p.al reason why Rust-fungi, for instance, vary so much in their vigour and reproductive power on different wheats and gra.s.ses, though the other factors of the environment must also be of influence on them as well as on the hosts.

But--and this is the second point--modern research is also showing that the various species of Rust-fungi have split up into different varieties or specialised races, according to the particular host plants they inhabit. For instance there are special varieties or races of the particular species known as _Puccinia graminis_, the wheat rust, each of which grows well on various kinds of grain and gra.s.ses but refuses to infect others. Thus, the variety which infects Wheat refuses to infect Barley or Oats, while that variety which grows on Rye will not take on Wheat and so forth. Now it is important to notice that these specialised races are indistinguishable one from another by their visible microscopic characters: they are all botanically of the species _Puccinia graminis_ which forms its aecida on the Barberry. We must therefore conclude that we have here the same phenomenon as that met with in culture-races of bacteria which, having been fed for several generations on media rich in proteids, refuse to grow on media rich in carbohydrates, or when attenuated races are developed by culture under special conditions.

Now since such physiological races as I have described are by no means confined to _Puccinia_ but are also known in _Melampsora_, _Gymnosporangium_ and other fungi, we must conclude from this and from what we know of variation in plants and animals generally, that variation and adaptation are common among parasites, insects as well as fungi.

These considerations will serve to show moreover that the question of breeding disease-proof varieties of our cultivated plants is complicated by the danger of our breeding at the same time adapted races of their pests. It appears at first sight extremely improbable that we should escape the danger by breeding from those specimens of our plants which have best survived a fungus epidemic. Still, it must not be forgotten that ”hardy varieties,” and races adapted to other exigencies of the non-living environment, have been bred by selection--and nevertheless this variable non-living environment is always with us. The matter is therefore simply and solely one of experiment, and the retort that a disease-resisting variety of any particular plant has not yet been raised is no more valid than the objection that a true blue primrose has not yet been obtained: whether the same remark can be made with regard to any hope of a _disease-proof_ plant may be another matter, but in any case it must be made more cautiously in the light of our present experience.

NOTES TO CHAPTER XVIII.

The reader will find more on this subject in Bailey's _Survival of the Unlike_ and the literature quoted in the notes to Chapter VIII.

For varieties of Indian Wheats, etc., see Watt, _Agricultural Ledger_, Calcutta, 1895.

For a discussion on so-called ”Disease-proof Wheats” consult Eriksson & Henning, _Die Getreideroste_.

Magnus' paper is in the _Berichte der Deutschen bot.

Gesellsch._, 1894, p. 39.

Concerning physiological races and adapted varieties of _Puccinia_, etc., see Eriksson, ”A General View of the Princ.i.p.al Results of Swedish Research into Grain Rust,”

_Botanical Gazette_, vol. 25, 1898, p. 26.

For an account of Wheat-rust see Marshall Ward, ”Ill.u.s.trations of the Structure and Life-history of _Puccinia graminis_, etc.,” _Ann. of Bot._, 1888, Vol. II., p. 215.

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