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 hitherto 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 quantity 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 passed 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 quantity 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 Peronosporaparasitica 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 galllike 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 principal reason why Rust-fungi, for instance, vary so much in their vigour and reproductive power on different wheats and grasses, 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 grasses 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 more over 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.
The reader will find more on this subject in Bailey's Survival of the Unlike and the literature quoted in the notes to Chapter 8.
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 Principal Results of Swedish Research into Grain Rust," Botanical Gazette, vol. 25, 1898, p. 26.
For an account of Wheat-rust see Marshall Ward, "Illustrations of the Structure and Life-history of Puccinia graminis, etc.," Ann. of Bot., 1888, Vol. II., p. 215.