The same conclusions are true for another class of substances, the sugars. They may be a food taken from without, or a reserve deposited in the tissues. The animal takes from without, in fruits for instance, the ordinary sugar which pleases its taste. Beetroot, when flowering and fructifying, draws this substance from its roots in which stores have been amassed. The sugar cane when running to seed takes the sugar from the stores which it possesses in its cane. Brewer’s yeast, the _saccharomyces cerevisiæ_, the agent of alcoholic fermentation, finds this same substance in the sugary juices favourable to its development. In the same way, identically fatty substances, either in the form of food or of reserve-stuff, serve for nutrition to animals and vegetables; and that is again true of the substances of the fourth class, albuminoids, identical in the two kingdoms, foods or reserve-stuff, equally utilizable in both after digestion. _Identity of the Digestive Agents and Mechanisms in Plants and Animals._—Now, the results of contemporary research have been to establish a surprising resemblance in the modifications experienced by these foods, or reserve stuffs, in animals and plants; and even resemblances in the agents which realize them, and in the mechanisms by which they are performed. There is a real unity. The flour accumulated in the tuber of the potato is liquefied and digested on the appearance of the buds or of the flower, just as the starch of the liver or the alimentary flour is digested by the animal. The fatty matter which is stored up in the oleaginous grain is digested at the moment of germination, just as the fat during a meal is digested in the animal’s intestine. As the beetroot begins to run to seed, the root gives up part of its store of sugar, and this reserve stuff is distributed throughout the stalk after having been digested, exactly as would have been the case in the digestive canal of man. Vegetables, then, really digest. The four classes of substances mentioned above are really digested in order to pass from their actual form, a form unsuitable for interstitial exchanges, to another form suitable for nutrition. As there are four kinds of foods, so there are four kinds of digestions, four kinds of ferment-producing agents—amylolytic,[14] proteolytic,[15] saccharine, and lipasic[16] diastases, identical in the animal and the plant. Identity of ferments implies identity of digestions. Going down to the very basis of things, the digestive act is nothing but the action of this ferment. This is the crux of the whole question. All else is only difference in scene, varying in the means of execution and in the accessories. The difference arises from the stage on which it takes place, but the piece which is being played is the same, and the actors are the same, and so is the action of the play. [14] Amylolytic ferments change starch and glycogen (_amyloses_) into sugar.—TR.