into amides and sugar, and, while the latter is used in accomplishing the structure of the plant by changing into cellulose, the amide is again transformed into albuminoid, and by decomposition reduced to amides. Thus, by continually assimilating carbonic acid and water, combining with them to form albuminoids, and giving them off again as sugar, the amides act a prominent part in the development of plants. New supplies of amides, in the mean time, are continually formed, while the albuminous matter is partly transferred to remote organs, where, exposed to light and other agents, it undergoes various decompositions, by which the deposits of solid proteids, alkaloids, and many other bodies are produced. A certain quantity of proteids becomes stored up in the seeds, modified into gluten, or legumin, for the purpose of hereafter entertaining and supporting the life and growth of the offspring. Vegetable albumen, gluten, and legumin so closely resemble animal albumen, fibrin, and casein, that the same names have been given to them.
In thus tracing the origin of albuminoids in plants, we see them partly dissolved as vegetable albumen in the juice (upon the heating of which coagulated albumen gathers as foam on the surface), partly as "plasma" forming the contents of cells, and partly as solids in various organs, but chiefly in the seed, which by their presence acquires more or less valuable properties as food for animals, the nutritive value of grain and leguminous products being due merely to the high percentage of gluten and legumin contained in them.
Plants, indeed, are the sole source on which most animals depend for their food, for they are incapable of assimilating the constituents of air and water, as vegetables do. Only in the most simple forms of animals, such as moneres and amœbæ, the question is undecided whether such an assimilation takes place or not; but our knowledge of the limits between these low forms of vegetable and animal beings is very imperfect, and, in view of the numerous parasitic plants which draw their food from other organisms, we can not declare the source and process of nourishment to be a correct and pervading mark of difference between the two classes. If, however, some one should raise the objection that animals of prey do not depend upon plants for food, he might easily be corrected by showing that, in feeding on vegetable-eating creatures, they "indirectly" live on the plants themselves.
The changes which vegetable proteids undergo by being taken up into the animal organism are insignificant at first; having been transformed into soluble bodies (peptones) by pepsin, the digestive agent of the stomach, we see them appearing again in the circulation of blood as albumen, globulin, fibrinogen, combinations of very similar character to the proteids of plants. There is, indeed, no practical difference existing between albumens and legumins. The proteid of beans, peas, lentils, etc., is identical with casein, the proteid of cheese, as to composition and properties.