8 THE VEGETABLE PROTEINS 



developing embryo. That this variety in the nature of the constituent 

 substances is shared by the proteins of the embryo tissues has been 

 shown by studies of the protein constituents of the wheat embryo 

 which will be later considered more in detail. 



In most dicotyledonous seeds the cells containing the reserve 

 protein are distributed among those of the embryo tissues. 



In roots, bulbs and tubers the undissolved reserve protein occurs 

 suspended in the cell sap, frequently in the form of crystals. 



Little that is definite is known concerning the- chemical properties 

 of any of the plant proteins except those of seeds, for the proteins 

 occurring in the physiologically active cells and fluids of plants have 

 been but little studied, owing to the relatively small quantities in which 

 they occur and the difficulty of separating them from each other. 



We have just seen that the total protein is contained in several differ- 

 ent parts of the seed, namely, in the endosperm cells as reserve protein, 

 in the protoplasm of these cells, and in the cells composing the tissues 

 of the embryo, both in the cytoplasm and in the nuclei. As it is not pos- 

 sible, in most cases, to separate these different parts of the seed 

 by mechanical means, in sufficient quantity to permit a study of the 

 proteins of each, extracts of the entire seed will consequently contain 

 a mixture of proteins from all the different parts of the seed, and may 

 be expected to contain a number of different proteins. Experience has 

 shown this to be the case, as a careful examination of the extracts of all 

 seeds thus far studied has shown the presence of a number of different 

 types of protein. The whole of the protein contained in seeds is, 

 therefore, not reserve protein. Extracts of seeds always contain in 

 addition to a relatively large amount of one or two types of protein, 

 which are manifestly the reserve protein of the seed, a certain small 

 proportion having distinctly different properties from those of the latter. 

 It is probable that most of the former protein is yielded by the cells 

 of the embryo as well as by the protoplasm of the endosperm cells. 

 With most seeds definite evidence of this has not yet been obtained, 

 but in the case of wheat the embryos are separated by the commercial 

 process of milling in a nearly pure condition, and a study of this pro- 

 duct has shown that those proteins which are obtained only in small 

 quantity from the entire seed are present in relatively large amounts 

 in this embryo meal. The proteins of this embryo both in chemical 

 and physical character differ from those of the endosperm and resemble 

 more nearly the physiologically active proteins of animal tissues. 

 These embryo proteins are globulin, albumin and proteose, and associ- 

 ated with them is a large quantity of nucleic acid, so that products 



