122 BOTANY: PRINCIPLES. AND PROBLEMS 



photosynthesis, with nitrogen and sulphur which have been taken 

 into the plant from the soil through the root-hairs. This union 

 probably occurs most commonly in the leaves. It is a noteworthy 

 fact that (with a few exceptions) plants alone seem to possess 

 the ability to bring about this synthesis of protein, an ability 

 almost as significant to the rest of the organic world as is that 

 which enables them to manufacture carbohydrates from inorganic 

 substances. Animals depend almost entirely upon plants for 

 their supply of proteins. If given the simple protein compounds, 

 however, animals can build therefrom new and characteristic 

 protein materials of all kinds. 



Proteins, with their highly complex molecules, are not produced 

 directly but are built up by an aggregation of simpler nitrogenous 

 compounds, the amino-acids, which have been called their 

 "building-stones". Over twenty of these compounds have been 

 isolated and studied. Among these are glycine, C2HBNO2; 

 leucine, CeHuNOa; glutamic acid, C5H9NO4; and tryptophane, 

 C11II12N2O2. From these amino-acids, with the addition of 

 sulphur, are constructed all the proteins in almost infinite variety, 

 — the albumins, globulins, glutelins, prolamins, and many others, 

 which differ in composition, solubility, stability, and other 

 respects. 



Most of the protein which is stored as a reserve food in plants 

 occurs in definite bodies, the aleur one-grains, which are secreted 

 by the protoplasm much as are starch-grains and which often 

 fill the sap-cavity. The storage of proteins in this form is fre- 

 quently confined to particular regions of the plant. Thus the 

 aleurone layer of cereal grains, a single layer of cells just under 

 the pericarp, is filled with aleurone and is thus rich in protein. 



Proteins are much less abundant as reserve foods than are 

 carbohydrates, and are relatively poor energy-producers, but 

 their composition makes them far more effective than any other 

 foods in the construction and renewal of living substance. As 

 "tissue-builders" proteins therefore play a vital part in the 

 nutrition of plants and animals. 



Digestion. — We have already seen that the physiological proc- 

 esses of a plant are concerned almost entirely with substances 

 which are in solution, and that in order to enter the plant body 

 from the soil or the air or to pass from one cell to another within 

 it, a substance must first be dissolved. Insoluble materials are of 

 little significance in the economy of protoplasm. Most of the 



