106 A TEXTBOOK OF BOTANY [Ch, III, 13 



proteins, composed of the elements C, H, (J, N, S, [P], in 

 diverse, hut always complicati'd, proportions, form the basis 

 of flesh in animals ; and it is because the seeds of the Pulse 

 Family (Peas and Beans) contain so much protein that they 

 approach near to meat in their food value. 



Like the carbohydrates and fatty oils, but perhaps even 

 more than they, the proteins arc good food for animals, which 

 take them in fodder, vegetables, fruits, and grains. To ani- 

 mals, however, they have this special importance, that while 

 muscles, nerve substance, and other essential tissues are 

 composed chiefljr of proteins,' the higher animals at least 

 have no power to construct them from simpler substances, 

 but must take them ready-made from plants, or from ani- 

 mals which have taken them from plants. It is for its con- 

 densed supply of such proteins that meat has such food value, 

 and it is, of course, for their value as protein-accumulators 

 from plants on his behalf that man keeps cattle and other 

 domestic animals which he eats. Unlike the case of the sug- 

 ars, starches, and fatty oils, however, man does not, because 

 of practical difficulties, extract the plant proteins and re- 

 fine them for use, though he can do so when he wishes ; but 

 he usually takes them with the other food materials which 

 they happen to accompanj-. 



3. THE LIVING PROTOPLASM . The living material, 

 the most important in all organic nature, has already been 

 described (page 3.5). It is chemically a mixture of a great 

 many substances, but its greater antl most essential part is 

 composed of proteins. The proteins, indeed, have their 

 great importance as reserve food because they are a step in 

 the formation of living proto])lasm. Some of these jiroteins 

 are very complex (one, for example, has the formula 

 C720H1134N218O048S5, and much more complicated kinds are 

 known) ; and they are consequentl>' unstable and labile, 

 changing into other forms with absorptions or releases of 

 energy whieli are the foundation of various phenomena of 

 fife. But our knowledge of llic chemistrv of the li\ing 



