360 PHYSIOLOGY 



The simplest carbohydrate which has been detected in plants is 

 formaldehyde, HCOH. This group will be recognized in the makeup of 

 all the more complex ones above (but see p. 375). While it has only a 

 transient existence and does not occur free, except in minute amounts, 

 it has its special significance in that it is probably the first substance 

 formed by the green cells from water and carbon dioxid. 



Fats. — Fats are apparently always secondary products, and consti- 

 tute a common form of surplus food. These storage products furnish 

 various commercial oils; e.g. olive oil, cotton oil, linseed oil, castor oil, 

 com oil, etc. They occur usually in fluid form as minute droplets in 

 the protoplast, only occasionally being solid at ordinary temperatures, 

 as in the seed of cacao. They are of very complex structure, being com- 

 pounds of glycerin and three molecules of fatty acid. 



Their structure may be understood from these formulas: 



CH2OH CH2 • R 



-I I 



Glycerin is: CHOH A fat is: CH- R 



I I 



CH2OH CH2 • R 



in which R may represent oleic acid (C18H34O2), linoleic acid (C18H32O2), hypogaeic 

 acid (C16H30O2), or any other member of a considerable series of fatty acids, minus 

 the acid ion H. The R radicals may be all alike or different. When digested, fats 

 break up into glycerin and the fatty acid or acids. The fats contain a notably 

 small proportion of oxygen. 



The lecithins are substances allied to the fats in their constitution, 

 containing phosphoric acid and cholin in place of one of the fatty acid 

 radicals, R. They are very widely distributed in plants, and probably 

 play an important role in the protoplasm, but just what is not known at 

 present. It may be that they determine what substances may pass 

 through the membranes; and it may be also that they are connected 

 with the formation of chlorophyll. 



Amides. — The name is here used loosely and not in its strict chemical 

 sense, for a group of substances of which none are popularly known. 

 For convenience, they may be distinguished as nitrogenous compounds 

 intermediate between carbohydrates and proteins. On the one hand, 

 they are derivatives of proteins, among whose decomposition products 

 various amino-acids always figure. On the other hand, they are deriv- 

 atives of the carbohydrates and their allies, from which, with proper 

 additions, they are readily formed. In addition to the carbon, hydrogen, 

 and oxygen of carbohydrates, they contain nitrogen, always combined 



