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, 
corn 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 : 
CH 2 OH CH 2 R 
Glycerin is : CHOH A fat is : CH - R 
CH 2 OH CH 2 -.R 
in which R may represent oleic acid (CigHsiO^, linoleic acid (Ci8H 32 O 2 ), hypogaeic 
acid (CisHsoOiz), 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 
