WHERE FOOD COMES FROM 113 



protein, fat, or carbohydrate. Tea and tobacco are used because 

 of the presence of an alkaloid^ that makes the leaves of these 

 plants interesting to human beings. 



The fact that plants throw large quantities of oxygen into 

 the air makes them valuable neighbors, especially in the cities, 

 where oxygen is used up relatively faster on account of the 

 crowded population and the many fires. 



Our domestic animals feed largely on the leaves of plants : 

 grass, beet tops, hay, alfalfa, clover, and corn fodder furnish 

 the principal green food of cattle and horses. 



The dead leaves of plants (whether they have dropped in the 

 autumn or have reached the ground through the death of herbs 

 etc.) form the basis of the humus of the soil. Humus is a mass 

 of decaying vegetable matter, with some animal matter and soil. 

 This forms a soil covering that is very helpful from the point 

 of view of retaining moisture in the soil, and to a certain extent 

 it also serves in returning nitrogen and other elements to the soil. 



92. Simple food- makers. There was life upon the earth (and 

 therefore some way of making food out of simpler substances) 

 long before there were any leaf-bearing plants. In some of 

 the simplest plants the whole body consists of but a single 

 cell. Among the commoner examples are the green slime (see 

 section 47 and Fig. 30) and the pond scum or "frog spit" 

 (Spirogyra) that we find floating on the surface of ponds. In 

 such plants each cell carries on all the activities that together 

 make up being alive — all the activities that in larger and more 

 complex plants are carried on by different special parts or 

 organs. In the most complex plants some activities are carried 

 on by every cell, but certain processes are specialized : there is 

 division of labor among the root, the stem, and the leaf. 



93. The root. This organ takes on many different forms, from 

 the thin, stringy roots of grasses to the massive fleshy or woody 

 roots of beets or trees (Fig. 65). But in a general way it may 

 be considered an organ of attachment and of absorption. 



^ An alkaloid (that is, something that is "like an alkaU") is an organic com- 

 pound containing nitrogen and capable of combining with acids. 



