&4 PHYSIOLOGY. 



regularly of diminishing density, and that this gives the appearance of alter- 

 nating lines. The starch formed by plants is one of the organic substances 

 which are manufactured by plants, and it is the basis for the formation of 

 other organic substances in the plant. Without carbon food green plants 

 cannot make any appreciable increase of plant substance, though a consider- 

 able increase in size of the plant may take place. 



