148 THE BIOLOGY OF A PLANT. 



satisfied. And this is due to tlie fact that in the manufacture 

 of starch in the chlorophyll-bodies the kinetic energy of sunlight 

 a was expended in lifting the atoms into position of vantage,' 

 thus endowing them with energy of position. In this way some 

 of the radiant and kinetic energy of the sun comes to be stored 

 up as potential energy in the starch. In short, Pteris^ like all 

 green plants, is able by co-operation with sunlight to use sunple 

 raw materials (carbon dioxide, water, oxygen, etc.) poor in en- 

 ergy or devoid of it, and out of them to manufacture food ^ i.e., 

 complex compounds rich in available potential energy. We 

 shall see hereafter that this power is possessed by green plants 

 alone ; all other organisms being de23endent for energy upon the 

 potential energy of ready-made food. This must in the first 

 instance be provided for them by green plants ; and hence with- 

 out chlorophyll-l)earing plants animals (and colorless plants as 

 well) apparently could not long exist. 



The plant absorbs also a small amount of kinetic energy, in- 

 dependently of the sunlight, in the form of heat ; this, however, 

 is probably not a source of vital energy, but only contributes to 

 the maintenance of the body temperature. 



Circulation of Foods. It is chiefly in the green (chloroj^hyll- 

 bearing) parts of the plants, and in the presence of sunlight, that 

 food-manufacture goes on. Somehow, then, the water absorbed 

 by the roots must be transported to the leaves, and the starch 

 made in the leaves must be conveyed to the subterranean tissues. 

 Exactly how these transfers of material are effected is uncertain, 

 but there is reason to believe that they take place mainly by the 

 slow processes of diffusion. It is certain that no distinct organs 

 of circulation or distribution, such as the blood-vessels of the 

 earthworm, exist in the fern. 



Metabolism. Starch, as has just been seen, is first formed in 

 the chlorophyll-bodies. But the formation of starch, all-imj)or- 

 tant as it is, is after all only the manufacture of food as a j^re- 

 liminary to the real processes of nutrition. These processes must 

 take place everywhere in ordinary protoplasm; for it is here 

 that oxidations occur and the need for a renewal of matter and 

 energy consequently arises (cf. pp. 32 and 33). Sooner or later 

 the starch grains are changed into a kind of sugar {glucose, 

 CgHj^Og), which, unlike starch, dissolves in the sap, and may 



