HISTORY OF MATTER AND ENERGY. 107 



a position of vantage, thus endowing them with energy of posi- 

 tion. 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 simple compounds, poor in energy or devoid of 

 it, and out of them to manufacture food, i.e., complex compounds 

 rich in potential energy. We shall see hereafter that this power 

 is possessed by green plants alone; all other organisms being 

 dependent 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 without chlorophyll-bearing plants ani- 

 mals (and colorless plants as well) could riot 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 bodily temperature (cf. p. 40). 



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

 the chlorophyll-bodies. But the formation of starch, all-impor- 

 tant as it is, is after all only the manufacture of food as a pre- 

 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. 33 and 34). Sooner or later 

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

 C 6 H 12 O 6 ), which, unlike starch, dissolves in the sap, and may thus 

 be easily transported to all parts of the plant. "Wherever there is 

 need for new protoplasm, whether to repair previous waste or 

 to supply materials for growth, the elements of the starch (or 

 glucose) are in some unknown way combined with nitrogen and 

 sulphur (probably also with salts, water, etc.), to form proteid 

 matter. The particles of this newly formed compound are incor- 

 porated into the protoplasm (by " intus-susception," p. 5) and, in 

 some way at present shrouded in mystery, are endowed with the 

 properties of life. AVe do not know how long they may re- 

 main in the living state, but sooner or later they are oxidized, 

 and as a result of the oxidation, that energy is set free which 

 enables the fern to do work and prolong its existence. The 

 oxidated products are afterwards eliminated from the cells. 



If a larger quantity of starch is formed in the chlorophyll- 

 bodies than is immediatelv needed by the protoplasm for pur- 



