CHAPTER Y. 



NUTRITION AND FOOD-STUFFS. 



THE continuation of protoplasmic life depends on certain chemi- 

 cal changes which are accompanied by a considerable loss of sub- 

 stance. This loss must be made good by the assimilation of 

 material from without, and the manner by which it is obtained 

 constitutes one great point of difference between Plants and Ani- 

 mals. In the majority of the former (certain fungi form the 

 main exceptions) the cells in those portions of the plant which 

 are exposed to the light and air, contain a peculiar green sub- 

 stance called chlorophyll, and through the agency of this sub- 

 stance they are able to obtain from the inorganic kingdom nearly 

 all the food they require. Water is taken up by the roots with 

 such salts as may happen to be in solution, and is carried through 

 the stem to the leaves; here the active chlorophyll-bearing cells, 

 under the influence of the sun's rays, cause it to unite with the 

 carbon dioxide present in the air, to form various substances, of 

 which we may take starch or cellulose as the simplest example. 

 This reaction may be represented chemically, thus: 



6C0 2 + 5H 2 = C 6 H 10 5 + H . 



Starch or cellulose. 



A large proportion of oxygen is thus set free and discharged into 

 the atmosphere. 



The most striking property of plant protoplasm is, then, the 

 power of using the energy of the sun's rays to separate the ele- 

 ments of the very stable compounds, carbon dioxide and water, 

 and from the elements thus obtained to make a series of more 

 complex and unstable compounds, which readily unite with more 

 oxygen, and change back to carbonic anhydride and water. 



The new carbon compounds made in and by the protoplasm of 

 the green plants are some of the so-called " organic compounds," 



