THE ROLE OF GREEN PLANTS 



265 



mint, wintergreen, menthol, eucalyptus, camphor, and the like ; va- 

 rious alkaloids ; poisonous substances such as nicotine and strych- 

 nine ; acids such as mahc, citric, and tartaric. Plant protoplasm, in 

 addition, as we have seen, manufactures many characteristic enzymes 

 and produces pigments like the 

 chlorophylls and carotins al- 

 ready mentioned. The carotin 

 present in green grass fed to 

 dairy cows gives the deeper 

 color so much desired in cream 

 and butter. Another interest- 

 ing substance found in carotin 

 is a precursor of Vitamin A 

 which exists in plant bodies as 

 a form of carotin and is prob- 

 ably transformed by the liver of 

 animals into Vitamin A. This 

 is another example showing how 

 closely the lives of plants and 

 animals are interwoven. (See 

 pages 277-279.) 



00 



A diagram of the outer portion of a 

 cross section of a wheat grain showing the 

 various layers of tissues : h, the different 

 integuments of the ovary and seed which 

 make up the husk ; o, the cells of " tileu- 

 rone layer" of the endosperm, which are 

 loaded with protein grains : and b, the layer 

 of starch-bearing cells. (After Cobb.) 



Respiration 



Respiration is essentially the 

 same process in plants as in 

 animals. In its simplest terms it is the release of potential energy 

 from foods by means of the process of oxidation, whereby oxygen 

 is used and carbon dioxide is given ofT. Glucose is perhaps the 

 chief fuel of the plant body, although fats also serve this purpose. 

 The latter are probal^ly changed to sugar before actually being 

 utilized in the respiratory process. 



In order to have respiration take place, there must be an exchange 

 of gases through a selectively permeable membrane. This means that 

 there will be an exchange of oxygen and carbon dioxide in the cells 

 where the oxidative process is taking place. Sin.ce respiration occurs in 

 all living cells and since there is a greater volume of carbon dioxide and 

 oxygen in parts of plants that are growing rapidly, it is obvious that 

 growing roots must have a supi)ly of oxygen. This is a reason for the 

 loosening of soil particles around plants in cultivation to allow air to 

 have access to the root hairs. The actual oxidative |)rocess is con- 



