THE ORGANIC COMPONENTS OF PLANTS 17 



particular species. Hence, it does not seem necessary, or desir- 

 able, in this study of the chemistry of plant growth, to present 

 as detailed a consideration of the differences in metabolic activity 

 of the different types of plants as complete accuracy of statement 

 in all cases might demand. We will, instead, discuss the organic 

 chemical components of plant tissues and the reactions which 

 they undergo, using the more common type of autotrophic plants 

 as the illustrative material in most cases. 



Hence, it will be understood that in all the following dis- 

 cussions of plant activities, except where specific exceptions are 

 definitely mentioned, it is the green, or autotrophic, plants to 

 which reference is made in each case. 



From the standpoint of the sum total of its activities, a green 

 plant is essentially an absorber of solar energy and a synthetizer 

 of organic substances. Each individual autotrophic plant takes 

 up certain amounts of the anergic foods which are discussed 

 in the preceding chapter and manufactures from them a great 

 variety of complex organic compounds, using the energy of the 

 sun's rays, absorbed by chlorophyll, as the source for the energy 

 necessary to accomplish these synthetic reactions. The ultimate 

 object of these processes is to produce seeds, each containing an 

 embryo and a sufficient supply of food for the young plant of the 

 next generation to use until it has developed its own synthetic 

 organs; or (in the case of perennials) to store up reserve food 

 materials with which to start off new growth after a period of 

 rest and often of defoliation. To be sure, animals and men 

 often interfere with the completion of the life cycle of the plant, 

 and utilize the seeds or stored food material for their own nutri- 

 tion, but this is a biological relation which has no influence upon 

 the nature of the plant's own activities. 



Since all of these synthetic reactions must go on at ordinary 

 temperatures, active catalyzers are necessary. These the plant 

 provides in the form of enzymes (see Chapter XIV) which are 

 always present in active plant protoplasm. Proper conditions 

 for rapid chemical action are further assured by the colloidal 

 nature (see Chapter XV) of the protoplasm itself. 



TYPES OF CHEMICAL CHANGES INVOLVED IN PLANT GROWTH 



The whole cycle of chemical changes which is involved in plant 

 growth represents the net result of two opposite processes; the 



