Cnapter 2 



THE EVOLUTION OF THE PLANT BODY 



As a result of the preceding discussion on the essential nature 

 of a plant organism, we can define a plant as an organized mass 

 of protoplasm which secures its food either by manufacturing it 

 from inorganic substances absorbed from the environment 

 (autotrophic metabolism) or by absorbing it from the surround- 

 ings (heterotrophic metabolism); in addition, like all living 

 things, a plant is capable of growth, response and reproduction. 

 Every plant known to botanists has these characteristics; but as 

 a result of millions of years of experimentation, various types of 

 plant bodies have evolved, each fitted to carry on these typical 

 plant activities under different circumstances and with varying 

 degrees of efficiency. It is our problem in this chapter to reduce 

 the great variety of body types to those which represent progres- 

 sive stages in adaptation of the plant pattern of existence to a 

 v^de range of living conditions. 



Most vital of all processes is the securing of food. In all cases 

 this is obtained from the environment by direct absorption 

 through cell walls and plasma mertibranes in marked contrast 

 to the animal method of ingestion with consequent digestion 

 and absorption into the body cells. The absorption of inorganic 

 materials from the environment and their synthesis into food is 

 a typical activity of all green plants, which therefore form the 

 useful majority of our forest and crop plants. Most important 

 of all the food-manufacturing functions of plants is the process 

 of photosynthesis. This requires an adequate supply of raw 

 materials and sunlight, at temperatures favorable to protoplas- 

 mic activity. Coincident with the necessary intake of carbon 

 dioxide and water is the need of oxygen for respiration. Also 

 fundamental in the absorptive phases of plant metabolism is 

 the intake of nitrates, sulphates, phosphates and other salts 



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