EVOLUTION OF PLANTS 395 



growth, and little or no integration of the body as a 

 whole, though they are true animals, living exclusively 

 on organic food. 



The powers of rapid locomotion and extreme sensi- 

 tiveness to varied stimuli, together with the compactness 

 and high integration of the body, are thus seen to be 

 not necessarily associated with the fundamental animal 

 character of living on solid organic food, but to be 

 merely a possible development of animal organisation. 



The early organisms which had the power of living 

 on liquid foods, whether simple mineral salts and 

 carbon dioxide dissolved in water, or more complex 

 solutes such as sugars and nitrogenous substances 

 derived from other organisms, were not forced to retain 

 or develop the means of ingesting solid food, and it 

 is these forms which have given rise to the class of 

 organisms we call plants. Those which could intercept 

 light energy by means of a pigment such as chlorophyll 

 were able to use the simplest forms of raw food material, 

 carbon dioxide and simple salts, and it is these forms 

 which have given rise to the green plants — the main 

 line of plant evolution. 



The green plants, as the result of their greenness, 

 have the unique power of building up protoplasm from 

 simple inorganic constituents — they can assimilate 

 carbon and nitrogen from carbon dioxide and nitrates. 

 These they find everywhere around them, the latter 

 in water and soil, the former in the air as well. Since 

 they are able to find their food wherever there is enough 

 water to dissolve it and to maintain their protoplasmic 

 structure, they are the only possible forerunners of 

 the extension of life upon the earth, living where other 

 organisms cannot live, and themselves providing organic 

 food for animals and for colourless plants. 



