PLANT AND ANIMAL EVOLUTION 105 



Some Physicochemical Contrasts Between Plant 

 AND Animal Evolution 



In their evolution, while there is a continuous specialization 

 and differentiation of the modes of obtaining energy, plants 

 may not attain a higher chemical stage than that observed 

 among the bacteria and alga?, except in the parasitic forms 

 which feed both upon plant and animal compounds. In the 

 energy which they derive from the soil plants continue to be 

 closely dependent upon bacteria, because they derive their 

 nitrogen from nitrates generated by bacteria and absorbed 

 along with water by the roots. In reaching out into the air 

 and sunlight the chlorophyllic organs differentiate into the 

 marvellous variety of leaf forms, and these in turn are sup- 

 ported upon stems and branches which finally lead into the 

 creation of woody tissues and the clothing of the earth with 

 forests. Through the specialization of leaves in connection 

 with the germ-cells flowers are developed, and plan-ts establish 

 a marvellous series of balanced relations with their life environ- 

 ment, first with the developing insect life, and finally with the 

 developing bird life. 



The main lines of the ascent and classification of plants are 

 traced by palgeobotanists partly from their structural evolu- 

 tion, which is almost invariably adapted to keep their chloro- 

 phyllic organs in the sunlight^ in competition with other plants, 

 and partly from the evolution of their reproductive organs, 

 which pass through the primitive spore stage into various 

 forms of sexuality, with, finally, the development of the seed 

 habit and the dominance of the sporophyte.' It is a striking 

 peculiarity of plants that the powers of motion evolve chiefly 

 in connection with their reproductive activities, namely, with 



1 Wager, Harold, 1915, p. 468. - M. A. Howe. 



