28 PROTOPLASMIC ACTION AND NERVOUS ACTION 



All organisms and all cells without exception possess 

 this power, that of transforming certain materials 

 selectively appropriated from the surroundings into 

 their own specifically organized and chemically active 

 living substance. The materials used by different 

 organisms vary widely in chemical character and accessi- 

 bility — contrast the case of a yeast cell growing in a 

 culture-medium with man in his complex social environ- 

 ment — but in every case the essential process is the 

 transformation of non-living environmental material 

 into living substance of a constant and characteristic 

 organization and activity. 



The general as well as the special features of the 

 organization of any living being are an index of the nature 

 and accessibility of the environmental materials required 

 for its maintenance. This is well illustrated by the 

 general morphological and physiological contrast between 

 animals and plants. Since in plants constructive 

 metabolism begins with simple mobile or diffusible 

 materials (CO2, water, salts), present everywhere in the 

 soil and atmosphere, there is no need for locomotion; 

 and these organisms lead typically a stationary existence, 

 remaining rooted to one spot where the necessary 

 materials can reach them by diffusion. The typical 

 radiating, branching, or dichotomous habit of growth, 

 reaching out into all directions of space and thus provid- 

 ing a large area of surface for interchange, is an '^ adapta- 

 tion" to this general environmental condition. Sessile 

 animals also tend to acquire a radiating plan of structure, 

 as illustrated in coelenterates and echinoderms. In the 

 great majority of animals, however, the food supplies 

 have to be selected from an environment containing 



