164 PLANT PHYSIOLOGY 



molecules of water enclosed, that the definite protoplasmic 

 structure will be strained or destroyed, and the massiye 

 movements will accordingly change or cease. 



Without going further into the subject now (see the next 

 chapter), it may be stated that the growth of the living 

 organism, like that of the crystal-, is in accordance with the 

 sensibility of its component molecules and groups of mole- 

 cules to physical and chemical influences. The difference 

 between the lifeless crystal and the living organism can be 

 suggested — not definitely stated, however — in this way : the 

 molecules of the crystal, and the crystal as a whole, are 

 entirely subject to the prevailing balance of physical forces ; 

 the living organism, on the contrary, is able to modify, 

 change, or maintain the balance of physical forces. When it 

 ceases to be able to do this it dies, it becomes like the 

 crystal. This power peculiar to living organisms, whatever 

 it may be dependent upon, does not necessarily imply any 

 greater sensitiveness to physical forces than would be repre- 

 sentable by the sum of the sensibilities of all the substances 

 composing and contained within the living organism (p. 

 185). But the organism is sensitive, and its growth cor- 

 responds to the forces or influences to which the organism 

 is subjected quite as much as to the matter with which it 

 is supplied. 



Wliat is growth? Of this no adequate definition can be 

 given, although for each mind the word possesses a certain 

 significa,nce. Physiologists have alwa^-s attempted to state 

 in what growth consists, and have always failed. Our con- 

 ception of growth should be clarified at once by distinguish- 

 ing this phenomenon from the others ordinarily accompany- 

 ing it. Growth is a part of the process of development. 

 Growth and differentiation together accomplish develop- 

 ment. Differentiation is that specialization in structure 

 which follows and contributes to specialization in function. 

 Differentiation is limited by the number of parts and of 

 cells, in other words, by the size of the body. In a small 

 body little differentiation is possible, in a large one much. 

 But it does not follow from this that the largest bodies are 

 the most highly differentiated in structure and function; 



