1 62 INDIVIDUALITY IN ORGANISMS 



rise to roots when this transportation falls below a 

 certain minimum or when they are isolated from it in any 

 way. This' alternative is more nearly in accord with the 

 views of most botanists, and it seems at present more 

 satisfactory than the assumption of two opposed and 

 overlapping gradients. If, however, this relation 

 between root system and other parts is transportative 

 rather than transmissive, McCallum's experiment de- 

 scribed above of bringing about physiological isolation of 

 the upper levels of the stem from the root system by 

 local anesthesia seems to indicate that the transportation 

 is not a simple physical process but is dependent in 

 some way and to some extent upon the metabolic 

 activity of living cells. 



If we accept this alternative and admit at the same 

 time the primary dominance of the stem-tip or tips and 

 the secondary dominance within the root system of the 

 root-tip or tips we must regard the root system as a sub- 

 ordinate specialized constituent individual of the com- 

 posite plant individual. The root, like the leaf, is 

 primarily determined by relations to other parts of the 

 plant, but requires certain external conditions for its 

 development and differentiation. Like the leaf also, the 

 root or root system shows a certain degree of second- 

 ary individuation among its parts. 



The formation of roots is the reaction of a plant 

 individual to a certain relation between internal and 

 external conditions, and this relation may apparently 

 be brought about either by the inhibition of activity in, 

 or absence of, the original root system, or in many cases 

 by changes in the external conditions, such as decrease 

 in light and increase in moisture, even though the 



