130 MORPHOLOGICAL DEVELOPMENT. 



In plants of the third order it is thus with the proximate 

 components: they are subject to mutual influences that 

 are unlike one another and are continually changing. The 

 earlier-formed units become mechanical supporters of the 

 later-formed units, and so experience modifying forces from 

 which the later-formed units are exempt. Further, these 

 elder units simultaneously begin to serve as channels through 

 which materials are carried to and from the younger units 

 another cause of differentiation that goes on increasing in in- 

 tensity. Once more, there arise ever-strengthening contrasts 

 between the amounts of light which fall upon the youngest or 

 outermost units and the eldest or innermost units; whence 

 result structural contrasts of yet another kind. Evidently, 

 then, along with the progressive integration of cells into 

 fronds, of fronds into axes, and of axes into plants still more 

 composite, there come into play sundry causes of differen- 

 tiation which act on the whole and on each of its parts, 

 whatever their grade. The forces to be overcome, the forces 

 to be utilized, and the matters to be appropriated, do not 

 remain the same in their proportions and modes of action for 

 any two members of the aggregate: be they members of the 

 first, second, third, or any other order. 



215. Nor are these the only kinds and causes of hetero- 

 geneity which we have to consider. Beyond the more 

 general changes produced in the relative sizes and shapes of 

 plants and their parts by progressive aggregation, there are 

 the more particular changes determined by the more par- 

 ticular conditions. 



Plants as wholes assume unlike attitudes towards their 

 environments; they have many ways of articulating their 

 parts with one another; they have many ways of adjusting 

 their parts towards surrounding agencies. These are causes 

 of special differentiations additional to those general differen- 

 tiations that result from increase of mass and increase of com- 

 position. In each part considered individually, there arises 



