THE SHAPES OP BRANCHES. 151 



of incident forces — a result and a cause which go on ever 

 complicating. 



§ 226. One conspicuous trait in the shapes of branches 

 has still to be named. Their proximal or attached ends 

 differ from their distal or free ends, in the same way that 

 the lower ends of trees differ from their upper ends. This 

 fact, like the fact to which it is here paralleled, has had its 

 significance obscured by its extreme familiarity. But it 

 shows in a striking way how the most differently-conditioned 

 parts become the most strongly contrasted in their struc- 

 tures. A phaenogamic axis is made up of homologous seg- 

 ments, marked off from one another by the nodes; and a 

 compound branch consists of groups of such segments. The 

 earliest-formed segments, alike of the tree and of each 

 branch, serve as mechanical supports and channels for sap to 

 the successive generations of segments that grow out of 

 them; and become more and more shaded by their progeny 

 as these increase. Hence the progressively-increasing con- 

 trasts which, while mainly due to the unlikenesses of bulk 

 accompanying differences of age, are in part due to the un- 

 likenesses of structure which differences of relation to the 

 environment have caused. 



§ 227. Thus, then, it is with the proximate parts of plants 

 as it is with plants as wholes. The radial symmetry, the 

 bilateral symmetry, and the asymmetry, which branches dis- 

 play in different trees, in different parts of the same tree, and 

 at different stages of their own growths, prove to be all con- 

 sequent on the ways in which they stand towards the entire 

 plexus of surrounding actions. The principle that the 

 growths are unequal in proportion as the relations of parts 

 to the environment are unequal, serves to explain all the 

 leading traits of structure. 



