136 MORPHOIJGICAL DEVELOPMENT. 



the lower ends of trees differ from tlieir upper ends. Tliis 

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

 significance obscared 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 phasnogamic axis is made up of homologous 

 segments, 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 pro- 

 geny as these increase. Hence the progressively-increasing 

 contrasts. If the trunk of a tree were sawn horizontally 

 into a series of slabs, each some two inches thick or there- 

 abouts ; if each of the main branches were similarly divided 

 transversely, and the like were done with all the branches 

 borne by it, down to their ultimate twigs, which would be se- 

 verally cut across at each internode ; then, morphologically 

 considered, any one of these slabs would be the homologue 

 of any internode of an ultimate twig, wath its leaf and axil- 

 lary bud. In the immense contrast between these oldest 

 and youngest units of composition, we should have exhibited 

 the cumulative result of continuous differentiation caused by 

 continuous action of modifjdng forces — the one unit ha^Tng 

 been originally just like the other. 



§ 227. Thus, then, it is with the proximate pai ts of plants as 

 it is with plants as wholes. The radial sjTnmetry, the bilateral 

 sjTnmetry, and the as^Tumetry, which branches display in 

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

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

 quent 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 to the 

 environment are unequal, serves to explain all the leading 

 traits of structure. 



