THE SHAPES OF BRANCHES. 133 



ihat its upward-bending termination has a partially-modified 

 radialness, at the same time that its drooping lateral branch- 

 lets give to the part nearer the tnuik a completely bilateral 

 character. 



Now in these few instances, which are typical of countless 

 instances that might be given, we see, as we saw in the case 

 of the fungi, that the same thing is true of the parts in 

 their relations to the whole and to one another, which is true 

 of the whole in its relations to the environment at large. 

 Entire trees become bilateral instead of radial, when exposed 

 to forces that are equal only on opposite sides of one plane ; 

 and in their branches, parallel changes of form occur under 

 parallel changes of conditions. 



§ 224. There remains to be said something respecting the 

 distribution of leaves. How a branch carries its leaves con- 

 stitutes one of its characters as a branch ; and is to be con- 

 sidered apart from the characters of the leaves themselves. 

 The principles hitherto illustrated we shall here find illus- 

 trated still further. 



The leading shoot and all the upper twigs of a fir-tree, 

 have their pin- shaped leaves evenly distributed all round, or 

 placed radially ;* but as we descend, we find them beginning 

 to assume a bilateral distribution ; and on the lower, hori- 

 zontally- growing branches, their distribution is quite bilateral. 

 Between the Irish and English kinds of Yew, there is a con- 

 trast of like significance. The branches of the one, shooting 

 up as they do almost vertically, are clothed with leaves 

 all round ; while those of the other, which spread laterally, 

 bear their leaves on the two sides. In trees with better- 

 developed leaves, the same principle is more or less manifest 

 in proportion as the leaves are more or less enabled by their 

 structures to maintain fixed positions. Where the foot-stalks 



* Here and throughout, the word radial is applied equally to the spiral and 

 the whorled structures. These, as heing alike on all sides, are similarly distin- 

 guished from arrangements that are alike on two sides only. 



