148 MORPHOLOGICAL DEVELOPMENT. 



decided bilateralness. A full-grown Araucaria, too, exhibits 

 in its lower branches modifications similarly caused; and in 

 each of such branches there may be remarked the further 

 fact, that its upward-bending termination has a partially- 

 modified radialness, at the same time that its drooping lateral 

 branchlets give to the part nearer the trunk a completely 

 bilateral character. 



Now in these few instances, typical of countless instances 

 which 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 th 

 distribution of leaves. How a branch carries its leaves 

 constitutes one of its characters as a branch, and is to be 

 considered apart from the characters of the leaves them- 

 selves. The principles hitherto illustrated we shall here find 

 illustrated 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, horizon- 

 tally-growing branches, their distribution is quite bilateral, f 

 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 



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

 and the whorled structures. These, as being alike on all sides, are similarly 

 distinguished from arrangements that are alike on two sides only. 



f It should be added that this change of distribution is not due to 

 change in the relative positions of the insertions of the leaves but to their 

 twistings. 



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