132 MORPHOLOGICAL DEVELOPMENT. 



arranged with tolerable regularity all round, in corre- 

 spondence witli its equal exposure on all sides, each branch- 

 let shows by its curve as much bilateral symmetry as 

 its simple form permits. The branch b, dissimilarly 

 circumstanced on the side next the main stem and on 

 the side away from it, has an approximate bilaterahiesa 

 as a whole, while the bilaterabiess of its branchlets varies 

 with their respective positions. And in the branch c, having 

 its parts still more differently conditioned, these traits ol 

 structure are still more marked. Extremely strong contrasts 

 of this kind occur in trees having very regular modes ot 

 growth. The uppermost branches of a Spruce-fir have radially 

 arranged branchlets : each of them, if growing vigorously, 

 repeats the type of the leading shoot, as shown in Fig. 203, 

 a, b. But if we examine branches lower and lower down the 

 tree, we find the vertically-growing branchlets bear a less and 

 less ratio to the horizontally-growing ones ; until, towards the 

 bottom, the radial arrangement has wholly merged into the 

 bilateral. Shaded and confined by the branches above them, 

 these eldest branches develop their ofishoots in those direc- 

 tions where there is most space and light : becoming finally 

 quite flattened and fan-shaped, as shown at Fig 2o3, c. And 

 on remembering that each of these eldest branches, when first 

 it diverged from the main stem, was radial, we see not only 

 that between the upper and lower branches does this contrast 

 in structure hold, but also that each branch is transformed 

 from the radial to the bilateral by the progressive change in 

 its environment. Other forces besides those which aid 



or hinder growth, conspire to produce this two-sided character 

 »n lateral branches. The annexed Fig. 204, sketched from 

 an example of the Pinus CoiUterii at Kew, shows very clearly 

 how, by mere gravitation, the once radiaUy-arranged branch- 

 lets may be so bent as to produce in the branch as a whole a 

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

 in its lower branches modifications similarly caused ; and in 

 each of such branches there maybe remarked the further fact, 



