§ 223. These few cases introduce us to the more familiar but more complex cases which plants of the third degree of aggregation present. At _a_, _b_, _c_, Fig. 202, are sketched three homologous parts of the same tree: _a_ being the leading shoot; _b_ a lateral branch near the top, and _c_ a lateral branch lower down. There is here a double exemplification. While the branch _a_, as a whole, has its branchlets arranged with tolerable regularity all round, in correspondence with its equal exposure on all sides, each branchlet 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 bilateralness as a whole, while the bilateralness of its branchlets varies with their respective positions. And in the branch _c_, having its parts still more differently conditioned, these traits of structure are still more marked. Extremely strong contrasts of this kind occur in trees having very regular modes of 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 offshoots in those directions where there is most space and light: becoming finally quite flattened and fan-shaped, as shown at Fig. 203, _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 in lateral branches. The annexed Fig. 204, sketched from an example of the _Pinus Coulterii_ at Kew, shows very clearly how, by mere gravitation, the once radially-arranged branchlets 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 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. [Illustration: Figs. 202–204.]