The Principles of Biology, Volume 2 (of 2)

by Herbert Spencer

Figs. 202–204.

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 the 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 themselves. 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;[32] but as we descend we find them beginning to assume a bilateral distribution; and on the lower, horizontally-growing branches, their distribution is quite bilateral.[33] Between the Irish and English kinds of Yew, there is a contrast 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 are long and slender, and where, consequently, each leaf, according to its weight, the flexibility and twist of its foot-stalk, and the direction of the branch it grows from, falls into some indefinite attitude, the relations are obscured. But where the foot-stalks are stiff, as in the Laurel, it will be found, as before, that from the topmost and upward growing branches the leaves diverge on all sides; while the undermost branches, growing out from the shade of those above, have their leaves so turned as to bring them into rows horizontally spread out on the two sides of each branch.

Fig. 205.

A kindred truth, having like implications, comes into view when we observe the relative sizes of leaves on the same branch, where their sizes differ. Fig. [205] represents a branch of a Horse-chestnut, taken from the lowermost fringe of the tree, where the light has been to a great extent intercepted from all but the most protruded parts. Beyond the fact that the leaves become by appropriate growths of their foot-stalks bilaterally distributed on this drooping branch, instead of being distributed symmetrically all round, as on one of the ascending shoots, we have here to note the fact that there is unequal development on the upper and lower sides. Each of the compound leaves acquires a foot-stalk and leaflets that are large in proportion to the supply of light; and hence, as we descend towards the bottom of the tree, the clusters of leaves display increasing contrasts. How marked these contrasts become will be seen on comparing a and b, which form one pair of leaves that are normally equal, or c and d, which form another pair normally equal.

Let us not omit to note, while we have this case before us, the proof it affords that these differences of development are in a considerable degree determined by the different conditions of the parts after they have been unfolded. Though those inequalities of dimensions whence the differentiations of form result, may be in many cases largely due to the inequalities in the circumstances of the parts while in the bud (which are, however, representative of inequalities in ancestral circumstances); yet these are clearly not the sole causes of the unlikenesses which eventually arise. This bilateralness resulting from the unequal sizes of the leaves, must be considered as due to the differential actions that come into play after the leaves have assumed their typical structures.

§ 225. How, in the arrangement of their twigs and leaves, branches tend to lapse from forms that are approximately symmetrical to forms that are quite asymmetrical, need not be demonstrated: it is sufficiently conspicuous. But it may be well to point out how the tendency to do this further enforces our argument. The comparatively regular budding out of secondary axes and tertiary axes, does not usually produce an aggregate which maintains its regularity, for the simple reason that many of the axes abort. Terminal buds are some of them destroyed by birds; others are burrowed into by insects; others are nipped by frost; others are broken off or injured during gales of wind. The environment of each branch and its branchlets is thus ever being varied on all sides: here, space being left vacant by the death of some shoot that would ordinarily have occupied it; and there, space being trenched on by the lateral growth of some adjacent branch that has had its main axis broken. Hence the asymmetry, or heterogeneity of form, assumed by the branch, is caused by the asymmetrical distribution of incident forces—a result and a cause which go on ever complicating.