First: In a great number of trees, the vertical trunk never ceases to be built. It is confused, at the top of the tree, among other radiating branches, being at first, of course, just as slender as they, and only prevailing over them in time. It shows at the top the same degree of irregularity and undulation as a sapling; and is transformed gradually into straightness lower down (see Fig. 50). The reader has only to take an hour's ramble, to see for himself how many trees are thus constructed, if circumstances are favorable to their growth. Again, the mystery of blossoming has great influence in increasing the tendency to dispersion among the upper boughs: but this part of vegetative structure I cannot enter into; it is too subtle, and has, besides, no absolute bearing on our subject; the principal conditions which produce the varied play of branches being purely mechanical. The point at which they show a determined tendency to spread is generally to be conceived as a place of _rest_ for the tree, where it has reached the height from the ground at which ground-mist, imperfect circulation of air, &c., have ceased to operate injuriously on it, and where it has free room, and air, and light for its growth. [Illustration: FIG. 50.] § 12. I find there is quite an infinite interest in watching the different ways in which trees part their sprays at this resting-place, and the sometimes abrupt, sometimes gentle and undiscoverable, severing of the upright stem into the wandering and wilful branches; but a volume, instead of a chapter or two, and quite a little gallery of plates, would be needed to illustrate the various grace of this division, associated as it is with an exquisitely subtle effacing of undulation in the thicker stems, by the flowing down of the wood from above; the curves which are too violent in the branches being filled up, so that what was at _a_, Fig. 50, becomes as at _b_, and when the main stem is old, passes at last into straightness by almost imperceptible curves, a continually gradated emphasis of curvature being carried to the branch extremities. [Illustration: FIG. 51.]