§ 1. We must be content, in this most complex subject, to advance very slowly; and our easiest, if not our only way, will be to examine, first, the conditions under which boughs would form, supposing them all to divide in one plane, as your hand divides when you lay it flat on the table, with the fingers as wide apart as you can. And then we will deduce the laws of ramification which follow on the real structure of branches, which truly divide, not in one plane, but as your fingers separate if you hold a large round ball with them.

The reader has, I hope, a clear idea by this time of the main principle of tree-growth; namely, that the increase is by addition, or superimposition, not extension. A branch does not stretch itself out as a leech stretches its body. But it receives additions at its extremity, and proportional additions to its thickness. For although the actual living shoot, or growing point, of any year, lengthens itself gradually until it reaches its terminal bud, after that bud is formed, its length is fixed. It is thenceforth one joint of the tree, like the joint of a pillar, on which other joints of marble may be laid to elongate the pillar, but which will not itself stretch. A tree is thus truly edified, or built, like a house.