[2] I find that the office and nature of cambium, the causes of the action of the sap, and the real mode of the formation of buds, are all still under the investigation of botanists. I do not lose time in stating the doubts or probabilities which exist on these subjects. For us, the mechanical fact of the increase of thickness by every leaf's action is all that needs attention. The reader who wishes for information as accurate as the present state of science admits, may consult Lindley's _Introduction to Botany_, and an interesting little book by Dr. Alexander Harvey on _Trees and their Nature_ (Nisbet & Co., 1856), to which I owe much help. [3] In the true sense a "mediator," ([Greek: mesitês]). [4] The gradual development of this radiating structure, which is organic and essential, composed of what are called by botanists medullary rays, is still a great mystery and wonder to me. CHAPTER VII. THE STEM. § 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.