3. Enlargement, or systematic change in arrangement, of a typical form, as in the growth of the members of an animal. 285. The growth of trees came under the second of these heads. A tree did not increase in stem or boughs as the wrist and hand of a child increased to the wrist and hand of a man; but it was built up by additions of similar parts, as a city is increased by the building of new rows of houses. Any annual shoot was most conveniently to be considered as a single rod, which would always grow vertically if possible. Every such rod or pillar was, in common timber trees, typically either polygonal in section, or rectangular. If polygonal, the leaves were arranged on it in a spiral order, as in the elm or oak. If rectangular, the leaves were arranged on it in pairs, set alternately at right angles to each other. Intermediate forms connected each of these types with those of monocotyledonous trees. The structure of the _arbor vitae_ might be considered as typically representing the link between the rectangular structure and that of monocotyledons; and that of the pine between the polygonal structure and that of monocotyledons. Every leaf during its vitality secreting carbon from the atmosphere, with the elements of water, formed a certain quantity of woody tissue, which extended down the outside of the tree to the ground, and farther to the extremities of the roots. The mode in which this descending masonry was added appeared to depend on the peculiar functions of cambium, and (the speaker believed) was as yet unexplained by botanists. 286. Every leaf, besides forming this masonry all down the tree, protected a bud at the base of its own stalk. From this bud, unless rendered abortive, a new shoot would spring next year. Now, supposing that out of the leaf-buds on each shoot of a pentagonal tree, only five at its extremity or on its side were permitted to develop themselves, even under this limitation the number of shoots developed from a single one in the seventh year would be 78,125. The external form of a healthily grown tree at any period of its development was therefore composed of a mass of sprays, whose vitality was approximately distributed over the _surface_ of the tree to an equal depth. The branches beneath at once supported, and were fed by, this orbicular field, or animated external garment of vegetation, from every several leaf of which, as from an innumerable multitude of small green fountains, the streams of woody fiber descended, met, and united as rivers do, and gathered their full flood into the strength of the stem.