§ 8. I say nearly regular—for there are subtleties of variation in plan which it would be merely tiresome to enter into. All that we need care about is the general law, of which the oak spray furnishes a striking example,—that the buds of the first great group of alternate builders rise in a spiral order round the stem (I believe, for the most part, the spiral proceeds from right to left). And this spiral succession very frequently approximates to the pentagonal order, which it takes with great accuracy in an oak; for, merely assuming that each ascending bud places itself as far as it can easily out of the way of the one beneath, and yet not quite on the opposite side of the stem, we find the interval between the two must generally approximate to that left between 1 and 2, or 2 and 3, in Fig. 8.[2]
| Fig. 9. |
§ 9. Should the interval be consistently a little less than that which brings out the pentagonal structure, the plant seems to get at first into much difficulty. For, in such case, there is a probability of the buds falling into a triangle, as at A, Fig. 9; and then the fourth must come over the first, which would be inadmissible (we shall soon see why). Nevertheless, the plant seems to like the triangular result for its outline, and sets itself to get out of the difficulty with much ingenuity, by methods of succession, which I will examine farther in the next chapter: it being enough for us to know at present that the puzzled, but persevering, vegetable does get out of its difficulty and issues triumphantly, and with a peculiar expression of leafy exultation, in a hexagonal star, composed of two distinct triangles, normally as at B, Fig. 9. Why the buds do not like to be one above the other, we shall see in next chapter. Meantime I must shortly warn the reader of what we shall then discover, that, though we have spoken of the projections of our pentagonal tower as if they were first built to sustain each its leaf, they are themselves chiefly built by the leaf they seem to sustain. Without troubling ourselves about this yet, let us fix in our minds broadly the effective aspect of the matter, which is all we want, by a simple practical illustration.
§ 10. Take a piece of stick half-an-inch thick, and a yard or two long, and tie large knots, at any equal distances you choose, on a piece of pack-thread. Then wind the pack-thread round the stick, with any number of equidistant turns you choose, from one end to the other, and the knots will take the position of buds in the general type of alternate vegetation. By varying the number of knots and the turns of the thread, you may get the system of any tree, with the exception of one character only—viz., that since the shoot grows faster at one time than another, the buds run closer together when the growth is slow. You cannot imitate this structure by closing the coils of your string, for that would alter the positions of your knots irregularly. The intervals between the buds are, by this gradual acceleration or retardation of growth, usually varied in lovely proportions. Fig. 10 shows the elevations of the buds on five different sprays of oak; A and B being of the real size (short shoots); C, D, and E, on a reduced scale. I have not traced the cause of the apparent tendency of the buds to follow in pairs, in these longer shoots.
| Fig. 10. |
| Fig. 11. | Fig. 12. |
§ 11. Lastly: If the spiral be constructed so as to bring the buds nearly on opposite sides of the stem, though alternate in succession, the stem, most probably, will shoot a little away from each bud after throwing it off, and thus establish the oscillatory form b, Fig. 11, which, when the buds are placed, as in this case, at diminishing intervals, is very beautiful.[3]
§ 12. I fear this has been a tiresome chapter; but it is necessary to master the elementary structure, if we are to understand anything of trees; and the reader will therefore, perhaps, take patience enough to look at one or two examples of the spray structure of the second great class of builders, in which the leaves are opposite. Nearly all opposite-leaved trees grow, normally, like vegetable weathercocks run to seed, with north and south, and east and west pointers thrown off alternately one over another, as in Fig. 12.