§ 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. [Illustration: FIG. 10.] [Illustration: FIG. 11.] § 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. [Illustration: FIG. 12.] This, I say, is the normal condition. Under certain circumstances, north and south pointers set themselves north-east and south-west; this concession being acknowledged and imitated by the east and west pointers at the next opportunity; but, for the present, let us keep to our simple form.