223 CHAPTER IX SUMMARY THE BUD THE bud is a young shoot-that is, a stem or branch with its rudimentary leaves closely packed together. It is normally situated either at the summit of the stem or in the axil of a leaf. The young leaves require protection against too great cold, heat, drought, or moisture, too vivid light, and the attacks of animals and plants. This is effected in various ways. The scales which protect the winter-buds fall into at least eight categories: 1. Pedestals of last year's leaves, as in Pyrus Aria. 2. Stipules of last year's leaves, as in Magnolia. 3. In many cases the young leaves are protected by modified leaves, as, for instance, in Willows and Lilac. 4. In others the protection is afforded by leaf-stalks, the leaf not being developed, as in Maples and the Sycamore; 5. In others by the leaf-base, as in the Rose, Portugal Laurel, &c.; 6. In others, again, by stipules, as in the Beech, Oak, Elm, Poplar, &c. ; 7. In others by pairs of stipules, as in the Elm, Spanish Chestnut, &c.; 8. Or by two connate stipules, not, however, belonging to the same leaf, as in the Hop. In many species the protection thus afforded is further enhanced (1) by the presence of a gummy or resinous secretion, as in the Horse Chestnut, Plane, Alder, Conifers; or (2) by furry hairs, as in the Plane, Horse Chestnut, Vine. STIPULES Stipules are normally more or less leaf-like structures at the base of, or just below, and one on each side of, the leaf-stalk. They are very variable in size and appear ance. A typical leaf may either be considered to consist of four parts: 1. The leaf base. 2. A pair of stipules. 3. The leaf-stalk or petiole. 4. The leaf-blade, which for shortness is often called the leaf. Or, and perhaps more philosophically, of two parts: 1. A leaf-base, which may or may not develop stipules. 2. A petiole, the upper part of which normally expands into a leaf-blade. In some cases, as in the Rose, all four are developed; in others one, two, or even three of these constituents may remain undeveloped. As a general rule stipules serve to protect the bud, and in most cases shelter younger leaves only; but in some, especially in quickly growing plants, as, for instance, the Pea, Hop, Vine, &c., they envelop the petiole and blade of their own leaf. In some cases the stipules of the last leaf of a year protect the young leaves of the following season, as, for instance, in some Magnolias, Petteria, Poplar. In most cases, however, these stipules drop off, and the outer, covering scales are an integral part of the bud. Where there are stipules, the outer leaves often develop no leaf-blades (Oak, Beech, &c.). When stipules serve, and serve only, to protect the bud, they are generally caducous, and drop off soon after the leaves have expanded. In the Oak, Beech, Hornbeam, &c., each of these outer bud-scales is a single stipule. In some cases, as in the Elm, Spanish Chestnut, &c., each consists of the two connate stipules belonging to the same leaf. In others, as in the Hop, each scale consists of the two stipules on one side of the shoot. The two halves, therefore, belong to different leaves. The general function of stipules being to protect the younger leaves, their presence or absence depends in great measure on the need for such protection. Hence we may almost lay it down as a general rule that where the leaf-stalk is broad and covers the bud there are no stipules, and that where it is too narrow to protect the bud stipules are present. The protection of the bud is, indeed, the general function of stipules, though by no means the only one. We may summarise the uses of stipules as follows: 1. To protect the bud. 2. To help in performing, or even, as in Lathyrus Aphaca, to perform, the function of leaves. 3. Sometimes they are hardened into spines, and thus serve to protect the whole plant, as in Robinia (so-called Acacia). 4. Sometimes they form hooks, and assist the plant in climbing (Paliurus). 5. Sometimes they are developed into tendrils, as in Smilax. 6. Sometimes they secrete gum, which helps to protect the bud; 7. Or a sweet secretion, which attracts wasps, &c., as in some species of Viburnum. 8. Sometimes they serve to hold water, as, for instance, in some species of Viola, Thalictrum and Polygonum. 9. Sometimes they act as reserves of nourishment, as in Gunnera. 10. Sometimes they serve for a support to the stem, as in some species of Polygonum. The view here suggested seems to apply well both to the cases where the stipules are very short-lived, and also to those in which they are very persistent. When they serve, and serve only, to protect the leaves to which they themselves belong, they often fall off when the leaves themselves expand. On the other hand, as a general rule, they protect the following leaf or leaves, as, for instance, in Magnolia, Liriodendron, and other Magnoliaceae. When the stipules of the terminal leaves of one year protect the next leaves, which do not emerge till the following year, they are much more persistent than the leaves themselves. Both cases sometimes occur in the same family. This, then, is the answer I should give to M. Vaucher's question (see p. 20), and the presence or absence of stipules is not determined, I think, as suggested by De Candolle, by any question of general symmetry, but rather by practical considerations connected with the wants and requirements of the plants. No doubt, also, there are some cases in which stipules have ceased to be of any use to the plants, and are merely the persistent rudiments of organs which performed a useful purpose to the ancestors of the existing |