195. It is but a step from the study of the bulb to that of the leaf-bud. Buds are of two kinds in respect to their contents; the leaf-bud containing the rudiments of a leafy stem or branch, the flower-bud containing the same elements transformed into the nascent organs of a flower for the purpose of reproduction.
196. The leaF-bud consists of a brief, cone-shaped axis with a tender growing point, bearing a protecting covering of imbricated scales and incipient leaves.
65. Branch of pear tree. The terminal bud a, having been destroyed, an axillary bud supplied its place, and formed the axis b. c, Thickened branch with flower-buds, d, branch with leaf-buds. 66. t, section of terminal bud; I, of axillary bud.
197. Nature of the scales. The scaly envelops of the bud appear to be either the rudimentary leaves or stipules of the preceding year, formed late in the season, arrested in their development by the frosts and scanty nourishment, and reduced to a sear and hardened state. If the bud of the rose, tulip-tree, or horse-chestnut be examined when swollen in the spring, the student will notice a gradual transition from the outer scales to the evident leaves or stipules within.
67, Bud of currant unfolding, - the scales gradually becoming leaves. 68, Bud of tulip-tree, - the scales unfolding into stipules.
198. It is an interesting illustration of designing Wisdom that buds are furnished with scales only in wintry climates. In the Torrid Zone, or in conservatories, where the temperature is equalized through the year, plants develop their foliage into buds immediately after formation, without clothing them in scales. In annual plants also, the buds are destitute of scales, not being destined to survive the winter. Hence it is evident that the transformation of autumnal leaves into scales, is a means ordained by the great Author of Nature to protect the young shoots in their incipient stages from sudden cold and moisture, - an office which they effectually fulfil by their numerous downy folds and their insoluble coat of resin.
199. How buds are protected. In many trees the bud-scales are clothed with dense, downy hairs. In others, as in the horse-chestnut, balm of Gilead, and other species of poplar, the buds are covered with a viscid, aromatic resin, resembling a coat of varnish. A considerable quantity may be separated from a handful of such buds in boiling water.
200. The parent bud. In regard to position, buds are either terminal or axillary - a distinction already noticed. The plumule of the embryo is the original parent bud, containing within its minute organization the manifold parts of the future plant - stem, leaves, flower, fruit - all to be successively unfolded in future months or years. The unfolding of this first terminal bud in the one direction of its point produces the simple stem.
201. Origin of branches. But in every plant a special provision is made for the development of branches. It is a general law that every expanding leaf shall subtend an infant bud in its axil, that is, in the upper angle of the insertion of the leaf-stalk; hence the plant may always have as many axillary buds as it has leaves.
202. Axillary buds are especially noted as being either active or latent. In the former case they are unfolded into branches at once, or in the spring following their formation. But latent buds suspend their activities from year to year, or perhaps are never quickened into growth.
203. Axillary buds become terminal so soon as their development fairly commences, therefore each branch also has a terminal bud, and, like the main axis, is capable of extending its growth as long as that bud remains unharmed. If it be destroyed by violence or frost, or should it be transformed into a flower-bud, the growth in that direction forever ceases.
204. The suppression of axillary buds tends, of course, to simplify the form of the plant. Their total suppression during the first year's growth of the terminal bud is common, as in the annual stem of mullein and in most perennial stems. When axillary buds remain permanently latent, and only the terminal bud unfolds year after year, a simple, branchless trunk, crowned with a solitary tuft of leaves, is the result, as in the palmetto of our southern borders.
205. A PARTIAL SUPPRESSION OF buds occurs in almost all species, and generally in some definite order. In plants with opposite leaves, sometimes one bud of the pair at each node is developed and the other is suppressed, as in the pink tribe (Caryophyllaceae). When both buds are developed, the branches, appearing in pairs like arms, are said to be brachiate, as in the Labiatae. In many trees the terminal buds are arrested by inflorescence each season, and the growth is continued by axillary buds alone, as in the Catalpa and horse-chestnut. In all trees, indeed, buds are suppressed more or less, from various causes, disguising at length the intended symmetry of the branches, to the utter confusion of twigs and spray.
206. Accessory buds, one or more, are sometimes found just above the true axillary bud, or clustered with it, and only distinguished from it by their smaller size: as in the cherry and honeysuckle.
69, Hypericum Sarothra, with brachiate branches. 70, Pink (Dianthus) - axillary buds alternately suppressed.
207. Adventitious or accidental buds are such as are neither terminal nor axillary. They occasionally appear on any part of the plant in the internodes of the stem or branches, on the root, or even leaves.
208. Causes and examples. Such buds generally result from some abnormal condition of the plant, from pruning or other destruction of branches or stem above, while the roots remain in full vigor; thus destroying the equilibrium of vital force between the upper and lower axis. The leaf of the walking-fern emits rootlets and buds at its apex; the leaf of Bryophyllum from its margin, each bud here also preceded by a rootlet. Some plants are thus artificially propagated in conservatories from the influence of heat and moisture on a leaf or the fragment of a leaf.
209. Vernation or praefoliation are terms denoting the mode of arrangement and folding of the leaf-organs composing the bud. This arrangement is definitely varied in different orders of plants, furnishing useful distinctions in systematic botany.
210. The vernation of the bud is exhibited in an interesting manner by making with a keen instrument a cross-section of it in its swollen state, just before expansion; or it may be well observed by removing one by one the scales.
211. The forms of vernation are entirely analogous to those of aestivation, and denoted by similar terms. We shall here notice only such as are more peculiar to the leaf-buds.
212. Vernation is considered in two different aspects, first, the manner in which the leaf itself is folded; second, the arrangement o; the leaves in respect to each other. This depends much upon the phyllotaxy. (§ 220.)
Vernation, 71, of oak leaf; 72, of Liriodendron (tulip tree); 73, of fern; 74, of carex;
75, sage; 76, iris.
213. Each leaf alone considered is either flat and open, as in the mistletoe, or it is folded or rolled, as follows:
Reclined, when folded crosswise with apex bent over forward towards the base as in the tulip-tree.
Conduplicate, when folded perpendicularly, with the lateral halves brought together, face to face, as in the oak.
Plaited or plicate, each leaf folded like a fan; vine, birch.
Circinate, when each leaf is rolled or coiled downwards from the apex, as in the sun-dew and ferns.
Convolute, the leaf wholly rolled up from one of its sides, as in the cherry. Involute, having both edges rolled inwards as in apple, violet Revolute, with both margins rolled outwards and backwards, as in the dock, willow, rosemary.
Vernation. 77, of birch leaf; 7S, of lilac, (imbricate); 79, cherry leaves, (convolute); 80, dock bud, (revolute); 81, balm of Gilead, (involute).
214. The general vernation is loosely distinguished in descriptive botany as valvate (edges meeting), and imbricate (edges overlapping), terms to be noticed hereafter. The valvate more often occurs in plants with opposite leaves. Imbricated vernation is
Equitant (riding astraddle), when conduplicate leaves alternately embrace - .the outer one the next inner, by its tinfolded margins, as in the privet and iris.
82. Vernation of Sycamore bud.
Obvolute, or half-equitant, when the outer leaf embraces only one of the margins of the inner, as in the sage.
Triquetrous, where the bud is triangular in section, and the leaves equitant at each angle, as in the Carices.
215. The principle of budding. Each leaf-bud may be regarded as a distinct individual, capable of vegetating either in its native position, or when removed to another, as is extensively practiced in the important operation of budding.
216. Bulblets. In the tiger-lily, Cicuta bulbifera, and Aspidium bulbiferum, the axillary buds spontaneously detach themselves, fall to the ground, and become new plants. These remarkable little bodies are called bulblets.
83,84, Showing the process of "budding."