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and to consist of a highly excitable mass of cellular substance, originating in the pith, and having a special power of extension in length. Under ordinary circumstances, the growing point clothes itself with leaves as it advances, and then it becomes a branch ; but sometimes it simply hardens as it grows, and forms a sharp conical projection called a spine, as in the Gleditschia, the Sloe, &c.
The spine must not be confounded with the prickle or aculeus already described, from which it differs in having a considerable quantity of woody tissue in its structure, and in being as much in communication with the central parts of a stem as branches themselves; while prickles are merely superficial concretions of hardened cellular tissue. Spines occasionally, as in the Whitethorn, bear leaves; in domesticated plants they often entirely disappear, as in the Apple and Pear, the wild varieties of which are spiny, and the cultivated ones spineless.
We ought to consider the spadix of the Arum, and several forms of disk hereafter to be described, as modifications of the growing point of the bud, and consequently as analogous to spines.
Linnæus called the bud Hybernaculum, because it serves for the winter protection of the young and tender parts; and distinguished it into the Gemma, or leaf-bud of the stem, and the Bulb, or leaf-bud of the root.
The leaf-bud has been compared by Du Petit Thouars and some other botanists to the embryo, and has even been denominated a fixed embryo. This comparison must not, however, be understood to indicate any positive identity between these two parts in structure, but merely an analogous function, both being formed for the purpose of reproduction; but in origin and structure they are entirely different. The leaf-bud consists of both vascular and cellular tissue, the embryo of cellular tissue only: the leaf-bud is produced without fertilization, to the embryo this is essential : finally, the leaf-bud perpetuates the individual, the embryo continues the species.
The usual, or normal, situation of leaf-buds is in the axil of leaves; and all departure from this position is either irregular or accidental. Botanists give them the name of regular when they are placed in their normal station, and they call all others latent or adventitious. The latter have been found in almost every part of plants; the roots, the internodes, the petiole, the leaf itself, have all been remarked producing them. On the leaf they usually proceed from the margin, as in Malaxis paludosa, where they form minute granulations, first determined to be buds by Henslow, or as in Bryophyllum calycinum and Tellima grandiflora; but they have been seen by Turpin (fig. 19.) proceeding from the surface of the leaf of Ornithogalum. (Fig. 20. represents a vertical section of one of these buds.)
We are unacquainted with the cause of the formation of leaf-buds; all we know is, that they proceed exclusively from cellular tissue; and if produced on the stem, from the mouths of medullary rays. It would seem as if certain unknown forces were occasionally so exerted upon a vesicle of cellular tissue as to stimulate it into a preternatural degree of activity, the result of which is the production of vessels, and the formation of a nucleus having the power of lengthening. There is, indeed, an opinion, which I believe is that of Mr. Knight, that the sap itself can at any time generate buds without any previously formed rudiment; and that buds depend, not upon a specific alteration of the arrangement of the vascular system, called into action by particular circumstances, but upon a state of the sap favourable to their creation. In proof of this it has been said, that if a bud of the Prunus Pseudo-cerasus, or Chinese Cherry, be inserted upon a cherry stock, it will grow freely, and after a time will emit small roots from just above its union with the stock; at the time when these little roots are formed, let the shoot be cut back to within a short distance of the stock, and the little roots will then, in consequence of the great impulsion of sap into them, become branches emitting leaves.
The leaf-buds of the deciduous trees of cold climates are covered by scales, which are also called tegmenta ; these afford protection against cold and external accidents, and vary much in texture, thickness, and other characters. Thus, in the Beech, the scales are thin, smooth, and dry; in
Willows they are covered with a thick down; in Populus balsamifera they exude a tenacious viscid juice. In herbaceous plants and trees of climates in which vegetation is not exposed to severe cold, the leaf-buds have no scales; which is also, but very rarely, the case in some northern shrubs, as Rhamnus Frangula.
The scales of the bud, however dissimilar they may be to leaves in their ordinary appearance, are nevertheless, in reality, leaves in an imperfectly formed state. They are the last leaves of the season, developed at a period when the current of vegetation is stopping, and when the vital powers have become almost torpid. That such is really their nature is apparent from the gradual transition from scales to perfect leaves that occurs in such plants as Viburnum prunifolium, Magnolia acuminata, Liriodendron Tulipifera, and Æsculus Pavia : in the latter the transition is, perhaps, most satisfactorily manifested. In this plant the scales on the outside are short, hard, dry, and brown; those next them are longer, greenish, and delicate; within these they become dilated, are slightly coloured pink, and occasionally bear a few imperfect leaflets at their apex; next to them are developed leaves of the ordinary character, except that their petiole is dilated and membranous like the inner scales of the bud; and, finally, perfectly formed leaves complete the series of transitions.
Among the varieties of root is sometimes classed what botanists call a bulb ; a scaly body, formed at or beneath the surface of the ground, emitting roots from its base, and producing a stem from its centre. Linnæus considered it the
leaf-bud of a root; but in this he was partly mistaken, roots being essentially characterised by the absence of buds. He was, however, perfectly correct in identifying it with a leafbud. A bulb has the power of propagating itself by developing in the axils of its scales new bulbs, or what gardeners call cloves, (Nucleus and Adnascens of the older botanists; Adnatum of Richard ;) which grow at the expense of their parent bulb, and eventually destroy it. Every true bulb is, therefore, necessarily formed of imbricated scales, and a solid bulb has no existence. The bulbi solidi, as they have been called, of the Crocus, the Colchicum, and others are, as we shall hereafter see (see Cormus), a kind of subterranean stem: they are distinct from the bulb in being, not an imbricated scaly bulb, but a solid fleshy stem, itself emitting buds. It has been supposed that they were buds, the scales of which had become consolidated; but this hypothesis leads to this very inadmissible conclusion, — that as the cormus or solid bulb of a Crocus is essentially the same, except in size and situation, as the stem of a Palm, the stem of a Palm must be a solid bulb also, which is absurd. In truth, the bulb is analogous to the bud that is seated upon the cormus, and not to the cormus itself; a bulb being an enlarged subterranean bud without a stem, the cormus a subterranean stem with buds on its surface.
Of the bulb, properly so called, there are two kinds.
are thin and membranous, and cohere in the form of a distinct covering, as in the onion; and, 2. the naked bulb (Bulbus squamosus) (fig. 22, 23.), in which the outer scales are not membranous and united, but distinct and fleshy like the inner scales, as in Lilium. The outer covering of a bulb of the first kind is called the tunic.
Besides the bulbs properly so called, there are certain leaf-buds, developed upon stems in the air, and separating spontaneously from the part that bears them, which are altogether of the nature of bulbs. Such are found in Lilium tigrinum, some Alliums, &c. They have been called bulbilli, propagines, bacilli, &c. Care must be taken not to follow some botanists, in confounding with them the seeds of certain Amaryllidaceæ, which have a fleshy coat; but which, with a vague external resemblance to bulbs, have in every respect the structure of genuine seeds.
The tegmenta, or scales of the bud, have received the following names, according to the part of the leaf of which they appear to be a transformation; such terms, are, however, but seldom employed :
1. foliacea, when they are abortive leaves, as in Daphne Mezereum.
2. petiolacea, when they are formed by the persistent base of the petiole, as in Juglans regia.
3. stipulacea, when they arise from the union of stipules, which roll together and envelope the young shoot, as in Carpinus, Ostrya, Magnolia, &c.
4. fulcracea, when they are formed of petioles and stipules combined, as in Prunus domestica, &c.—(Rich. Nouv. Elem. 134. ed. 3.)
The manner in which the young leaves are arranged within the leaf-bud is called foliation, or vernation. The names applied to the various modifications of this will be explained in Glossology; they are of great practical importance both for distinguishing species, genera, and even natural orders; but have, nevertheless, received very little general attention. The vernation of Prunus Cerasus is conduplicate ; of Prunus domestica, convolute ; of Ferns and Cycadaceæ, circinate, and so on.