BUCKU. (Drossra.] BUD, or LEAF-BUD, in Vegetable Physiology, is the organised rudiment of a branch. Whatever becomes a branch is, when first organised, a bud ; but it does not therefore follow that all buds become branches ; on the contrary, owing to many disturbing causes, buds are subject to transformations and deformities which mask their real nature.
A Leaf-Bud is constructed thus :—In its centre it consists of a minute conical portion of delicate cellular tissue, and over the surface of this are arranged rudimentary leaves, in the form of scales. These scales are closely applied to each other ; those on the outside are the largest and thickest, and the most interior are the smallest and most delicate. In cold countries the external scales are often covered with hair, or a resinous varnish, or some other contrivance, which enables them to, prevent the tWeetlit of frost to the young and tender centre which they protect; but in warm countries, where such a provision is not required, they are green and smooth and much less numerous. The cellular centre of a bud is the seat of its vital activity; the scales that cover it are the parte towards the development of which its vital energies are first directed.
A Leaf-Bud usually originates in the axil of a leaf; indeed there are no leaves in the sail of which one or more buds are not found either in a rudimentary or a perfect state. Its cellular centre com municates with that of the woody centre of the stem, and its scales are in connection with the bark of the latter. When stems have the structure of Exogens, the bud terminates one of the medullary processes ; in Eodogens it is simply in communication with the cellular matter that lies between the bundles of woody tissue in such stems. It is moreover important to observe that this is true not only of what are called normal buds, that is to say, of buds which originate in tho axil of the leafy organs, but also of adventitious buds, or such as are occasionally developed in unusual situations. It would seem as if, under favourable circumstances, buds may be formed wherever the cellular tissue is present ; for they occur not only at the end of the medullary processes of the root and stem of Exogens, but on the margins of leaves, as in Bryophyllum, .3fala.ris paludosa, and many others; and occasionally on the surface of leaves, as in the case of an Ornithogalum published by Turpin, and not very uncom monly in ferns.
A Leaf•Bud has three special properties, those of growth, attraction, and propagation. In warm damp weather, under the influence of light, it has the power of increasing in size, of developing new parts, and so of growing into whatever body it may be eventually destined for. In effecting this it lengthens by the addition of new matter to its cellular extremity, and it increases in diameter partly by a lateral addition to the same kind of tissue, and partly by the deposit of woody matter emanating from the bases of the scales or leaves which clothe it. Aa soon as growth commences the sap which a bud con
tains is either expended in forming new tissue or lost by evaporation. In order to provide for such loss the bud attracts the sap from that part of the stem with which it is in communication ; that part so acted upon attracts sap in its turn from the tissue next it, and so a general movement towards the buds is established ns far as the roots, by which fresh sap is absorbed from the soil. Thus is caused the phenomenon of the flow of the sap. Every leaf-bud is in itself a complete body, consisting of a vital centre covered by nutritive organs or hairs. Although it is usually called into life while attached to its parent plant, yet it is capable of growing as a separate portion, and of producing a new individual in all respects the same as that from which it was divided ; hence it is a propagating organ as much as a seed, although not of the name kind ; and advantage has been taken of this for horticultural purposes. [BUDDING, in Ann AND Sc. DIY.] • In general a bud is developed into a branch, but that power is inter fered with or destroyed by several cruises. This must be evident from the following consideration independently of all others. Every one knows that leaves are arranged with great symmetry upon young branches; as buds are axillary to leaves, the branches they produce ought therefore to be as symmetrically arranged as leaves; and this we see does not happen. We may account for this in two or three ways : accidental injuries will doubtless destroy some ; from want of light others will never be called into action ; and of those which are originally excited to growth a part is always destroyed by the superior vigour of neighbouring buds, which attract away their food and starve them. There is moreover iu manyplants a special tendency to pro duce their leaf-buds in a stunted or altered state. In fir-trees the side buds push forth only two or a small number of leaves, and never lengthen at all; in the Cedar of Lebanon they lengthen a little, bear a cluster of leaves at their points, and resemble short spurs ; in the aloe, the whitethorn, and many other plants, they lengthen more, produce no leaves except at their very base, and grow into hard sharp-pointed spines. The knobs seen on beech and other trees, which have been called by Dntrochet embryo-buds and by Dr. Lankester abortive branches, take their origin in buds which are not normally developed. The bulbilli which are found iu the nails of many Lille mous plants originate in the bud. Bulbs are nothing but leaf-buds with unusually fleshy scales, and with the power of separating spon taneously from the mother-plant; and flower•buda are theoretically little more than leaf-buds without the power of lengthening, but with the organs that cover them in a special state. Hence flowers are modified branches. (FLowza.] Schleiden regards the changed bud; hence in his Prineiples of Scientific Botany' he calls this organ the seed-bud. [Ovut.E.]