EXOGENS AND ENDOGENS.

43

separate them; and in Endogens they are all mixed together, in consequence of the manner of growth of those plants not requiring the same kind of arrangement of parts as is indispensable in Exogens.* This will be sufficiently illustrated by the comparison of the stems of an Oak, a Cabbage, and an Asparagus.

Tubers, the root-stock of the Iris and Ginger, what are called the roots (corms) of the Colchicum and Crocus, are all so many different forms of stem.

It is the property of a stem, during its growth, to form upon its surface, at irregularly increasing or diminishing distances, minute vital points of the same nature as that in which the stem itself originated. Each of those points becomes, or may become, a leaf-bud, capable of forming other stems or branches like that on which it appeared; and each is protected and nourished by a leaf which springs from the bark immediately below the bud. Such leaf-buds are the parts that enable a stem, when reduced to the state of a cutting, to produce a new individual like itself; and, without them, propagation by portions of the stem is, under ordinary circumstances, impossible.

Leaf-buds are capable, under fitting circumstances, of growing when separated from their mother branch, whether they are planted in the earth, or inserted below the bark of a kindred species. In the former case, they emit roots into the soil; in the latter they produce wood, which adheres to the

As this work excludes everything botanical that does not directly bear upon horticultural purposes, I have not explained the difference between Exogens and Endogens ; wishing the reader to refer for information upon such points to works upon pure botany. Nevertheless as these words are of frequent occurrence, I may as well state that they denominate the two greatest classes in the vegetable kingdom, to one or other of which almost all the flowering plants of common occurrence are referable, and that they derive their names from the peculiarity of their manner of growth. EXOGENS (literally, outside-growers) are plants whose woody matter is augmented annually by external additions below the liber; and, consequently, they are continually enclosing within their centre the woody substances formed in previous years; to such plants, a lateral communication between the centre and the circumference, by means of medullary rays, seems necessary. ENDOGENS (literally, inside-growers) are plants whose woody matter is augmented annually by internal additions to their centre; and, consequently, they are continually pushing to their circumference the woody substance formed in previous years.

44

LEAF-BUDS AND BULBS.

wood on which they may be placed. Under ordinary circumstances, leaf-buds will not form anywhere except at the axils * of leaves; but occasionally they appear from other parts, such as the root (see page 31), the spaces of the stem which lie between the leaves (the internodes), and even from the leaves themselves (see page 23). In all such cases, they are termed adventitious, because of the uncertainty of their appearance. A very remarkable state of them is the embryo-bud, a name applied to the knaurs, knurs, nodules, or hard concretions, found in the bark of various trees, which seem to have, occasionally, the power of propagating the individual, notwithstanding their deformed and indurated state.

The connection between the formation of timber and the action of buds will be considered hereafter, when speaking of the wood-forming power of leaves, which are organs resulting wholly from the development of buds.

BULBS are buds of a particular kind, larger than common, containing an unusual quantity of organizable matter, and separable, spontaneously, from the part which bears them.

B

Fig. X.-A. Bulb of Tiger Lily contrasted with B. a Leaf-bud.

They are magazines in which certain plants store up the nutritive matter assimilated by the leaves. The identity of a bulb and a bud, in all essential circumstances, is obvious, if the

* The acil is the acute angle formed by a leaf and stem, at the origin of the former ; all bodies growing within that angle are said to be axillary.

COURSE OF THE SAP.

45

bud of any tree (Fig. X. B.) is compared with the bulbs of the Tiger Lily (Fig. X. A.)

Since leaf-buds are thus the parents of wood, one of the means of propagating the individual to which they belong, the origin of branches, and consequently the source of the development of leaves themselves, they may be considered the most important organs of vegetation, so far as any one organ can be called most important where all are so mutually dependent the one on the other, and so powerfully concur in maintaining the system of vegetable life, that it is difficult to abstract one part without impairing the efficiency of the remainder.

The office of the stem is, to convey the crude fluid obtained by the roots from the soil, and called sap, into the leaves for elaboration, and then to receive it back again. Sap is, originally, water holding in solution gaseous matter, especially carbonic acid, together with certain earths and salts, but as soon as it enters the stem, it dissolves the vegetable mucilage it finds there, and becomes denser than it was before; it is further changed by the decomposition of a part of its water, acquires a saccharine character, and, rising upwards through the whole mass of wood, and more especially the alburnum, takes up any soluble matter it passes among. Its specific gravity keeps thus increasing till it reaches the summit of the branches; by degrees, it is wholly distributed among the leaves. In the leaves it is altered, and then returned into the general system, more especially into the fruit, and the bark, through which it falls, passing off horizontally through the medullary rays into the interior of the stem, and fixing itself in the interior of the bark, especially of the root, when it undergoes various changes, the results of which are known under the name of vegetable secretions.

It may be said, that, in trees, the alburnum and liber have each two equally important offices to perform the alburnum giving strength and solidity to the stem, and chiefly conveying sap upwards; the liber not only conveying sap downwards, but covering over the alburnum, protecting it from the air, and enabling it to form without interruption. The central wood is of little consequence, and may be destroyed, as it constantly is

46

COURSE OF SAP NOT PREVENTED BY WOUNDS.

in hollow trees; and the outer rind is of comparatively small importance, for it is continually perishing under the influence of the atmosphere: but liber and alburnum are the seats of vitality in trees and cannot be permanently injured without destruction to the plant.

If indeed this were absolutely the case it would be indispensable that liber and alburnum should be most carefully guarded; and so they are in nature by the thick integument of mere bark, which overlies them. But it continually happens that the usual vegetative processes are interrupted by accidents, while the power of repairing injuries is so great that many of the usual functions of a plant may be destroyed without serious injury, such functions being performed ad interim by other organs until the injury is repaired; so that although, under ordinary circumstances, the sap of exogens rises through the wood and descends through the liber, yet the simplicity of structure in plants is such, that, together with the permeability of their tissue, it enables them to propel their fluids by lateral instead of longitudinal communications. The trunk of a tree has been sawed through beyond the pith in four opposite directions; namely, from north to south, from west to east, from south to north, and from east to west, at intervals of a foot, so as completely to cut off all longitudinal communication between the upper and lower parts of the stem, as effectually as if those two parts had been dissevered; and yet the propulsion of the sap from the roots into the head of the tree, and vice versâ, went on as before: which could only have been effected by a lateral transmission of this fluid through the woody tissue. So when "ringing' is practised, and the alburnum is partially destroyed, the descending fluid diverges into the stratum of wood beneath the annulation; and when it has passed by, it again returns into its accustomed channels.

A striking example of this was given by Mr. Curtis in the Gardeners' Chronicle for 1846 (p. 597). It was the case of a Pollard Ash-tree (Fig. XI.) struck with lightning on the 7th or 8th of May, 1845, and thus deprived of the bark all round the trunk for a space of eight feet at fig. 3, and of three feet two inches at fig. 1. Nevertheless in July 1846

DECORTICATED TREES.

47

the tree was in full leaf as shown in the cut. In this case the descending sap must have either accumulated above the wound, or, which is

more probable, must have reached the lower part of the system by passing through the wood itself, till it reached the point 2; as indeed Knight showed must happen in other but less striking instances of complete decortication (Physiol. Papers, p. 130), such as ringing.