« PreviousContinue »
bundles. Is this the case? The lower part of their bark, too, must be much harder, that is, much more filled with woody bundles, than the upper. Is that the fact?. The hardness of the exterior of Palm stems cannot be owing to the pressure of new matter from within outwards, but to some cause analogous to the formation of heartwood in Exogens. Is there any proof that such a cause is in operation? These inquiries. have been partially answered by Mr. George Gardner, from observations made by him in Brazil. He made a vertical section of a Palm tree four inches in circumference, and he was able plainly to trace woody bundles proceeding from the base of the leaves to the centre of the stem, at an angle of 18°; they then turned downwards and outwards to within a few lines of the external cortical part of the stem, running parallel with its axis. The distance between the ends of the arcs was about two and a half feet. He adds, that the wood of Palm trees is much harder at the bottom than in any other part of the stem, the inhabitants of tropical climates using only this part for economical purposes. (Taylor's Magazine, xi. 553.)
The epidermis of an Endogenous stem seems capable of very little distension. In many plants of this kind the diameter of the stem is the same, or not very widely different, at the period when it is first formed, and when it has arrived at its greatest age: Palms are, in particular, an instance of this; whence the cylindrical form that is so common in them. That the increase in their diameter is really inconsiderable, is proved in a curious, and at the same time very conclusive, manner, by the circumstance of gigantic woody climbing plants sometimes coiling round such stems, and retaining them in their embrace for many years, without the stem thus tightly wound round indicating in the slightest manner, by swelling or otherwise, that such ligatures inconvenience it. A specimen illustrative of this is preserved in the Museum of Natural History at Paris, and has been figured, both by Mirbel in his Elémens (tab. xix.), and De Candolle in his Organographie (tab. iv.). We know from the effect of the common Bindweed upon the Exogens of our hedges, that the embrace of a twining plant is, in a single year, destructive of
the life of every thing that increases in diameter; or at least produces, above the strangled part, extensive swellings, which end in death.
It is, however, certain that other Endogens do increase extensively in diameter up to a certain point; sometimes this is effected with great rapidity; and the horizontal growth once stopped appears never to be renewed: thus, in the Bamboo, stems are sometimes found as much as two feet in circumference, which were originally not more than half an inch in diameter. Others would seem to have an unlimited power of distension in the Dracænas, called in French colonies in Africa Bois-chandelles, the first shoot from the ground is a turio (sucker), an inch in diameter, and perhaps fifteen feet high; but in time it distends so much that sometimes two men can scarcely embrace it in their extended arms. (Thouars, Essais, p. 3.)
As Endogenous stems contain no concentric zones, there is nothing in their internal structure to indicate their age; but in the opinion of some botanists, there are sometimes external characters which will afford sufficient evidence of it. It is said that the number of external rings which indicate the fall of leaves from the trunk of the Palm tribe coincides with the number of years that the individual has lived. There is, however, no proof of this at present; such statements must therefore be received with caution. It may further be remarked, with reference to this subject, that in many Palms these rings disappear after a certain number of years.
In arborescent Endogens it usually happens that only one terminal leaf-bud developes; and in such cases the stem is cylindrical, or very nearly so, as in Palms. If two terminal leaf-buds constantly develope, the stem becomes dichotomous, but the branches are all cylindrical, as in Pandanus and the Doom Palms of Egypt; but if axillary leaf-buds are regularly developed, as in the Asparagus, Dracæna Draco, or in arborescent grasses, then the conical form which prevails in Exogens exists in Endogens also.
In Endogens there are but few important anomalies in structure; and of these the most striking, namely that of Grasses, is more peculiar than anomalous. Yucca appears,
from a figure of Meneghini (Ricerche sulla Struttura del Caule nelle Piante Monocotyledoni, t. ix. f. 1. c.), to arrange its woody bundles in concentric layers when old; Smilax has a stem strictly endogenous, and a root which approaches in structure the stem (not root) of Exogens; and, in the article Endogens, in the Penny Cyclopædia, I have shown that the stem of Barbacenia is composed of roots of an endogenous nature, held together by the adhesion of their cortical integument, and that of a very slender central true stem on which they are moulded. In Grasses the stem is hollow except at the nodes, where transverse partitions intercept the cavity, dividing it into many cells. In the Bamboo these cells and partitions are so large that, as is well known, lengths of that plant are used as cases to contain papers. But if the gradual developement of a grass be attentively observed, it will be found that the stem is originally solid; that it becomes hollow in consequence of its increasing in diameter more rapidly than new tissue can be formed; so that its deviation from the ordinary characters of Endogenous structure is much less considerable than it seems to be at first sight.
According to Mohl, the structure of an Exogenous and an Endogenous stem, during the first year of their growth, is altogether the same; but in the second year the wood and the liber of the former separate, and new matter is then interposed, while, on the contrary, in Endogens no such separation occurs, and consequently the newly-formed matter of the stem is forced towards the centre, through which it passes, with a constant tendency, however, to reach the outside. I confess, however, I do not perceive this analogy; on the contrary, if we compare the new shoot of an Asparagus and that of an Elder-bush, the difference between them will be too great to be thus explained away. M. Dutrochet thinks that in the globular rhizoma of Tamus an argument may be found to show the identity of Exogens and Endogens in the first period of their growth; and this may, perhaps, be admitted; but it is equally evident, from the same example, that they become entirely different immediately after the first period. Nor, indeed, is the anatomy of the woody tissue, which constitutes the ligneous wedges of Exogens, the same as that
which forms the woody bundles of Endogens. In the latter each woody bundle is, when divided transversely, described by Mohl as consisting of the following parts (fig. 45.):
a. Thick-sided woody tissue (cellulæ libri).
c. Thin-sided parenchyma (vasa propria).
e. A variety of bothrenchymatous tissue (cellule ligni punctata). f. Spiral vessels.
But this is quite unlike the anatomy of the smaller portions of woody tissue in Exogens, in which there is no such arrangement of woody tissue, so called vasa propria, or spiral vessels.
Endogens have no bark. They have a cortical integument composed of an epiphloeum and an inner layer, analogous perhaps to liber; and the woody part of which, according to Mohl, is formed in Palms by the introduction of the ends of the woody arcs of the stem. In Tamus elephantipes the epiphlæum acquires the nature of cork, but splits into pyramidal laminated area. This approximates the cortical integument of Endogens very little to true bark, which is essentially characterized by being separable from the wood; and having its woody tissue parallel with that of the stem, and formed altogether in an independent, though parallel, direction.
SECT. III. Of the Root, or Descending Axis.
AT or about the same time that the ascending axis seeks the light and becomes a stem, does the opposite extremity of the seed or bud bury itself in the earth and become a root, with a tendency downwards so powerful, that no known force is sufficient to overcome it. Correctly speaking, nothing can be considered a root except what has such an origin; for those roots which are emitted by the stems of plants are in reality the roots of the buds above them, as will be hereafter explained. Nevertheless, nothing is more common than even for botanists to confound subterranean stems or buds with roots, as has been already seen. (See Bulb, Tuber, Soboles, &c. &c.)
Independently of its origin, the root is to be distinguished from the stem by many absolute characters. In the first place, its ramifications occur irregularly, and not with a symmetrical arrangement: they do not, like branches, proceed from certain fixed points (buds), but are produced from all and any points of the surface. Secondly, a root has no leafbuds, unless indeed, as is sometimes the case, it has the power of forming adventitious ones; but, in such a case, the irregular manner in which they are produced is sufficient evidence of their nature. Thirdly, roots have no scales, leaves, or other appendages; neither do they ever indicate upon their surface, by means of scars, any trace of such : all underground bodies upon which scales have been found are stems, whatever they have been called. A fourth distinction between roots and stems is, that the former have never any stomates upon their epidermis; and, finally, in Exogens the root has never any pith. It has been also said that roots are always colourless, while stems are always coloured; but aërial roots are often green, and all underground stems are colourless.
The body of the root is sometimes called the caudex; the minute subdivisions have been sometimes called radicles, — a term that should be confined to the root in the embryo;