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tissue coated by a stratum of woody tissue, enclosing, at irregular distances from the centre, very unequal
37 portions of the vascular system. The pith is exceedingly excentrical; and the medullary rays, which are imperfectly formed, do not all radiate from the pith, but on the thickest side form curves passing from one side of the stem to the other, their concavities turned towards the pith.
In the stem of a Bignonia in my possession, from Colombia (fig. 38.), the vascular system is divided into four
nearly equal parts, by four short
thick plates radiating from the
into the cavity. In Stauntonia latifolia
39 (fig. 39.), which has a twining stem, there are no concentric circles, and the medullary rays are curved, part from right to left, and part from left to right, diverging at one point and converging at another: the bark is pierced with extensive longitudinal perforations.
In Euonymus tingens (fig. 40.) the vessels near the centre of the stem are arranged in concentric interrupted circles, but towards the bark there is no trace of such circles; the surface of the stem is deeply cut into lobes parallel with the stem, and the vessels are all
confounded in an uniform mass. Gaudichaud represents the stem of some Malpighiaceous plants to be in like manner divided into a number of regular lobes, which however, actually reach the axis; and, in consequence of the twining habit of the stem, are twisted into the appearance of a cable externally. In Menispermum laurifolium (fig.
41 41.) the concentric lines evidently belong to the medullary system; they are extremely interrupted and unequal, often only half encircling the stem, or even less, and they anastomose in various ways; the medullary rays are unusually large, and lie across the wood like parallel bars; and, finally, the plates of which the wood consists each contains but one vessel, which is situated at the external edge of the plate. 42
None of the anomalous forms of Exogenous stems are, however, more remarkable than an unknown Burmese tree (fig. 42.), for a specimen of which I am indebted to Dr. Wallich. In a section of this, the general appearance is so much that of an Endogenous stem, that without an attentive examination it might be actually mis
taken for one. The diameter of this stem is two inches seven lines; it is nearly perfectly circular, and has a very thin but distinct bark, with a central pith surrounded by very compact woody tissue. There are neither zones nor medullary rays; but the vascular system consists of an uniform mass of vessels and woody tissue, disposed with great symmetry, and of the same degree of compactness at the circumference as in the centre. Amongst this wood are interspersed, at the distance of about half a line, with great regularity, passages containing loose cellular tissue. These passages are convex at the back and rather concave in front, run parallel with the vessels, and do not seem to have any kind of communication with each other. They, no doubt, represent the medullary rays of the cellular system of this highly curious plant. It must be remarked, that the resemblance borne by this stem to that of an Endogenous plant is more apparent than real; for whilst, in the latter, the vascular system is separated into bundles surrounded by the cellular system, in this, on the contrary, the cellular system consists of tubular passages, surrounded by masses of the
It will be observed that, in all those cases of irregular developement, the part next the centre is but little affected; and such seems to be the general rule. In the Penny Cyclopædia, article Exogens, there are figures of several cases of structure still more anomalous than the preceding, with the woody matter contorted excessively; but even in them the centre is in the normal condition of exogenous wood.
Such examples show the student that it is neither medullary rays nor concentric zones in the wood that are the certain indications of Exogenous growth, both the one and the other being sometimes absent; but that the presence of a central pith, and a greater degree of hardness in the wood next the centre, than in the circumference, are the signs from which alone any absolute evidence can be derived.
Plants of an arborescent habit having this structure being almost exclusively extra-European, and most of them natives only of the tropics, botanists have had much fewer opportunities of examining them, and, consequently, our knowledge concerning them is more limited. Nevertheless, the investigations of Mohl and others have thrown great light upon their real organisation.
In Endogenous plants the vascular and cellular systems are as distinct as in Exogenous, but they are differently arranged. The cellular system, instead of being distinguishable into pith, bark, and medullary rays, is a uniform mass, in which the vascular system lies imbedded in the form of thick fibres, seldom having any tendency to collect into zones or wedges resembling wood. The fibrous bundles consist of woody tissue, enclosing spiral or other vessels.
The following is an explanation of the opinions generally entertained concerning the formation of an Endogenous stem. Its diameter is supposed to be increased by the constant addition of fibrous bundles to the centre, whence the name; those bundles displace such as are previously formed, pushing them outwards; so that the centre, being always most newly formed, is the softest; and the outside, being older, and being gradually rendered more and more compact by the pressure exercised upon the bundles lying next it by those forming in the centre, is the hardest. In Endogenous plants that attain a considerable age, such as many Palms, this operation goes on till the outside becomes sometimes hard enough to resist the blow of a hatchet. It does not, however, appear that each successive bundle of fibres passes exactly down the centre, or that there is even much regularity in the manner in which they are arranged in that part: it is only certain that it is about the centre that they descend, and that on the outside, below the growing point, no new formation takes place from the circumference. This appears from the manner in which the bundles cross and interlace one another, as is shown in the figure of Pandanus odoratissimus given by De Candolle in his Organographie (tab. vi.), or still more clearly in the lax tissue of the inside of the stems of Dracæna Draco.
The investigations of Mohl appear, however, to show that this view of the structure of Endogens requires some modification. According to this observer, every one of the woody bundles of a Palm stem originates in the leaves, and is at first directed towards the centre; arrived there, it follows the course of the stem for some distance, and then turns outward again, finally losing itself in the cortical integument. In the course of their downward descent, the woody bundles gradually separate into threads, till at last the vascular system, which for a long time formed an essential part of each of them, disappears, and there is nothing left but woody tissue. In this view of the growth of Endogens, the trunk of such plants must consist of a series of arcs directed from above inwards, and then from within outwards; and consequently the woody bundles of such plants, instead of being parallel with each other, must perpetually intersect each other. There are, however, some difficulties in the way of this theory, which we do not find adverted to by its author. If Mohl's view of the structure of Endogens be correct, they must after a time lose the power of growing, in consequence of the whole of the lower part of their stems being choked up by the multitude of descending woody