« PreviousContinue »
It may be here remarked, that stolo has given rise to the name stool, which is applied to the parent plant from which young individuals are propagated by the process of layering, as it is technically called by gardeners. The branch laid down was termed propago by the older botanists, and the layer was called malleolus, which literally signifies a hammer; the name being thus applied, because, when the layer is separated from its parent, its lower end resembles a hammer head, of which the new plant represents the handle.
The Offset, fig. 31. (propagulum, Link), is a short lateral branch in some herbaceous plants, terminated by a cluster of leaves, and capable of taking root when separated from the mother plant, as in Sempervivum. It differs very little from the runner.
The Rootstock, fig. 29. (rhizoma), is a prostrate thickened rooting stem, which yearly produces young branches or plants. It is chiefly found in Iridaceæ and epiphytous Orchidaceæ, and is often called caudex repens. The old botanists called it cervix, a name now forgotten.
The Vine, fig. 32. (viticula, Fuchs.), is a stem which trails along the ground without rooting, or entangles itself with other plants, to which it adheres by means of its tendrils, as the Cucumber and the Vine. This term is now rarely employed. De Candolle refers it to the runner or sarmentum ; but it is essentially distinct from that form of stem, because it does not root.
The Pseudobulb is an enlarged aerial stem, resembling a tuber, from which it scarcely differs, except in its being formed above ground, in having an epidermis that is often extremely hard, and in retaining upon its surface the scars of leaves which it once bore. This is only known in Orchidaceous plants, in which it is very common.
The term stem (caulis) is generally applied to the ascending caudex of herbaceous plants or shrubs, and not to trees, in which the word trunk is employed to indicate their main stem; sometimes, however, this is called caulis arboreus. From the caulis, Linnæus, following the older botanists, distinguished the culmus or straw, which is the stem of Grasses; and De Candolle has further adopted the name Calamus for all fis
tular simple stems without articulations, as those of Rushes; but neither of these differ in any material degree from common stems, and the employment of either term is superfluous. This has already been remarked with respect to culmus by Link, who very justly inquires (Linnæa, ii. 235.) "cur Graminibus caulem denegares et culmum diceres?"
If a plant is apparently destitute of an aerial stem, it is technically called stemless (acaulis), a term which must not however be understood to be exact, because it is, from the nature of things, impossible that any plant can exist without a stem in a greater or less degree of developement. All that the term acaulis really means, is that the stem is very short.
3. Of its Internal Modifications.
The internal structure of the stems of Flowering plants, is subject to two principal and to several subordinate modifications. The former are well illustrated by such plants as the Oak and the Cane, specimens of which can be easily obtained for comparison. A transverse slice of the former exhibits a central cellular substance or pith, an external cellular and fibrous ring or bark, an intermediate woody mass, and certain fine lines radiating from the pith to the bark, through the wood, and called medullary rays; this is called EXOGENOUS structure. In the Cane, on the contrary, neither bark, nor pith, nor wood, nor medullary rays, are distinguishable; but the transverse section exhibits a larger number of holes irregularly arranged, and caused by the section of bothrenchymatous and vascular tissue, and the mass of woody and cellular substance in which they lie imbedded. This kind of structure is named ENDOGenous.
In both cases there is a cellular and vascular system distinct from each other; it is only by a diversity in their respective arrangement that the differences above described are caused. In explaining in detail the peculiar structure of Exogenous and Endogenous stems, it will be more convenient to consider them with reference to those two systems, than to follow the usual method of leaving the fact of there being two distinct systems out of consideration.
The cellular system in an Exogenous stem chiefly occupies the centre and the circumference, which are connected by thin
vertical plates of the same nature as themselves. The central part (a, fig. 34.) is the pith, that of the circumference (b) is the bark, and the connecting vertical plates (c) are 6 medullary rays.
The pith is a cylindrical or angular column of cellular tissue, arising at the point of separation between the root and stem and ter
minating at the leaf-buds, with all of which, whether they are lateral or terminal, it is in direct communication. Its tissue, when cut through, almost always exhibits an hexagonal character, and is frequently larger than in any other part. When newly formed, it is green, and filled with fluid; but its colour gradually disappears as it dries up, and it finally becomes colourless. After this it undergoes no further change, unless by the deposition in it, in course of time, of some of the peculiar secretions of the species to which it belongs. It has been contended, indeed, by some physiologists, that it is gradually pressed upon by the surrounding part of the vas
cular system, until it is either much reduced in diameter or wholly disappears; and in proof of this assertion, the Elder has been referred to, in which the pith is very large in the young shoots, and very small in the old trunks. Those, however, who entertain this opinion, seem not to consider that the diameter of the pith of all trees is different in different shoots, according to the age of those shoots; that in the first that arises after germination, the pith is a mere thread, or at least of very small dimensions that in the shoots of the succeeding year it becomes larger- and that its dimensions increase in proportion to the general rapidity of developement of the vegetable system: the pith, therefore, in the firstformed shoots, in which it is so small compared with that in the branches of subsequent years, is not small because of the pressure of surrounding parts; it never was any larger.
The pith is always, when first forming, a uniform compact mass, connected without interruption in any part; but the vascular system sometimes developing more rapidly than itself, it occasionally happens that it is either torn or divided into irregular cavities, as in the Horse Chestnut, the Rice-paper plant, and many others; or that it is so much lacerated as to lose all resemblance to its original state, and to remain in the shape of ragged fragments adhering to the inside of the vascular system: this is what happens in Umbelliferous and other fistular-stemmed plants.
Sometimes the pith is much more compact at the nodes than in the internodes, as in the Ash; whence an idea has arisen that it is actually interrupted at those places: this is, however, a mistake; for in general there is no interruption of continuity, but a mere alteration in compactness. It does however sometimes happen, that the pith takes a large developement at the nodes, so as to cut off the vascular system of the internodes into almost distinct parts. This occurs in what are called articulated stems, as in Piper, Viscum, &c., and in the Vine when young. Dutrochet regards such cases as evidence that each internode is an independent creation in the beginning, and that it is only after having been growing for a period of time, varying in different cases, that the internodes become connected by woody formations.
It seldom happens that any part of the vascular system intermixes with the pith, which is usually composed of cellular tissue exclusively; but in Ferula and the Marvel of Peru, it has been proved by Mirbel and De Candolle, that bundles of woody fibre are intermixed; in Nepenthes there is a considerable quantity of spiral vessels scattered among the cellular tissue of the same part; and many other cases of a similar kind are now known. In Nyctaginaceae generally, in Piperaceæ, Cycadaceæ, Chloranthaceæ, &c., this occurs, and has been made by Professor Schultz the character of a large division in his Natural System of Botany, called by him Synorgana dichorganoidea; but such cases may be found in Loranthus, and are not uniform in the orders quoted: in Boerhaavia repanda, for example, the pith contains no bundles of vascular tissue, but is filled with fistula containing very soft, lax, spheroidal, cellular tissue, surrounded by smaller, harder, and more cubical tissue, which passes into the medullary rays; a most curious organisation.
The Bark is the coating of the stem immediately above the wood, to which it forms a sort of sheath, and from which it is separable without difficulty at certain seasons. But, although it appears as an independent formation, it is, in reality, organically connected with the wood by the processes of cellular tissue, which, under the name of medullary rays, pass through the wood, and lose themselves in the thickness of the bark. Formerly bark was distinguished into cortical or cellular integument, under which name was comprehended the whole of the external parenchymatous part, and liber or inner bark, a name used to denominate the fibrous woody portion lying next the alburnum. But it is necessary to look at the organisation of the bark with more precision, if we are to understand all the peculiarities found in its many modifications. It appears to me that the observations of Mohl are the best and most complete which have hitherto been made upon this very important subject: they, and many more of considerable value, by Dutrochet, Link, and others, render a peculiar nomenclature for the parts of the bark indispensable; so many false or indefinite ideas are there which attach to the older terms. Bark may be described anato