« PreviousContinue »
tapering to each extremity, and overlapping each other in various degrees, these are consolidated into a mass which considerable force is insufficient to break. Any one who will examine a single thread of the finest flax, with a microscope which magnifies only 180 times, will find that what to the eye appears a single thread is in reality composed of a great number of distinct tubes.
It is also the tissue from which roots are emitted. Unlike the leaf-buds, roots are always prolongations of the woody tissue of the stem, as may be seen by tracing a young root to its origin. The woody tissue, when applied to this purpose, is, however, always covered with cellular tissue.
The real nature of the functions of the VASCULAR SYSTEM has been the subject of great difference of opinion. Spiral vessels have been most commonly supposed to be destined for the conveyance of air; and it seems difficult to conceive how any one accustomed to anatomical observations, and who has remarked their dark appearance when lying in water, can doubt that fact. Nevertheless, many observers, and among them Dutrochet, assert that they serve for the transmission of fluids upwards from the roots. This physiologist states that, if the end of a branch be immersed in coloured fluid, the latter will ascend in both the spiral vessels and articulated bothrenchyma; but that in the former it will only rise up to the level of the fluid in which the branch is immersed, while through the latter it will travel into the extremities of the branches. But from this statement it does not appear that spiral vessels convey fluid; it only shows that M. Dutrochet confounds one kind of tissue with another, when he infers that trachenchyma performs certain functions, because such functions are proper to bothrenchyma in a particular state. It has also been asked, how the opinion that spiral vessels are the sap-vessels is to be reconciled with the fact of their non-existence in multitudes of plants in which the sap circulates freely. To which might have been, or perhaps has been, added the question, why they do not exist in the wood, where a movement of sap chiefly takes place in exogenous trees.
And further, it has always been remarked, that, if a tranverse section of a Vine, for instance, or any other plant, be put under water, bubbles of air rise through the water from the mouths of the spiral vessels. But then, it has been urged, that coloured fluids manifestly rise in the spiral vessels; a statement which has been admitted when the spiral vessels are wounded at the part plunged in the colouring fluid, but denied in other circumstances. Indeed, to persons acquainted with the difficulty of microscopic investigations, the obscurity that practically surrounds a question of this sort must be apparent enough.
Kützing, adopting the views of Schulthess and Oken, has recently asserted that the spiral vessels represent the nervous system of plants; and that both they and the tubes of pleurenchyma perform the same office in the system of vegetable vitality, as metallic wires in conducting electromagnetical currents. (Linnæa, xii. 26. Schulthess, Drei Vorlesungen über Electromagnetismus, gehalten in der naturforschenden Gesellschaft zu Zurich : 1835.)
The use of spiral vessels has been investigated with care by Bischoff, who instituted some delicate and ingenious experiments, for the purpose of determining the real contents and office of the spiral vessels. It is impossible to find room here for a detailed account of his experiments, for which the reader is referred to his thesis, De vera Vasorum Plantarum Spiralium Structura et Functione Commentatio : Bonnæ, 1829. It must be sufficient to state, that, by accurate chemical tests, by a careful purification of the water employed from all presence of air, and by separating bundles of the spiral vessels of the Gourd (Cucurbita Pepo), and of some other plants from the accompanying cellular substance, he came to the following conclusions, which, if not exactly, are probably substantially, correct:-“ That plants, like all other living bodies, require, for the support of their vital functions, a free communication with air; and that it is more especially oxygen which, when absorbed by the roots from the soil, renders the crude fluid fit for the nourishment and support of a plant, just as blood is rendered fit for that of animals. But, for this purpose, it is not sufficient that the external surface should be surrounded by the atmosphere; other aëriferous organs are provided, in the form of spiral vessels, which are placed internally, and convey air containing an unusual proportion of oxygen, which is obtained through
the root, by their own vital force, from the earth and water. In a hundred parts of this air twenty-seven to thirty parts are of oxygen, which is in part lost during the day by the surface of plants under the direct influence of the solar rays.”
With such evidence of the aëriferous functions of the spiral vessels, it will doubtless to many appear probable that this question is settled, as far as spiral vessels, properly so called, are concerned. Whether or not ducts have a different function is uncertain ; it is probable, however, from the extreme thinness of their sides, that they are really filled with fluid when
have been the case when they were first generated.
Link, who formerly considered trachenchyma a part of the aëriferous system, now declares its function to be that of conveying nutritious secretions. (Element. ed. 2. i. 188.) He considers his new opinion proved by certain plants, which he had grown in a solution of prussiate of potash, having had their spiral vessels stained blue when afterwards grown in sulphate of iron. (See APPENDIX.)
Gaudichaud has also been appealed to, as being of the same opinion ; but this botanist seems to have ascertained nothing more than that articulated bothrenchyma conveys fluid, which is a very different thing. (See Ann. des Sc.)
It requires no argument to prove that the office of CinenChyma is to convey the elaborated sap of a plant to the places where it is needed, and especially down the inner parts of the bark of oxygens.
For all details concerning this matter, we must wait for the
appearance of Professor Schultz's memoir. In regard to the functions of air-cells and lacune, it may be sufficient to remark, that in all cases in which they form a part of the vital system, as in water plants, they are cavities regularly built up of cellular tissue, and uniform in figure in the same species; while, on the other hand, where they are not essential to vitality, as in the pith of the Walnut, the Ricepaper plant, the stems of Umbelliferæ, and the like, they are ragged, irregular distentions of the tissue. In the former case they are intended to enable plants to float in water ; in the latter, they are caused by the growth of one part more rapidly than another.
When the elementary organs combine themselves into an organised structure endowed with life, they produce a body, 1. invariably bounded by curved lines, and, 2., having its parts balanced with great symmetry. In these respects they agree with the animal, and differ from the mineral, kingdom.
There is no such thing as an angle in vegetation: the points of the most acuminated leaves, the so-called angles of leaves and stems, the teeth of serrated leaves, are all in reality so many curves. Even the apparently flat surfaces of leaves and petals are only segments of large circles. This is a necessary consequence of the primitive spheroidal form of vegeable tissue, which, in whatever way it may develope, must always be bounded externally by the curved sides of the parenchyma.
In like manner, it will always be found, that every part of a plant is balanced by some other part. The stem is equipoised by the root; one leaf or pair of leaves is counterbalanced by the next leaf or pair; one side of a leaf answers to another; of the anthers, one lobe has its fellow on the opposite side of the connective; and this kind of comparison may be carried into the minutest part of vegetable structure. It is true, that it appears in some cases of irregularity to be departed from; as in Labiatæ and other irregular flowers, where the balance among the several parts seems to be destroyed; in such plants as Goldfussia anisophylla, in which one of every pair of opposite leaves is much smaller than the other; and in such leaves as those called by botanists oblique, in which one side of the leaf is much smaller than the other : but, even here, the symmetry is only destroyed in appearance. If in Labiatæ the force of developement preponderates in the anterior segments of the corolla, it is counterbalanced by the fuller developement of the posterior stamens and sepals; and in Leguminosæ, in which the posterior petal overbalances those in front, the anterior sepals and stamens restore the equipoise. If in Goldfussia it is the right hand-leaf of one pair which is the smaller, it is the left leaf of the next pair.
In the arrangement of the organs round the axis of a plant, the same careful provision of antagonistic developement is manifest. If one leaf grows on the right of a stem, the next appears on the left, the next on the right; if a pair of opposite leaves points N. and S., the next pair points E. and W., the next N. and S. again; and, if a whorl of 4 leaves is directed to the cardinal points, the next whorl is directed to the intermediate points of the compass, and so on. The same laws will be found to be observed, with little variation, through all the organs of a flower.
Messrs. Chatin and Moquin Tandon have been led, by such considerations as these, to assume that there are in the vegetable, as in the animal, kingdom, both a centripetal and centrifugal force of developement; the former appertaining to Exogens and Endogens, and the latter to Acrogens. I know nothing of the evidence upon which M. Chatin thinks this proposition may be maintained. M. Moquin Tandon has, however, given a sketch of his ideas upon the same subject, and I must confess they do not appear to me conclusive. (See Comptes rendus, v. 691.)