Oldalképek
PDF
ePub

stretched out cells: for, in the first place, it is very difficult to make a section completely parallel with the vessels, without cutting through a vertical wall; but where the tubes are cut through, they commonly assume the appearance of a transverse wall. In the second place, we may be easily deceived by air bubbles, the sides of which may assume the appearance of organic cross walls. In the third place, we cannot always conclude that there exists a perfect partition wall, where we believe we see it; for it may be only the two sides of the tube approaching each other, where a fold in the canal swells out the vessel itself. That, in fact, the cross walls are not always complete where they seem to be so appears to me to be proved by the confervas, in which we think we perceive true partition walls, and yet we see the green matter make its way from one articulation to another. All these considerations have induced me to believe in the existence of continued vessels wherever the sap clearly flows, even though anatomy should seem to decide to the contrary. That the sap, which flows with such impetuosity in the stem of a birch or maple, when it is punctured in the spring, should not proceed from open vessels, but from the so called small vasa spiralia, is quite incredible. The absorption of coloured liquid by plants seems to establish my opinion; for the coloured liquor is confined to what modern physiologists have been pleased to call stretched out cells, and is not to be found in the tela cellulosa, though supposed a part of it. Who can in such a case believe that there are no vessels or continued canals?

These views and observations allow me to speak of the vessels of mosses, algæ, &c. Indeed, it appears to me a very partial proceeding to refuse vessels to these fine plants, which vegetate with such rapidity and vigour. In the conferva elongata very distinct canals or tubes may be observed below the bark. In the ribs of the leaves of leafy mosses we often speak of ductuli, and we mean by this word real vessels. In the Jungermanniæ, which grow so rapidly, and assume such beauty, vessels may be observed with the greatest facility on that account I shall pass them over.

The causes why observers have been unwilling to recognise vessels in these plants, and likewise in the perfect plants, are the following:-They paid so much attention to the spiral vessels, that they conceived they must meet with something similar before they were at liberty to speak of vessels at all. It has an appearance of accuracy and precision not to speak of vessels unless they be as distinctly marked as the spiral vessels. But in a physiological point of view, the subject becomes darkened and imperfect. According to every analogy, we must give the name of vessels to those organs in which the sap flows, which nourishes the whole body; and those

* Dr. Afzelius has informed me that when the stem of the tetracera potatoria is cut, people can satisfy their thirst with the pure water contained in it. I have examined this wood microscopically, and find therein very large woody vessels, from which this water proceeds, and certainly no stretched out cells.

tubes, which carry a more local and less remarkable liquid, and which in the anatomy of animals are called ducts, as the salivary ducts, the seminal ducts, &c. In the anatomy of plants philosophers, without observing it, have nearly fallen into an abuse of language. That the spiral vessels nourish plants, is not very probable. They are exceedingly few, and often altogether wanting. In Guaiac wood we see very distinctly that the false tracheæ contain resin, which is not a substance that nourishes plants, but an excretion; but the true spiral vessels are only modifications of the false trachea, and other similar vessels situated in the wood. To attempt to draw a distinct line between them would be the same thing as in the human body not to admit the veins without valves to be real veins, but to constitute them a distinct class of vessels. The smallest stripe is sufficient to constitute a spiral vessel, and a false trachea members of quite a distinct system; and the duct lying hard by, where the cross stripe is distinct, is called a lacuna, as if it were altogether fortuitous. Here, where no different functions can be discovered, we abound in distinctions and names; yet we do not choose to distinguish the woody, cortical, and radiating vessels, in which distinct functions are very evident, from the general tela cellulosa.

On these grounds I call the fine canals containing nourishing sap, namely, the woody and cortical vessels, true vessels: and, on the contrary, name the larger canals, containing materials already brought to a state of perfection, ducts. So that in my language the spiral vessels become spiral ducts. However, I call Hedwig's ductuli in the leaves of mosses, &c. vessels, an expression by no means inconsistent with the old and more generally received names, but contrary to the new ones.

I shall now give a sketch of the different kinds of ducts, or rather point out the way in which these canals may be arranged.

The finer canals, namely, the woody vessels, carry thin, liquid, nourishing sap, to the cellular texture, as we have already seen. The more consistent and viscid sap, which approaches gum and resin in its nature, could not flow in so fine tubes. Therefore larger ducts have been constructed for them, which constitute a vascular system quite different from the system employed in nourishing the plant. But in order that this viscid sap may move freely, the walls of the vessels containing it could not be composed of a single thin membrane, but must be strong, and not liable to be torn. On this account they are wound round with spiral fibres, by the contraction of which the resinous sap is driven on, or at least prevented from accumulating. These spiral fibres, in young twigs and in herbs, in which no thick sap exists, are usually isolated, and run at a distance from each other. In the finest filaments, and other parts of the blossom, we find spiral vessels of the most delicate and beautiful structure, and no other larger ducts. In older parts of plants these spiral vessels grow together, and nothing more remains of their fine spiral structure than some cross stripes. They are then called false

trachea. We can still perceive the cross stripes very distinctly in these ducts; for example, in guaiacum wood, in which pretty consistent resin is contained. In red sanders wood the cross stripes themselves in the false trachea are contracted, so that the red extractive is collected in grains. Their analogy with the spiral ducts in structure and functions cannot in this case be perceived. In older parts of plants the cross stripes are accumulated on the walls of the ducts; so that the whole assumes the appearance of a thick, confused web and this is peculiarly the case in those places where greater strength is necessary, or where the thickest resin is to be coveyed along. In the tribe of pines observers have in vain searched for spiral ducts, and yet they constitute the trees richest in resin that we are acquainted with. A fine spiral duct would be speedily torn by the viscid liquid that moves in it; but nature always takes care to produce a stronger structure where uncommon resistance is necessary.

It appears to me very probable that the fine spiral ducts communicate at first with the woody vessels, and that when they proceed further they change into false trachea, from which new spiral ducts proceed, constituting a bundle; and that at last in the oldest parts of the plant the false trachea are changed into those large ducts called cylindric lacunæ. These three ducts are usually found near each other lying in a bundle, and commonly so that the spiral ducts are nearest to the woody vessels. It is quite impossible, indeed, to demonstrate all this anatomically, as we cannot follow a single spiral vessel through a complete plant, or even a complete branch. I consider it as probable (and in the present case it is allowable to offer a bare probability) that in these vessels there is a kind of retrograde motion of the materials of plants: that the most recently formed resinous sap is contained in the uppermost and smallest twigs, where we find separate spiral ducts; and that it flows down slowly and gradually till we come to the thick resin in the roots. We see at least that the roots abound most in large ducts filled with resin.

From all this it appears very probable that the spiral ducts, false trachea, and cylindrical lacunæ, are gradations of the same series. On that account it would be proper to distinguish them by a general name. I have given to them all the common name of ductus ligni, or ducts situated in the wood; I call each of them separately ductus spirales, subspirales, &c. as subspecies. The name ductus ligni is simple, and I do not see why we should give complicated names to so very simple organs.

The part which the spiral ducts and their varieties perform in the wood is performed in the bark by other ducts of an equally simple nature. We find quite in the neighbourhood of the bundles of cortical vessels small ducts which contain a milky juice, and which I call ductus guttiferi.

In other plants the same ducts seem to pass towards the exterior parts into larger canals, which clearly lie in the tela cellulosa. In

our pinus sylvatica it is very evident that the smaller internal ductus corticis contain a thin resinous sap, which in the larger ducts acquires a thicker consistency. In the trees which contain a milky juice (arbores guttiferae), as, for example, the mammea Americana, we perceive distinctly the fine ducts, as vasa guttifera, but the larger resiniferous ducts do not present themselves. In like manner the lactescent plants seem to have only finer ducts, which appear scarcely to differ from the cortical vessels themselves. On that account I have spoken of them distinctly from the ductus guttiferi, as a variety of the cortical ducts. But that the milky juice, especially in the bark, comes from such ducts, is to me very evident.

Whether all the ducts of the bark, even though they may contain the same materials with the ducts of the wood, are yet always destitute of every trace of spiral fibres, is a point that cannot be determined with precision. They certainly always lie on the outside of the bundle of cortical vessels; (and not in that bundle itself as the ducts of the wood do). Perhaps there were not materials there for such spiral fibres, which in the ducts of the wood may have some analogy with the vessels or fibres of the wood itself. The cortical ducts lie always in the tela cellulosa, and probably their walls are composed of that matter, and not of fibres. These considerations induced me to give them the name of cellular ducts (ductus cellulosi), especially as some similar canals present themselves in the pith. Perhaps it would have been better to have called them ductus corticis. Their different layers, and their peculiar structure probably proceeding from that circumstance, show us that the system of the bark in dicotyledonous plants is always distinct from the system of the wood, although both show a strong analogy to each other. The reason why nature has placed the cortical ducts on the outside of the bundles of vessels is probably that in such a position it is less injurious to the plant if they happen to be ruptured, and that they can stretch with more facility to admit an increase of matter.

Yet to affirm with certainty that all these things are so, is quite impossible. When we have a great object before our eyes, we must not be stopped by small difficulties, otherwise we shall be exhausted before the object is attained.*

* Dr. Wahlenberg, at the reading of this paper, exhibited to the Society various preparations of slips of wood and bark, in which the different vessels could be distinctly seen with a glass, and still better by means of a compound microscope. (Note of the society entitled Friends of Natural History in Berlin.)

ARTICLE V.

Analysis of Rice. By M. Henri Braconnot, Professor of Natural History, Director of the Garden of Plants at Nancy, &c.*

As rice has not hitherto been analyzed, and as it is one of the most important grains, since it serves for food to a great part of the human species, I thought it worth while to subject it to some experiments.

Parmentier appears to me the only person who has made some experiments on rice. † His results induced him to consider it as a peculiar substance, which he placed between starch and gum, doubtless on account of its horny translucency, and the difficulty of reducing it into a powder, which has neither the fineness, the creaking sound, nor the feel of starch, and which falls quickly to the bottom when diffused in water. But we shall find that this species of grain is more complex than had been supposed.

Action of Water on Rice.

A hundred grammes of Carolina rice, unground, lost by drying five grammes of humidity. They were then macerated with water at the temperature of 122°. The grains absorbed the water with avidity, and almost at the same time split by several transverse sections, which did not happen nearly so quickly if the rice had not been previously well dried. These grains, thus split, were easily squeezed between the fingers into a very fine powder. They were pounded in a glass mortar, adding to them in successive portions the liquid in which they had been macerated. Thus a milky liquid was obtained, which was thrown upon a filter. The greatest part of the substance of the rice remained upon the filter. Being well washed in water, in order to take up every thing soluble, and then dried, it weighed 93.67 grammes. The water in which it was washed was set aside for examination. These 93.67 grammes, when diffused through water, passed completely through a silk seirce; but the milky liquid contained at least two distinct substances: the one, very white, constituting about two-thirds of the total weight, remained for some time suspended in the liquid; the other, less white, was specifically heavier. It was easily separated from the first substance by the affusion of a great quantity of water, and by repeatedly decanting off the emulsive liquid. This liquid in a few days let fall a very white deposite, which had acquired a kind of density by the approach of its particles to each other. When dried, it was of a brilliant white, light, and was easily reduced to an impalpable powder, which adhered readily to the fingers, and emitted a particular sound when pressed.

* Translated from the Ann, de Chim. et Phys. iv. 370, April, 1817,
+ Ann. de Chim. xl. 33.

« ElőzőTovább »