« PreviousContinue »
through the textures generally, like a stream through a porous rock, and not in fixed and definite channels.
The movement of the elaborated sap or latex in the laticiferous vessels, to which the name of cyclosis is given, has been fully investigated by Professor Schultz. It is the counterpart of the capillary circulation in the higher animals in the earlier periods of their fætal history, or of that which remains permanently in some of the rudimentary entozoid forms; that is to say, it is not a single and constant stream, proceeding and returning to and from a common fixed centre or heart, but a complex movement, consisting of many distinct currents directed to and from certain independent centres, the positions of which are subject to continual change. This is the plan observable in the vascular area of animal bodies prior to the formation of the heart, and therefore there is nothing special and peculiar in the phenomena of cyclosis.
The second special movement of the sap, or rotation, is that which is observed in the contents of certain cells, and the phenomena are always limited to one of these rudimentary bodies. The law is not constant, and there are great and manifest variations, not only in different plants, but even in contiguous cells of the same plant. In one instance, the movement is directed along one wall of the cavity, and in a contrary direction down the other side, the line of the current being in accordance with the longitudinal axis of the cell:
in another instance it follows a spiral course in a transverse direction: in a third it is vague, and to all appearance unobservant of any particular law.
In attempting to detect the causes of these several movements, we propose to commence our task with the study of the phenomena of cyclosis, for here we secure an intermediate position between the vague and imperfectly-understood movements confined to individual cells, and that more complicated circulation of the sap which extends to the entire plant.
I. OF THE MOVEMENTS IN THE MILK VESSELS OF
THE PLANT (CYCLOSIS).
In this, as in every phenomenon of vital movement, we have to seek for the efficient cause in agents which proceed from surrounding objects, and in agents which reside within the organism.
1. Of extra-organic force as the agent in the
phenomena of cyclosis. On watching the movements of the sap in the laticiferous vessels, Dr. Lindley found them to cease in a low temperature, and to recommence when the plant was carried into a warm room; and in this fact we have evidence of a partial dependence upon the influence we designate as heat. Let us begin our examination, then, by inquiring into the manner in
which this physical agent may be concerned in these changes; and to this end, let us divest ourselves of those ideas of mysterious and unknown powers with which the explanation is presumed to be connected, and at the same time banish the notion that the causes of vital movement must, of necessity, be different from those which determine motion in inanimate bodies, while we consider, on ordinary principles, the necessary effects of heat under these circumstances.
Motion is one of the effects of heat in ordinary matter, repulsion of the particles marking the communication, and attraction the withdrawal, of the influence. If the body acted upon be arranged in a tubular form, an alteration in the capacity of the cavity must result, the calibre being diminished or extended, as the case may be. Such, also, may be presumed to be the effect of heat in a vessel composed of the simpler organic fabrics, for we have already seen that the particles of these tissues are affected like those of inorganic bodies.
Let it be supposed, then, that a latex vessel is acted upon by heat, and what, we may ask, will be the consequences. The vessel, as we have already seen, is composed of matter possessed of a remarkable susceptibility to movement under slight alterations of heat, much greater in degree than that which belongs to water, or blood, or the allied fluids, and therefore
we suppose that the expansion of the latex will not be commensurate with the change experienced by the coats of the vessel; and hence, the primary operation of heat upon the vessel will be the production of a vacuum between the coats and the contents.
The operation of heat, therefore, may be the cause of the primary act of absorption, for to fill the vacuum which is thus formed, it is evident that there must be a rushing inwards of aëriform or watery fluids from neighbouring parts—a phenomenon readily permitted by the porosity, which is one of the properties of organic membrane—and thus the filling of the shrunk and empty laticiferous web, on the return of spring, may be nothing more than the necessary effect of the increased warmth of the season.
It does not appear, however, by what means the solar heat can be concerned in the production of the focal movements by which the latex is agitated, for this agency operates uniformly and equally upon all portions of the web. Nor in the construction of the vessels themselves does there appear to be any peculiarity which could localize the operation, inasmuch as the focal points are subject to change, and not confined to any fixed and permanent positions. At this stage of our inquiry, also, we do not find any aid in the physical agencies which are correlative of heatnamely, light, electricity, and the rest—and therefore we leave our present point of view, and proceed to
study the phenomena of cyclosis in relation to the agents, which reside within the organism.
2. Of intra-organic force as the agent in the
phenomena of cyclosis. In addition to vital and peculiar principles, we find within the organism the several forces of ordinary matter-namely, heat, chemical affinity, electricity, and the rest. Of these agents, at this stage of our inquiry, heat is the only one whose effects are tangible and intelligible, and therefore we leave the others in abeyance for the present.
We know from the history of the animal economy that the production of organic heat will be commensurate with the molecular changes of the nutrient fluid, and the solid textures in relation to this fluid; and hence we may argue that the quantity of latex present in any part of the laticiferous web, will be a fitting measure of the capacity of that part for the production of heat. A greater amount of this influence, we may argue, will be extricated where the vessels are dilated and congested with sap, (vasa expansa,) than in the places where the vascular calibre is contracted, and the contents evacuated (vasa contracta).
The variable sizes of the laticiferous tubes, which is characteristic of the system, may therefore be the cause of some irregularities in the origination and