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are determined by the very same agents which bring about the special movements of the cell and latex vessel.
1. Of extra-organic force as the agent in this
A statement of M. Biot, quoted by Dr. Lindley, affords a true clue to the discovery of the secret action of these forces. According to this observer, there is an abundant exudation of sap from the spongioles in frosty weather, which continues until a thaw supervenes, when it gives place to an opposite process of absorption, of which the vigour and activity is directly related to the degree of warmth and heat. In other words, there are changes which seem to show that the tissues of the plant contract when exposed to the influence of cold, and expand when heated, just as any inorganic substance; the waters being expelled from or absorbed into the porous textures, according to circumstances.
The powerful upward current of the sap, during the warm weather, would also seem to point to the same order of changes, and to meet with its explanation in them; for at this time the trunk and branches, with their several appendages, will be expanded by the heat, and vacua produced, into which fluids may be supposed to rush. The agent, also, which produces the expansion will vaporize a large quantity of these fluids, and the vapours, passing off into the atmosphere, will leave their former spaces to be filled from the same sources; and hence (the condition of the atmosphere being such as to favour evaporation and radiation) the same agent which causes the primary rush of fluids from the earth will provide for the continuance, and convert the rush into a continuous current.
The downward movement of the sap during the colder months of winter, when the temperature of the air is too low to produce any considerable expansion in the tissues of the aërial system of the plant, and when, for the same reason, the evaporation of fluids will be retarded, would seem also to be of similar significance; for at this time we may suppose the underground portions of the plant to be relatively warmer and more expanded than the aërial; and, consequently, on the same principles we may account for the change of current which has taken place.
Other powers may, and do, operate in these movements; but the actual exudation and absorption of sap from the roots, witnessed by Biot, on the supervention and cessation of a frost, together with the changes in the main current which characterize the circulation of the plant in summer and winter,—all point to heat as a grand and important agent.
In the circulation of the plant it has been already stated that the fluids move indifferently through the cells and vessels, or through the insterstitial spaces, and that no part is specially concerned in the process. Each cell, as it would appear, aids in the production of the general current; and this, therefore, is compounded of an infinite number of minor currents. The sap, indeed, may be said to rise as through a porous rock; and the analogy would be nearly perfect, if we could imagine the component crystals to be hollow and porous, and the stream to filter through their cavities as well as in the interstices. That the leaves or spongioles have no special power in determining these movements, though at times these are the main entrances to fluid, is evident from the changes in the main current of the sap which take place in winter and summer; for whatever function seems to belong to the leaf at one time is transferred to the rootlet at the other, so that the function of either, in relation to the absorption or exhalation of fluid cannot be regarded as absolute. In the arguments, also, which show the essential unity of the various parts of the plant, which have been set forth elsewhere,* there is a strong reason for the belief in the possibility of a change of function in these parts.
The entire subject however is very imperfect, and many new observations must be made before it can be fully known. At the same time, it is sufficiently clear that the phenomena of the general circulation in the plant are not always definitely and absolutely the same; and from what is known, it may be argued that the changes which take place are due to the operation of causes analogous to those which determine changes in the centres of the laticiferous vessel. In particular, the statement of Biot is very clear as to the existence of general expansion under the influence of heat, and of contraction under the opposite condition of cold; and hence we may infer that these effects will always follow when the plant is subjected to these agents.
* “ Proteus; or, the Law of Nature.” London: J. Churchill, 1850. Pages 9, &c.
2. Of intra-organic force as an agent in the general
movement of the sap. · The evidence which appertains to this division of our subject, above what has been cited in connexion with the cell and latex vessel, is very obscure and imperfect; and to enter upon it would involve the discussion of questions which will be more advantageously considered when we speak of vascular movements in animal bodies. As, however, the general movement of the sap is in great part the aggregate expression of the cellular and vascular movements, we may argue that the influence of intra-organic force will be the same as in the cell and latex-vessel,- and as to the rest, we must be content to reason from analogy, from what we shall find hereafter in animal bodies.
OF VITAL MOVEMENTS IN THE VESSELS AND CELLS
OF ANIMAL BODIES.
In creatures which remain at the foot of the scale of existence, and in the early transitional phases of beings which ultimately attain to a higher grade, the vascular movements are in every respect similar to those of the laticiferous system in the plant; but in more perfect states the plan is greatly altered, and instead of the ever-changing centres, and the oscillatory currents connected with them, there is a single heart and a perfect circulation. In this movement the heart is undoubtedly the prime agent, but it is not less true, that the vessels, and especially the smaller ones, possess an important complementary power,—and this it is our present purpose to investigate.
The cellular movements of animal bodies are, in many respects, similar to those of plants, but in addition to these is one that is exhibited in certain processes to which the name of cilia is given, and which has attracted considerable attention in these days of microscopic investigation. All these demand attention in proper order.