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the difference in the two cases being simply this,-that, whereas every cell among the latter possesses the same attributes, and is accordingly capable of sustaining its own existence, and of living independently of its fellows, the component cells in the higher plants are so diversely endowed, as to be mutually dependent upon each other. Thus the cells of the leaves will not grow, unless the due supply of fluid be imbibed by the cells of the roots; the secreting cells cannot separate their characteristic deposits, unless these have been already prepared by the elaborating agency of the leaves; and every fresh development, or growth, presupposes the due performance of their functions by all the organs which supply the requisite material.
In this manner the nutritive operations of plants, however varied their nature and complicated their relations, essentially consist of cell-life; the tendency of which is to build up and sustain a fabric to whose extent and duration no definite limit can be assigned. In the meantime it is observable that the growth of the more permanent substance of the structure is only attained by the continual development, decay, and renewal of other parts. The elaborating cells, which compose the soft tissue of the leaves, are a striking proof of this. They obviously have a fixed and definite term of life, during which their powers are exerted for the benefit of the remainder of the plant; and when this has been accomplished, they die and are cast off. The death of the leaves is not, as is sometimes supposed, the consequence of their fall; nor is it occasioned by their nutriment being cut off, in consequence of the vessels of the leaf-stalk becoming clogged-up towards the end of the summer. The truth is, they have lived their time; have completed the term of functional activity allotted to them. We may always notice that, when this activity has been peculiarly great, in consequence of an unusually high temperature, the death of the leaves takes place earlier. They have ceased to perform their special function-that of fixing carbon from the atmosphere-some time before they decidedly change colour: and their subsequent exuviation is in accordance with the general tendency of all living parts, to detach from themselves such as are dead. But the correspondence in all essential particulars between the simplest Cryptogamia and the highest forms to which
vegetation can attain, is not confined to the nutritive process. The complex phenomena of reproduction in the flowering plants appear mainly to consist in a repetition of what occurs in the lowest instances of vegetable life. According to the latest and most exact observations on the interesting problem of the nature of the fertilising action, the formation of the embryo is the result of the mixture of the contents of one of the pollen-grains or cells, with those of the embryo-sac, which is a large cell contained within the ovule or immature seed; in which case it essentially corresponds with the conjugation of the lower algæ. The differences presented by the comparison of the two products, are quite subordinate to their essential conformity. Thus, while it would seem that any two cells of a palmella or a protococcus may conjugate for the production of a new generation, the action is restricted, in the flowering plant, to certain groups of cells specially endowed; an arrangement quite in conformity with the general plan of the division of labour,' peculiar to the higher groups. Again, whilst the new product of the simple cellular plant can readily provide for its own livelihood, and soon developes itself into the form and constitution of its parent, the embryo of the flowering plant passes through a long series of progressive phases before the parental form is evolved, and during the early part of the series it is dependent upon its parent for the materials of its growth. In all instances, however, the vegetable embryo has at first the condition of a simple cell; this cell multiplies itself by subdivision, and the new cells continue adherent to each other, so that a cluster of cells is generated, strongly resembling in their condition some of the lower algæ or fungi. From this cluster, a rudiment of the cotyledon or seed-leaf is evolved; and then the development of the stem and root begins. During the whole of this process, the young fabric consists of nothing but cells; and it is not until the end of the period of germination, when the true leaves are unfolded and come into play, that the woody tissue, vessels, &c., are generated.
Thus the condition of the highest forms of vegetable life is ascertained to be the same at their origin with that of the lowest. And, we are entitled to affirm that the whole history of the growth and reproduction of the most perfect
plant is essentially a repetition of the development and propagation of that simple cell, to which we have so often referred as the type of all organization.
Of the facts which constitute the basis of this generalization, the greater number have been, for some time, familiar to vegetable physiologists. But it has been only within a very recent period, that their mutual relationship has been completely developed, by the discovery of the essential conformity between the process of reproduction, in the highest and lowest forms of vegetative existence. That some conformity would be found to exist between the instrumental structures concerned in the growth and propagation of plants, and those by whose agency the corresponding functions of animals are effected, was a speculation in much favour with those physiologists who had the clearest view of the intimate connection subsisting between these two great hemispheres of the world of organization. But no one, we venture to say, was in the least degree prepared for the surprising discoveries which the microscope has recently brought to light, with regard to the elementary structure of the animal tissues, and the early stages of embryonic development. These discoveries have followed one another in such rapid succession, that but a few years elapsed between the announcement of the first of the series, and the completion and generalization of the whole. Various inquirers had detected the existence, in several parts of the animal body, of true cellular tissues; and had shown that the history and offices of the component cells were, in all essential respects, the same with those of the elementary parts of the vegetable fabric. These, however, were usually considered exceptions; and it was still the rule, that the elementary tissues of animals are entirely different from those of plants. The first great step in advance was made by Schwann. In the work, whose title we have placed at the head of this article, he first announced it as a general fact, that the animal tissues are all developed from cells; although the various metamorphoses which these undergo, in their progress towards their complete form, more or less obscure their original character. A host of followers immediately devoted themselves to the re-investigation of these elementary tissues.
His statements, limited and corrected on some points, have been so greatly extended and confirmed on others,—and his generalization, premature in some respects, has been so fruitful in novel results and widely-extended applications, that he must be acknowledged to have done more than had been previously attempted or even conceived, towards elevating Physiology in the scale of sciences. Some of the general results may be made intelligible to all our readers.
The animal body contains many parts which are subservient to its mechanical functions alone. Thus the office of the bony skeleton is simply to afford support and protection to the softer parts,―to supply points of attachment to the muscles, by which the motive force is generated,—and to act as a system of levers, by which that force may be advantageously applied to the movement of the entire fabric. So, again, the purpose of the teeth is purely mechanical; and their solidity and durability are such, as almost to remind us of the homogeneity and permanence of inorganic masses. Further, in order to hold together the parts of the bony skeleton, and at the same time to allow them free movement upon each other, ligamentous cords and bands are interposed; which are composed of simple homogeneous fibres, some of them elastic, others non-elastic. The tendinous cords, by which the muscular power is brought to bear upon the bone; and the various fibrous membranes which are introduced among the muscles and other organs, for the purpose of binding them in their places, are of the same materials; as is also the case, though worked up in a different manner, with the tissue, so widely diffused through the body, which was formerly called cellular, but is now more correctly distinguished as areolar. In this tissue, the elastic and non-elastic fibres are woven into a network, the meshes or areola of which freely communicate with each other; and the one or the other kind of fibre predominates, according as an unyielding resistance to tension, or a certain measure of elasticity, is required. Its office is to hold together the elementary parts of other tissues (as bricks in a wall are held together by cement), and to form a sort of bed for the passage of the vessels and nerves, which it everywhere accompanies in their course and distribution. The same tissue, in a more condensed
state, forms, with blood-vessels, nerves, and lymphatics, the substance of the true skin which invests the exterior of the body; of the mucous membranes, which are prolonged from this into the alimentary canal, the respiratory cavity, and many other passages in the interior of the body; and of the serous membranes, which line the cavity of the chest and abdomen, and other shut sacs.' Now the function of these fibrous tissues is purely mechanical.
We do not find them in plants, because plants, not being constructed for active movement, have no need of them; and we may regard them as superadded in animals, merely to bring their structure into conformity with the physical conditions of their existence. They possess no properties which can be properly designated as vital; and it need not surprise us, therefore, if they should constitute an exception to the general rule of the development of the tissues from cells. In fact, the solid homogeneous fibres of the fibrous tissues are decidedly beneath cells in the scale of organization; and it would even seem that the solid material of bone, possibly also of dentine, is formed by the chemical union of calcareous salts with similar gelatinous fibres. The complex apparatus of cavities and tubes, however, by which the nutrition of these tissues is provided for, is probably generated (as we should expect) by the growth and transformation of cells.
Turning now to those portions of the animal fabric which are the instruments of the truly vital functions; and first, directing our attention to the phenomena of organic life, which are essentially the same with those of plants; we have no difficulty in tracing out the general fact that all these functions are performed, in animals as in plants, by the agency of cells. Thus the selection and absorption of the liquid aliment which has been prepared by the digestive process (itself a truly chemical alteration), is effected by the agency of the multitudes of cells contained within the extremities of the villi, which act as so many little roots, not spread out, indeed, through the soil in search of food, but suspended from the walls of the cavity containing it. The food so taken up is delivered over to the absorbent vessels, which, like the vessels that carry upwards the ascending sap of plants, convey the crude aliment to the organs in which it is to undergo concentration and