CHAPTER V. COMPARISON OF VEGETABLE WITH ANIMAL PHYSIOLOGY. 79. It is impossible to consider the subject of Vegetable Physiology and organization, without being struck by the analogy which it presents in so many points to that of Animals. Yet, however strong may be that analogy, it never in any instance becomes identity, and the marked fact, noticed in the Introduction, that the latter in all cases convey their food by the mouth to a stomach, is alone sufficient to establish a boundary between them;* the comparison, however, between the two, is so interesting and instructive, that a few words may be well bestowed upon the subject. The whole range of functions both of animals and plants, that is to say as far as nutrition and reproduction are concerned, affords ample illustrations of the near approach to similarity in the two kingdoms -a few examples of each may prove the truth of * There does indeed appear to be one group, about which some doubt exists in the mind of some physiologists as to its reference to the animal or vegetable kingdom. "They are mostly," says Dr. Carpenter, " formed of cells jointed together, as the Confervæ ; but some of them seem to possess a different interior structure; and others exhibit very curious motions, which can scarcely be distinguished with certainty from those of animals." (Carpenter's Veg. Phy., p. 44.) power of this assertion, while the difference will also in general be equally perceptible. In the entire course of that function by which the individual is nourished, the main point holds good in both cases; i. e., that matter fitted for its food is taken into the system by the appointed organs, thence conveyed through the necessary channels, assimilated and converted into the requisite substance for continuing and replenishing the tissue of the body, and furnishing the needful secretions, while such as is unavailing to any of these purposes, is excreted. In the plant, however, the juices are not conveyed to a single receptacle, there to be elaborated, but, according to the process detailed in the foregoing pages, are gradually in their progress converted from the crude into the nutritive sap. The circulation of this sap, and the the glands to convert it into peculiar secretions, suggests immediately to the mind the idea of an analogy with the circulation of the blood in animals, and a fanciful imagination might see a degree of further likeness to the venous and arterial blood in the two states of the sap. The similarity, however, though it does exist, is but very partial, no one general circuit of the sap throughout the system, as there is of the blood originally propelled from the heart, really taking place. Again the tissue produced and nourished in the two kingdoms, though very analogous in some respects, is by no means identical:-the cellular texture of animals differing from the cellular tissue of plants by its structure, which is not actually composed of individual cells, united together by the cohesion of their walls, but of "a congeries of extremely thin laminæ or plates, variously connected together by fibres, and by other plates, which cross them in different directions, leaving cavities or cells."* This cellular texture, however, forms the essential material of the animal fabric generally, as the cellular tissue does of the vegetable. The important chemical difference between animal and vegetable organized tissue has already been noticed, viz., the presence of nitrogen in the one case and its absence in the other (45). 80. Perhaps, however, the most curious and interesting analogy between animal and vegetable organization is that which relates to the process of reproduction-which in some of the lowest tribes of animals approaches more nearly to identity with that of plants than in any other function. In several of the most minute of the Infusoria, in which nevertheless, small as they are, the patient investigation of Ehrenberg has discovered a series of stomachs, we meet with frequent examples of multiplication by the spontaneous division of the body of the parent into two or more parts. "Many species of Monads, for instance, which are naturally of a globular shape, exhibit at a certain period of their development a slight circular groove round the middle of their bodies, which by degrees becoming deeper, changes their form to that of an hour-glass; and the middle part becoming still more contracted, they present the appearance of two balls united by a mere point. The monads in this state are seen swimming irregularly in the fluid; as if animated by two different volitions; and apparently for the purpose of tearing asunder the last connecting fibres, darting through the thickest of the crowd of surrounding animalcules; and the moment this slender ligament is broken, each is seen moving away from * Roget's "Anim. and Veget. Physiol.," vol. i. p. 99. the other and beginning its independent existence."* -Now although we have not in the vegetable world any instance of this voluntary division, yet, in the all but spontaneous action, the reproduction of plants by the division of their parts bears a strong analogy to it, and in the cases to be further mentioned, the resemblance is still stronger. The Hydra, or fresh water Polype, "is capable of indefinite multiplication by simple division: thus, if it be cut asunder transversely, the part containing a head soon supplies itself with a tail; and the detached tail soon shoots forth a new head, with a new set of tentacula. If any of the tentacula, or any portion of one of them be cut off, the mutilation is soon repaired; and if the whole animal be divided into a great number of pieces, each fragment acquires, in a short time, all the parts which are wanting to render it a complete individual." In this same animal (the Hydra) which is thus capable of being increased by what would in a plant be slips or cuttings, the natural method of propagation is analogous to that of many plants-such as the Duckweed: "At the earliest period at which the young of this animal is visible, it appears like a small tubercle, or bud, rising from the surface of the parent hydra; it grows in this situation, and remains attached for a considerable period; at first deriving its nourishment as well as receiving its mechanical support, from the parent. . . . this mode of multiplication, in its first period, corresponds exactly with the production of a vegetable by buds; .... although at a later stage, it differs from it in the complete detachment of the offspring from the parent." An instance of reproduction occurs in the sponge, which * Roget, Anim. and Veget. Physiol., p. 583. + Ib. p. 590. bears a near resemblance to the spontaneous fructification and bursting of the theca of many of the Cryptogamic plants. "The parts of the Spongia panicea, which are naturally transparent, contain at certain seasons a multitude of opaque yellow spots visible to the naked eye, and which, when examined by a microscope, are found to consist of groups of ova, or more properly gemmules, since we cannot discover that they are furnished with any envelop. In the course of a few months these gemmules enlarge in size, each assuming an oval or pear-like shape, and are then seen projecting from the sides of the internal canals of the parent, to which they adhere by their narrow extremities. In process of time, they become detached, one after the other; and are swept along by the currents of fluid, which are rapidly passing out of the larger orifices."* "When two gemmules, in the course of their spreading on the surface of a watch-glass, come into contact with each other, their clear margins unite without the least interruption,— in a few days we can detect no line of distinction between them, and they continue to grow as one animal. The same thing happens, according to the observations of Cavolini, to adult sponges, which on coming into mutual contact, grow together, and form an inseparable union. In this species of animal grafting we again find an analogy between the constitution of zoophytes and that of plants."+ 81. With respect to the higher orders of vegetable life, the Phanerogamic, or flowering plants, the whole analogy in their method of increase with that of the larger part of the animal creation has been so long known, and so much insisted on, that it is superfluous to dwell on it. Enough has been said to show * Roget, Anim. and Veget. Physiol., p. 156. + Ib. p. 159. |