« AnteriorContinuar »
remain gaseous under the ordinary condition of the atmosphere.
The hygroscopic property of organic substances, which is illustrated in the expansion or contraction of the belts of sea-wrack which occasionally serve as hygrometers, or in the shortening or lengthening of a lady's ringlets, according to the moisture of the atmosphere is another circumstance which may in part account for the changes of volume in these tissues. It is probable that the heat which determines the evaporation of water from the earth operates, remotely, in preparing these bodies to receive a portion of this fluid within their interstices; but whatever the explanation may be, the phenomenon is of a physical character, inasmuch as it is exhibited when the subjects of it are removed altogether from the operations of vitality. This hygroscopic property, therefore, is another reason why the remarkable changes of volume in the rudimentary tissues of the organism, under slight changes of temperature, may be nothing more than the natural and physical consequence of the constitution of these tissues.
Such would appear to be some general facts in connexion with the operation of one form of external agent, in some of the simpler phenomena of vital movement. They show that the motions in the tissues of the plant and animal, of which mention has been made, may be referred in part to causes that are
not within the pale of life; and also that the movements themselves, in so far as they are related to heat, are analogous to those which are brought about by the same agent in inorganic substances, any apparent difference being only one that may be accounted for by the mere physical constitution of the tissue.
OF VITAL MOVEMENTS.
These phenomena may be arranged under three heads :-Under the first, are those which are seen in vessels and cells; under the second, all such movements as occur in the irritable tumours of the sensitive plant, in the coats of the alimentary canal, and in the voluntary muscles; and under the third, the rhythmical beatings of the heart. We shall speak of the several varieties of vital motion according to this arrangement, and then add some separate considerations on the influence of the nerves and mind in these phenomena.
A. OF VITAL MOVEMENTS IN VESSELS
AND CELLS. These movements are less complex in the plant than in the animal, for here they are not confused by the cardiac impulse; and it is in the plant, therefore, that we institute our first inquiries.
SECTION I. OF VITAL MOVEMENTS IN THE VESSELS, CELLS, AND
INTERCELLULAR PASSAGES OF THE PLANT.
Preliminary Considerations. In plants, the sap exhibits very positive motion, but there is no true circulation. There is a general movement which belongs to the plant as a whole, and to which all parts contribute; and there is a special movement, which is itself divided into two varieties, the one belonging to the laticiferous vessels, and the other to certain single cells.
The plan of the general movements of the sap would seem to vary at different times and places. When examined during the summer, the main current is found to pass from the roots, through the fibres and ducts of the newly-formed woody zones of the stem, to the upper surface of the leaves. It then tends towards the lower surface of the leaf, and thence, in a downward direction, through the fibrous structures of the bark, passing at the same time inwardly and horizontally through the tissues which form the medullary rays. “Very little of the elaborated sap reaches the roots, from which the motion commenced; and none of it, except that small quantity which mixes with the ascending current, is again transmitted through the system.” The movement is not confined to any special tissue, but it seems to pervade indiscriminately the
intercellular passages as well as the cavities of the cells, and to pass without difficulty through organic membranes, except these be thickened and altered by solid deposits. During the winter, however, or, at any rate, in times of severe frost, (as we learn from M. Biot,) the sap of the same plant flows chiefly from the branches towards the roots, so that at these times the former organs would seem to exercise the office which generally belongs to the latter. In plants, also, where the roots are sparingly developed, and infixed in arid and moving sand, or in parasites, whose dry, aërial roots are every-way unfitted to the function of absorption, it is clear that the fluids necessary to the wants of the economy must find entrance by other channels. In the lower cellular plants, also, which are adherent to dry and barren rocks, the surface which exercises the function of absorption is that which corresponds to the aërial system of more perfect plants. It would appear, therefore, that the stem and root, with their several organs respectively, may be the channel of absorption, a varying degree of importance being assigned to either in different plants; and this we may understand to be possible, from the law of analogy which has been already set forth in another work. It would appear, also, that there are periodical changes in the same plant, so that no one plan can be considered as constant — a supposition which is the more probable from the fact, that the sap infiltrates