any way commensurate with that which is witnessed in these organic solids.

Notwithstanding this peculiarity, however, it is evident that the simple vital movements of which we speak are connected with, and in some degree determined by, ordinary heat, and on further examination. we find no reason to doubt this conclusion.

It is found, indeed, that various inorganic solids are affected differently by heat, some being acted upon to a much greater extent than others: and this is sufficient to show that there may be a still further difference in organic substances, without any departure from the mere physical character of the phenomenon.

It may be observed, also, that the solids in question are composed of several elements, as oxygen, hydrogen, nitrogen, and carbon, which for the most part remain gaseous when in an uncombined state, and of which the mutual affinity is so feeble that they readily pass into an aëriform condition,-and hence it may be supposed that such solids will undergo a greater change of volume under the action of heat than an inorganic substance, in which the constitution is simple and the molecular affinity more stable. And for the same reason it need be no ground of wonder that the change should be greater than in water, or in any other fluid whose particles do not tend to

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.

CHAPTER II.

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