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causes.

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In the Mimosa sensitiva, for example, where the life of the plant is quickened into its highest intensity, the vital motion is distinctly referrible to those associated influences which proceed from the

The leaves open and expand in the morning, and become raised on their footstalks; and in the evening they again close and droop: or, in other words, the irritable tissues pass into a state equivalent to expansion under the operation of light and warmth, and return to the contracted condition when these influences are withdrawn. They are in fact acted upon in the same manner as the rudimentary structures with which we were concerned in the first chapter.

The sudden contraction when the plant is touched would seem to point to an external agent, and to meet its explanation without the assumption of any mystical and inherent principle. The first question in this matter concerns the nature of the agent, and then we have to inquire whether there is an abstraction or a communication of influence when the contact is made. The agent, whatever it is, must be physical in its character, for it may be connected with an inorganic instrument, and therefore we have to choose which of the several correlated expressions of physical force is the one concerned. It is not heat, or light, or chemical affinity, because there may be no evidence of the presence of these in the instrument;—it is not the mere communication of ordinary motion, for the results are

peculiar;—and if not these, we have to ask if it be electricity? That it may be so we may argue for many reasons, but especially from the history of the blade electrometer, where the metal leaflets are seen to diverge on the communication of the influence which they are intended to measure, and to collapse when touched—phenomena so singular that we are led to ask if the instrument is not a coarse and clumsy imitation of the leaf of the sensitive plant. It is not difficult to suppose indeed that this leaf, of whose extreme susceptibility to motion we have evidence, may be a natural electrometer, and also that it may become charged with electricity, for it is acted upon by light and heat and other forces, with which electricity is associated as a correlative agent. It is also difficult to believe otherwise than that something has been abstracted, and not communicated, when the plant is touched, for we have evidence in the undisturbed natural changes of the plant that contraction is consentaneous with the withdrawal of the diurnal stimulus, while at the same time the possibility of an instantaneous abstraction, and the production of similar results, is evident in the movement of the common electrometer, so that this very fact of sudden contraction is a strong argument in favour of the force concerned being of the nature of electricity.

In this manner we may fancy the sensitive plant to be an exquisitely delicate natural electrometer adapted

to measure changes which are far beyond the perceptions of the common instrument of this name.

The phenomena of motion in other irritable plants are of a similar character, and with few exceptions, the contraction occurs at night, or at times of contact with any foreign body, while the opposite condition of expansion is cotemporaneous with the day. The difference in other cases is, that the movements are not susceptible of rapid alterations, and merely respond slowly to the sun, but still the general law is the same, and leaves and flowers open in the day, and close at night. There are some flowers, however, whose movements seem to obey a different law, and to open at night; but these it must be observed do not close in the day, in the same sense that the leaflets of the sensitive plant do, but wither; and therefore the difference may be accounted for by a greater sensibility in these plants, the nocturnal stimulus being sufficient to awaken life, while, on the other hand, the full beams of the sun may be said to scorch and destroy the mobile textures. Indeed, in some cases it may appear that the plants which exhibit these exceptional peculiarities are those which have been brought from climes less genial than ours, to whose days our nights may be nearly equivalent, and it is possible, if the phenomena of motion were observed in their own homes that these would cease to differ from others.

Under any circumstances the general phenomena of motion in plants conform to the law which rules in the sensitive plant, and as to the rest, it may be doubted whether there are any exceptional cases that are truly so.

2. Of intra-organic force as the agent in these

movements. Of these agents we have little knowledge, but there is reason to believe from the history of other movements of the plant, with which we have already been occupied, that they are of comparatively trifling importance. At present, indeed, we must be content to understand that the agency of extra-organic force is sufficient to account, in some measure at least, for the phenomena under consideration, and we must leave the darker and more mysterious operations of vitality until we can study the clearer manifestations which occur in animal bodies.

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Preliminary Considerations. In relation to these phenomena there are two initial considerations which demand our attention. The first is,—that all modes and forms of the contractile

state are related to each other,—that the mode to which the term irritability is applied, where the contraction is sudden and transitory, and the mode called tonicity, where the contraction is gradual and its duration indeterminate, are but varieties of the same state. This opinion has been gaining many supporters of late, and it is advocated in one of the most recent and important text-books of physiology.*

The second and more important of these considerations is, that the state opposed to muscular contraction is something more than mere passive relaxation.

This view has been advocated by more than one writer, and it is distinctly enunciated in the works of Bichật. In regard to the voluntary muscles, however, the opinion of this physiologist is somewhat obscure, for he speaks of these muscles being restored to the state in which they were previous to contraction by the action of the antagonist muscles. But at the same time, he cites some facts which show the presence of a positive power of extension :—thus, he speaks of the contraction and subsequent dilatation of a muscle after its removal from the body, when there can be no extraneous traction, and of the convulsive movements of a similar character which may be seen in divided muscles when these have been

*

Physiological Anatomy and Physiology of Man: Todd and Bowman, vol. ii. p. 172—1845.

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