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Show 258 DROSERA ROTUNDIFOLIA. CHAP. X. though rejected by most others, is that the whole cell, including the walls, actively contracts. If the walls are composed solely of non-nitrogenous cellulose, this view is highly improbable; but it can hardly be doubted that they must be permeated by proteid matter, at least whilst they are growing. Nor does there seem any inherent improbability in the cell-walls of Drosera contracting, considering their high state of organisation; as shown in the case of the glands by their power of absorption and secretion, and by being exquisitely sensitive so as to be affected by the pressure of the most minute particles. The cell-walls of the pedicels also allow various impulses to pass through them, inducing movement, increased secretion and aggregation. On the whole the belief that the walls of certain cells contract, some of their contained fluid being at the same time forced outwards, perhaps accords best with the observed facts. If this view is rejected, the next most probable one is that the fluid contents of the cells shrink, owing to a change in their 1nolecular state, with the consequent. closing in of the walls. .. A.nyhow, the movement can hardly be attributed to the elasticity of the walls, together with a previous state of tension. With respect to the nature of the motor i1npulse which is transmitted from the glands down the pedicels and across the disc, it seems rrot ilnprobable that it is closely allied to that influence which causes the protoplasm within the cells of the glands and tentacles to aggregate. We have seen that both forces originate in and proceed from the glands within a few seconds of the same tirne, and are excited by the sa1ne causes. The aggregation of the protoplasm lasts almost as long as the tentacles remain inflected, even though this be for more than a week · l:ut the ' CHAP. X. NATURE OF THE MOTOR IMPULSE. 259 protoplasm is redissolved at the bending place shortly before the tentacles re-expand, showing that the ex-. citing cause of the aggregating process has then quite ceased. Exposure to carbonic acid causes both the latter process and the motor impulse -to travel very slowly down the tentacles. We know that the aggregating process is delayed in passing throng h the cellwalls, and we have good reason to believe that this holds good with the motor impulse; for we can thus understand the different rates of its transmission in a longitudinal and transverse line across the disc. Under a high power the first sign of aggregation is the appearance of a cloud, and soon afterwards of extremely fine granules, in the homogeneous purple fluid within the cells ; and this a pparen tl y is due to the union of molecules of protoplasm. Now it does not seem an improbable view that the same tendency-namely for the molecules to approach each other-should be communicated to the inner surfaces of the cell-walls which are in contact with the protoplasm ; and if so, their n1olecules would approach each other, and the cell-wall would con tract. To this view it' may with truth be objected that when leaves are immersed in various strong solutions, or are subjected to a heat of above 130° Fahr. (54°·4 Cent.), aggregation ensues, but there is no moyement. Again, various acids and some other fluids cause rapid movement, but no aggregation, or only of an abnormal nature, or only after a long interval of time; but as most of these fluids are 1nore or less injurious, they may check or prevent the aggregating process by injuring or killing the protoplasm. There is another and more important difference in the two processes : when the glands on the disc are excited, they transmit some influence up the surrounding s 2 |