Page:Popular Science Monthly Volume 8.djvu/64

54 glands are stimulated, and the extreme marginal tentacles become inflected, the motor impulse is transmitted across half the diameter of the disk. It passes not along the vascular system, but through the cellular tissue, travelingtravelling [sic] more rapidly and easily in a longitudinal than in a transverse line, probably for the reason that the cells are elongated longitudinally, and some obstruction is encountered at each cell-wall through which the motor impulse must pass.

A molecular change of the protoplasm within the cells, to which Mr. Darwin has given the name of aggregation, precedes and accompanies all motion. When a leaf which has not been excited or inflected is examined, the cells forming the pedicels are seen to be filled with an homogeneous purple fluid. If the tentacle be examined some hours after having been excited, the purple matter is found to be aggregated into masses of various shapes suspended in a colorless fluid. The change begins within the glands and travels downward, being arrested for a short time at each cell-wall; the aggregated masses perpetually changing form, separating and uniting. After the cause of the excitement has been removed, and the tentacles have reëxpanded, the colored masses of protoplasm are redissolved, and the purple fluid again becomes homogeneous and transparent. This process of aggregation is not dependent upon the inflection of the tentacles or increased secretion of the glands—a most remarkable feature of the phenomenon being that in the tentacles which are inflected by an indirect irritation, conveyed by motor impulse from other glands, some influence is sent up to the glands, as their secretion is increased and becomes acid; then the glands thus excited send back some other action, causing the protoplasm to aggregate in cell beneath cell. There can actually be seen a molecular change proceeding, which may be somewhat similar to the molecular change which is supposed to be sent from one end of a nerve to another when sensation is felt. We have here a reflex action, and the only known case thereof in the vegetable kingdom. The rate at which the motor impulse is transmitted is much slower than in animals. This fact, as well as that of the motor impulse not being specially directed to certain points, are both, no doubt, due to the absence of nerves. Nevertheless, we perhaps see the prefigurement of the formation of nerves in animals in the transmission of the motor impulse being much more rapid down the confined space within the tentacles than elsewhere, and somewhat more rapid in a longitudinal than in a transverse direction across the disk.

Of course, there is not in this, or in the reflex action, any thing comparable with the nervous systems of animals, and, as Mr. Darwin says, "the greatest inferiority of all is the absence of a central organ, able to receive impressions from all points, to transmit their effects in any definite direction, to store them up and reproduce them." That is to say, Drosera seems to be without even the