Two slits near each other, both parallel to the midrib, were next made in the same manner as before, one on each side of the base of a filament, on five distinct leaves, so that a little slip bearing a filament was connected with the rest of the leaf only at its two ends. These slips were nearly of the same size; one was carefully measured; it was .12 of an inch (3.048 mm.) in length, and .08 of an inch (2.032 mm.) in breadth; and in the middle stood the filament. Only one of these slips withered and perished. After the leaf had recovered from the operation, though the slits were still open, the filaments thus circumstanced were roughly touched, and both lobes, or one alone, slowly closed. In two instances touching the filament produced no effect; but when the point of a needle was driven into the slip at the base of the filament, the lobes slowly closed. Now in these cases the impulse must have proceeded along the slip in a line parallel to the midrib, and then have radiated forth, either from both ends or from one end alone of the slip, over the whole surface of the two lobes.
Again, two parallel slits, like the former ones, were made, one on each side of the base of a filament, at right angles to the midrib. After the leaves (two in number) had recovered, the filaments were roughly touched, and the lobes slowly closed; and here the impulse must have travelled for a short distance in a line at right angles to the midrib, and then have radiated forth on all sides over both lobes. These several cases prove that the motor impulse travels in all directions through the cellular tissue, independently of the course of the vessels.
With Drosera we have seen that the motor impulse [page 316] is transmitted in like manner in all directions through the cellular tissue; but that its rate is largely governed by the length of the cells and the direction of their longer axes. Thin sections of a leaf of Dionaea were made by my son, and the cells, both those of the central and of the more superficial layers, were found much elongated, with their longer axes directed towards the midrib; and it is in this direction that the motor impulse must be sent with great rapidity from one lobe to the other, as both close simultaneously. The central parenchymatous cells are larger, more loosely attached together, and have more delicate walls than the more superficial cells. A thick mass of cellular tissue forms the upper surface of the midrib over the great central bundle of vessels.
When the filaments were roughly touched, at the bases of which slits had been made, either on both sides or on one side, parallel to the midrib or at right angles to it, the two lobes, or only one, moved. In one of these cases, the lobe on the side which bore the filament that was touched moved, but in three other cases the opposite lobe alone moved; so that an injury which was sufficient to prevent a lobe moving did not prevent the transmission from it of a stimulus which excited the opposite lobe to move. We thus also learn that, although normally both lobes move together, each has the power of independent movement. A case, indeed, has already been given of a torpid leaf that had lately re-opened after catching an insect, of which one lobe alone moved when irritated. Moreover, one end of the same lobe can close and re- expand, independently of the other end, as was seen in some of the foregoing experiments.
When the lobes, which are rather thick, close, no trace of wrinkling can be seen on any part of their upper [page 317] surfaces, It appears therefore that the cells must contract. The chief seat of the movement is evidently in the thick mass of cells which overlies the central bundle of vessels in the midrib. To ascertain whether this part contracts, a leaf was fastened on the stage of the microscope in such a manner that the two lobes could not become quite shut, and having made two minute black dots on the midrib, in a transverse line and a little towards one side, they were found by the micrometer to be 17/1000 of an inch apart.