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Show 214 DION lEA MUSCIPULA. CHAP. XIII. tissue) seems at first sight to favour this belief; for they run up the midrib in a great bundle,. sending off small bundles almost at right angles on each side. These bifurcate occasionally as they extend towards the margin, and close to the margin small branches from adjoining vessels unite .and enter. the marginal spikes. At some of these points of union the vessels form curious loops, like those described under Drosora. A continuous zigzag line of vessels thus runs round the whole circumference of the leaf, and in the midrib all the vessels are in close contact; so that all parts of the leaf seem to be brought into some degree of communication. Nevertheless, the presence of vessels is not necessary for the transmission of the motor impulse, for it is trans1nitted from the tips of the sensitive filaments (these being about the 2~ of a.n inch in length), into which no vessels enter; and these could not have been overlooked, as I made thin vertical sections of the leaf at the bases of the :filaments. On several occasions, slits about the T\, of an inch in length were made with a lancet, close to the bases of the filaments, parallel to the midrib, and, therefore, directly across the course of the vessels. These were made sometimes on the inner and sometimes on the outer sides of the filaments; and after several days, when the leaves had reopened, these filaments were touched roughly (for they were always rendered in so1ne degree torpid by the operation), and the lobes then closed in the ordinary manner, though slowly, and sometimes not until after a considerable interval of time. These cases show that the motor impulse is not transmitted along the vessels, and they further show that there is no necessity for a direct line of communication from the filament which is CHAP. XIII. TRANSMISSION OF MOTOR IMPULSE. 315 touched towards the midrib and opposite lobe, or towards ~he outer parts of the same lobe. 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, ~o 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 ·OS of an inch (2·032 mm.) in breadth; and in the middle stood the filament. Only one of these slips withered and perished. After tho 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 fila1nent, 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 slow 1 y 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. rrhese 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 |
