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Show 316 DION1EA MUSCIPULA. CHAP. XIII. 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 Dionma were made by my son, and the cells, both those of the central and of the more super:ficial 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 !eaf that h~d lately re-opened after catching an Insect, of wh1ch one lobe alone moved when irritated. Moreover, one end of the same lobe can close andreexpand, independently of the other end as was seen 0 ' 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 th~ir upper CHAP. XIII. TRANSMISSION OF MOTOR IMPULSE. 317 su~faces. 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 con tracts, 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 1 6 b 0 of an inch apart. One of the filaments was then touched and the lobes closed; but as they were prevented from meeting, I could still see the two dots, which now were 1 ~ ~ 0 of an inch a part, so that a small portion of the upper surface of the midrib had contracted in a transverse line 1 0 2 0 0 of an inch (·0508 mm. ). We know that the lobes, whilst closing, become slightly incurved throughout their whole breadth. This movement appears to be due to the contraction of the super:ficial layers of cells over the whole upper surface. In order to observe their contraction, a narrow strip was cut out of one lobe at right angles to the midrib, so that the surface of the opposite lobe could be seen in this part when the leaf was shut. After the leaf had recovered from the operation and had re-expanded, three minute black dots were made on the surface opposite to the slit or window, in a line at right angles to the midrib. The distance between the dots was found to be 1 ~ ~ 0 of an inch, so that the two extreme dots were 1 ~ ~ 0 of an inch apart. One of the filaments was now ·touched and the leaf closed. On again measuring the distances between the dots, the two next to the midrib were nearer together by ~ ~~ ~ of an inch, and the two further dots by ~ ;oo ~ of an inch, than they were before ; so that the two extrmne |
