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
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