Cuar. XIII.] TRANSMISSION OF MOTOR IMPULSE. 255 
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 Jobe 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 surfaccs. 
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 yy 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 
rio> of an inch apart, so that a small portion of the upper 
surface of the midrib had contracted ina transverse line ,j°55 
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 superficial 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 tu 
the midrib, The distance between the dots was found to be 
