Cnar. XII. TRANSMISSION OF MOTOR IMPULSE. 317 
surfaces. It appears therefore that the cells must con- 
tract. 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 ~14, 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 715, of an inch apart, 
so that a small portion of the upper surface of the 
midrib had contracted in a transverse line —2,, 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 nar- 
row 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 4%, of an inch, so that the 
two extreme dots were 4%, 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 
135% of an inch, and the two further dots by +44 of 
an inch, than they were before ; so that the two extreme 
