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 17/1000 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 15/1000 of an inch apart, so that a small portion
of the upper surface of the midrib had contracted in a transverse line
2/1000 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 to the midrib. The distance
between the dots was found to be 40/1000 of an inch, so that the two
extreme dots were 80/1000 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 1
to 2/1000 of an inch, and the two further dots by 3 to 4/1000 of an
inch, than they were before; so that the two extreme [page 318] dots
now stood about 5/1000 of an inch (.127 mm.) nearer together than
before. If we suppose the whole upper surface of the lobe, which was
400/1000 of an inch in breadth, to have contracted in the same
proportion, the total contraction will have amounted to about 25/1000
or 1/40 of an inch (.635 mm.); but whether this is sufficient to
account for the slight inward curvature of the whole lobe, I am unable
to say.
Finally, with respect to the movement of the leaves, the wonderful
discovery made by Dr. Burdon Sanderson* is now universally known;
namely that there exists a normal electrical current in the blade and
footstalk; and that when the leaves are irritated, the current is
disturbed in the same manner as takes place during the contraction of
the muscle of an animal.
The Re-expansion of the Leaves. This is effected at an insensibly slow
rate, whether or not any object is enclosed.** One lobe can re-expand
277