To this view it may with truth be objected that when leaves are
immersed in various strong solutions, or are subjected to a heat of
above 130 Fahr. (54 .4 Cent.), aggregation ensues, but there is no
movement. Again, various acids and some other fluids cause rapid
movement, but no aggregation, or only of an abnormal nature, or only
after a long interval of time; but as most of these fluids are more or
less injurious, they may check or prevent the aggregating process by
injuring or killing the protoplasm. There is another and more important
difference in the two processes: when the glands on the disc are
excited, they transmit some influence up the surrounding [page 260]
tentacles, which acts on the cells at the bending place, but does not
induce aggregation until it has reached the glands; these then send
back some other influence, causing the protoplasm to aggregate, first
in the upper and then in the lower cells.
The Re-expansion of the Tentacles. This movement is always slow and
gradual. When the centre of the leaf is excited, or a leaf is immersed
in a proper solution, all the tentacles bend directly towards the
centre, and afterwards directly back from it. But when the point of
excitement is on one side of the disc, the surrounding tentacles bend
towards it, and therefore obliquely with respect to their normal
direction; when they afterwards re-expand, they bend obliquely back, so
as to recover their original positions. The tentacles farthest from an
excited point, wherever that may be, are the last and the least
affected, and probably in consequence of this they are the first to
re-expand. The bent portion of a closely inflected tentacle is in a
state of active contraction, as shown by the following experiment. Meat
was placed on a leaf, and after the tentacles were closely inflected
and had quite ceased to move, narrow strips of the disc, with a few of
the outer tentacles attached to it, were cut off and laid on one side
under the microscope. After several failures, I succeeded in cutting
off the convex surface of the bent portion of a tentacle. Movement
immediately recommenced, and the already greatly bent portion went on
bending until it formed a perfect circle; the straight distal portion
of the tentacle passing on one side of the strip. The convex surface
must therefore have previously been in a state of tension, sufficient
to counter-balance that of the concave surface, which, when free,
curled into a complete ring.
The tentacles of an expanded and unexcited leaf [page 261] are
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