cells have very delicate walls, and are so few in number that they
could hardly cause a tentacle to bend with precision to a definite
point. Though I carefully looked, I could never detect any wrinkling of
the surface at the bending place, even in the case of a tentacle
abnormally curved into a complete circle, under circumstances hereafter
to be mentioned.
All the cells are not acted on, though the motor impulse passes through
them. When the gland of one of the long exterior tentacles is excited,
the upper cells are not in the least affected; about halfway down there
is a slight bending, but the chief movement is confined to a short
space near the base; and no part of the inner tentacles bends except
the basal portion. With respect to the blade of the leaf, the motor
impulse may be transmitted through many cells, from the centre to the
circumference, without their being in the least affected, or they may
be strongly acted on and the blade greatly inflected. In the latter
case the movement seems to depend partly on the strength of the
stimulus, and partly on [page 255] its nature, as when leaves are
immersed in certain fluids.
The power of movement which various plants possess, when irritated, has
been attributed by high authorities to the rapid passage of fluid out
of certain cells, which, from their previous state of tension,
immediately contract.* Whether or not this is the primary cause of such
movements, fluid must pass out of closed cells when they contract or
are pressed together in one direction, unless they at the same time
expand in some other direction. For instance, fluid can be seen to ooze
from the surface of any young and vigorous shoot if slowly bent into a
semi-circle.** In the case of Drosera there is certainly much movement
of the fluid throughout the tentacles whilst they are undergoing
inflection. Many leaves can be found in which the purple fluid within
the cells is of an equally dark tint on the upper and lower sides of
the tentacles, extending also downwards on both sides to equally near
their bases. If the tentacles of such a leaf are excited into movement,
it will generally be found after some hours that the cells on the
concave side are much paler than they were before, or are quite
colourless, those on the convex side having become much darker. In two
instances, after particles of hair had been placed on glands, and when
in the course of 1 hr. 10 m. the tentacles were incurved halfway
towards the centre of the leaf, this change of colour in the two sides
229