Nothing could be more striking than the appearance of the above four
leaves, each with their tentacles pointing truly to the two little
masses of the phosphate on their discs. We might imagine that we were
looking at a lowly organised animal seizing prey with its arms. In the
case of Drosera the explanation of this accurate power of movement, no
doubt, lies in the motor impulse radiating in all directions, and
whichever side of a tentacle it first strikes, that side contracts, and
the tentacle consequently bends towards the point of excitement. The
pedicels of the tentacles are flattened, or elliptic in section. Near
the bases of the short central tentacles, the flattened or broad face
is formed of about five longitudinal rows of cells; in the outer
tentacles of the disc it consists of about six or seven rows; and in
the extreme marginal tentacles of above a dozen rows. As the flattened
bases are thus formed of only a few rows of cells, the precision of the
movements of the tentacles is the more remarkable; for when the motor
impulse strikes the base of a tentacle in a very oblique direction
relatively to its broad face, scarcely more than one or two cells
towards one end can be affected at first, and the contraction of these
cells must draw the whole tentacle into the proper direction. It is,
perhaps, owing to the exterior pedicels being much flattened that they
do not bend quite so accurately to the point of excitement as the [page
247] more central ones. The properly directed movement of the tentacles
is not an unique case in the vegetable kingdom, for the tendrils of
many plants curve towards the side which is touched; but the case of
Drosera is far more interesting, as here the tentacles are not directly
excited, but receive an impulse from a distant point; nevertheless,
they bend accurately towards this point.
FIG. 11. (Drosera rotundifolia.) Diagram showing the distribution of
the vascular tissue in a small leaf.
On the Nature of the Tissues through which the Motor Impulse is
Transmitted. It will be necessary first to describe briefly the course
of the main fibro-vascular bundles. These are shown in the accompanying
sketch (fig. 11) of a small leaf. Little vessels from the neighbouring
bundles enter all the many tentacles with which the surface is studded;
but these are not here represented. The central trunk, which runs up
the footstalk, bifurcates near the centre of the leaf, each branch
bifurcating again and again according to the size of the leaf. This
central trunk sends off, low down on each side, a delicate branch,
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