_Drosera longifolia_ folds it leaves entirely around its victim, from
the apex down to the petiole after the manner of its vernation, but
in _Drosera rotundifolia_, whose marginal tentacles are longer, the
tentacles simply curve around the object, the glands touching the
substance, like so many mouths receiving nourishment. Experimented
upon with raw beef, the tentacles of healthy leaves, from within to
without, but in periods of time varying from six to eight or nine
hours, clasp firmly the beef, almost concealing it from view. Equally
vigorous leaves, however, made no move towards clasping a bit of dry
chalk, a chip of flint, or a lump of earth. Bits of raw apple cause
a curving of the tentacles, but very few of the glands are seen
touching them. It would seem, therefore, that these plants are really
carnivorous, preferring animal substances, which they, by the aid of
some ferment analogous to pepsin, which is secreted by the glands, are
able to absorb. A minute quantity of already soluble animal matter is
the exciting cause, and this must be taken in by the glands, or there
is no secretion of the fermenting material.
In all ordinary cases the glands alone are susceptible to excitement.
When excited, they do not themselves move or change form, but transmit
a motor impulse to the bending part of their own and adjoining
tentacles, and are thus carried towards the centre of the leaf.
Stimulants applied to the glands of the short tentacles on the disc
indirectly excite movement of the exterior tentacles, for the stimulus
of the glands of the disc acts on the bending part of the latter
tentacles, near their bases, and does not first travel up the pedicels
to the glands, to be then reflected back to the bending place. Some
influence, however, does travel up to the glands, causing them to
secrete most copiously, and the secretion to become acid, just such
an influence as that which in animals is transmitted along the nerves
to glands, modifying their power of secretion, independently of the
condition of the blood-vessels. Over organic substances that yield
soluble matter the tentacles remain clasped for a much longer time
than over those not acted upon by the secretion, or over inorganic
objects. That they have the power of rendering organic substances
soluble, that is, that they have the power of digestion, is no longer
a question of dispute. They certainly have this power, acting on
albuminous compounds in exactly the same manner as does the gastric
juice of mammals, the digested matter being afterwards absorbed. In
animals the digestion of albuminous compounds is effected by means of
a ferment, pepsin, together with weak hydrochloric acid, though almost
any acid will serve, yet neither pepsin nor an acid by itself has any
such power. It has been observed that when the glands of the disc are
excited by the contact of any object, especially of one containing
nitrogeneous matter, the outer tentacles and often the blade become
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