Insectivorous Plants | Page 9

odour of the secretion, I know not. I suspect from
the number of insects caught by the English species of Drosera, and
from what I have observed with some exotic species kept in my
greenhouse, that the odour is attractive. In this latter case the leaves
may be compared with a baited trap; in the former case with a trap laid
in a run frequented by game, but without any bait.
That the glands possess the power of absorption, is shown by their
almost instantaneously becoming dark-coloured when given a minute
quantity of carbonate of ammonia; the change of colour being chiefly
or exclusively due to the rapid aggregation of their contents. When
certain other fluids are added, they become pale-coloured. Their power
of absorption is, however, best shown by the widely different results
which follow, from placing drops of various nitrogenous and
non-nitrogenous fluids of the same density on the glands of the disc, or
on a single marginal gland; and likewise by the very different lengths
of time during which the tentacles remain inflected over objects, which
yield or do not yield soluble nitrogenous matter. This same conclusion
might indeed have been inferred from the structure and movements of
the leaves, which are so admirably adapted for capturing insects.
The absorption of animal matter from captured insects explains how
Drosera can flourish in extremely poor peaty soil,--in some cases where
nothing but [page 18] sphagnum moss grows, and mosses depend
altogether on the atmosphere for their nourishment. Although the
leaves at a hasty glance do not appear green, owing to the purple colour
of the tentacles, yet the upper and lower surfaces of the blade, the
pedicels of the central tentacles, and the petioles contain chlorophyll, so
that, no doubt, the plant obtains and assimilates carbonic acid from the
air. Nevertheless, considering the nature of the soil where it grows, the

supply of nitrogen would be extremely limited, or quite deficient,
unless the plant had the power of obtaining this important element from
captured insects. We can thus understand how it is that the roots are so
poorly developed. These usually consist of only two or three slightly
divided branches, from half to one inch in length, furnished with
absorbent hairs. It appears, therefore, that the roots serve only to imbibe
water; though, no doubt, they would absorb nutritious matter if present
in the soil; for as we shall hereafter see, they absorb a weak solution of
carbonate of ammonia. A plant of Drosera, with the edges of its leaves
curled inwards, so as to form a temporary stomach, with the glands of
the closely inflected tentacles pouring forth their acid secretion, which
dissolves animal matter, afterwards to be absorbed, may be said to feed
like an animal. But, differently from an animal, it drinks by means of
its roots; and it must drink largely, so as to retain many drops of viscid
fluid round the glands, sometimes as many as 260, exposed during the
whole day to a glaring sun. [page 19]

CHAPTER II.
THE MOVEMENTS OF THE TENTACLES FROM THE CONTACT
OF SOLID BODIES.
Inflection of the exterior tentacles owing to the glands of the disc being
excited by repeated touches, or by objects left in contact with
them--Difference in the action of bodies yielding and not yielding
soluble nitrogenous matter--Inflection of the exterior tentacles directly
caused by objects left in contact with their glands--Periods of
commencing inflection and of subsequent re-expansion--Extreme
minuteness of the particles causing inflection--Action under
water--Inflection of the exterior tentacles when their glands are excited
by repeated touches--Falling drops of water do not cause inflection.
I WILL give in this and the following chapters some of the many
experiments made, which best illustrate the manner and rate of
movement of the tentacles, when excited in various ways. The glands
alone in all ordinary cases 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. Strictly speaking, the
glands ought to be called irritable, as the term sensitive generally
implies consciousness; but no one supposes that the Sensitive-plant is
conscious, and as I have found the term convenient, I shall use it
without scruple. I will commence with the movements of the exterior
tentacles, when indirectly excited by stimulants applied to the glands of
the short tentacles on the disc. The exterior tentacles may be said in this
case to be indirectly excited, because their own glands are not directly
acted on. The stimulus proceeding from the glands of the disc acts on
the bending part of the [page 20] exterior tentacles, near their bases,
and does not (as will hereafter be proved) first travel