plants, in no way closely related together, have independently acquired
this same power.
As there exist several plants the glands of which cannot, as far as is
known, digest animal matter, yet can absorb salts of ammonia and animal
fluids, it is probable that this latter power forms the first stage
towards that of digestion. It might, however, happen, under certain
conditions, that a plant, after having acquired the power of digestion,
should degenerate into one capable only of absorbing animal matter in
solution, or in a state of decay, or the final products of decay,
namely the salts of ammonia. It would appear that this has actually
occurred to a partial extent with the leaves of Aldrovanda; the outer
parts of which possess absorbent organs, but no glands fitted for the
secretion of any digestive fluid, these being confined to the inner
parts.
Little light can be thrown on the gradual acquirement of the third
remarkable character possessed by the more highly developed genera of
the Droseraceae, namely the power of movement when excited. It should,
however, be borne in mind that leaves and their homologues, as well as
flower-peduncles, have gained this power, in innumerable instances,
independently of inheritance from any common parent form; for instance,
in tendril-bearers and leaf-climbers (i.e. plants with their leaves,
petioles and flower-peduncles, &c., modified for prehension) belonging
to a large [page 364] number of the most widely distinct orders, in the
leaves of the many plants which go to sleep at night, or move when
shaken, and in the irritable stamens and pistils of not a few species.
We may therefore infer that the power of movement can be by some means
readily acquired. Such movements imply irritability or sensitiveness,
but, as Cohn has remarked,* the tissues of the plants thus endowed do
not differ in any uniform manner from those of ordinary plants; it is
therefore probable that all leaves are to a slight degree irritable.
Even if an insect alights on a leaf, a slight molecular change is
probably transmitted to some distance across its tissue, with the sole
difference that no perceptible effect is produced. We have some
evidence in favour of this belief, for we know that a single touch on
the glands of Drosera does not excite inflection; yet it must produce
some effect, for if the glands have been immersed in a solution of
camphor, inflection follows within a shorter time than would have
followed from the effects of camphor alone. So again with Dionaea, the
313