Encyclopaedia Britannica > Volume 13, Infant-Kant

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INSECTIVOROUS PLANTS
INSECTIVOROUS PLANTS. Insectivorous or, as
they are sometimes more correctly termed, carnivorous
plants are, like the parasites, the climbers, or the succulents,
a physiological assemblage belonging to a number of
distinct natural orders. They agree in the extraordinary
habit of adding to the supplies of nitrogenous material
afforded them in common with other plants by the soil and
atmosphere, by the capture and consumption of insects and
other small animals. The curious and varied mechanical
arrangements by which these supplies of animal food, are
obtained, the ways and degrees in which they are utilized,
and the remarkable chemical, histological, and electrical
phenomena which accompany these processes of prehension
and utilization, can only be understood by a separate and
somewhat detailed examination of the leading orders and
genera. It is convenient to follow the order adopted by
Mr Darwin in his work on Insectivorous Plants (Lond.,
1875), to which our knowledge of the subject is mainly
due, incorporating, however, as far as possible the leading
observations of other writers on the subject. We must
preface this, however, by a brief summary of the facts of
taxonomy and distribution.
Taxonomy.—The best known and most important order
—the Droseracex—~is placed among the calycifloral exogens,
and has obvious affinities with the Saxifragacex. It
includes six genera—Byblis, Roridula, Drosera, Droso-
phyllum, Aldrovanda, and Dionxa, of which the last
three are monotypic, i.e., include only one species. The
curious pitcher-plant, Cephalotus follicularis, is usually
raised to the dignity of a separate natural order Cephalotese,
though Bentham and Hooker (Gen. Plant.) place it among
the Ribesiacex. The Sarraceniacex are thalamiflorals,
and contain the genera Sarracenia, Darlingtonia, Heliam-
phora, while the true pitcher plants or Nepenthacese,
consisting of the single large genus Nepenthes, are placed
near the Aristolochiacex among the Apetalx. Finally the
genera Pinguicula, Utricularia, Genlisea, and Polypom-
pholix belong to the gamopetalous order Utricidarix. Thus
all the four leading divisions of the exogenous plants are
represented by apparently unrelated orders; certain
affinities, however, are alleged between Droseracex,
Sarraceniacex, and Nepenthacex.
Distribution.—While the large genus Drosera has an all
but world-wide distribution, its congeners are restricted
to well-defined and
usually compara¬
tively small areas.
Thus Drosophyllum
occurs only in
Portugal and Mo¬
rocco, Byblis in
tropical Australia,
and, although Al¬
drovanda is found
in Queensland, in
Bengal, and in
Europe, a wide dis¬
tribution explained
by its aquatic habit,
Dionxais restricted
to a few localities
in North and South
Carolina, mainly
around Wilming¬
ton. Cephalotus
occurs only near
Albany in Western pia —Leaf of Sundew (Drosera rotundifolia). x4.
Australia, Heliam- (After Darwin.)
phora on the Roraima Mountains in Venezuela, Darling¬
tonia on the Sierra Nevada of California, and these three
genera too are as yet monotypic; of Sarracenia, however,
there are six or eight known species scattered over the
eastern States of North America. The 36 species of
Nepenthes are mostly natives of the hotter parts of the
Indian Archipelago, but a few range into Ceylon, Bengal,
Cochin China, and some even occur in tropical Australia
on the one hand, and in the Seychelles and Madagascar on
the other. Pinguicula is abundant in the north temperate
zone, and ranges down the Andes as far as Patagonia; the
150 species of Utricularia are mostly aquatic, and some are
found in all save polar regions; their unimportant congeners,
Genlisea and Polypompholix, occur in tropical America and
south-western Australia respectively. It is remarkable that
all the insectivorous plants
agree in inhabiting damp
heaths, bogs, marshes, and
similar situations where water
is abundant,—a peculiarity
perhaps due to their habit of
copious secretion and conse¬
quent need of water.
Drosera. — The Common
Sundew (D. rotundifolia) has
extremely small roots, and
bears five or six radical leaves
horizontally extended in a
rosette around the flowers talk.
The upper surface of each leaf
is covered with gland-bearing
filaments or “ tentacles,” of
which there are on an average
about two hundred. Each Fig. 2.—Leaf of Sundew, enlarged,
gland IS Surrounded by a large fleeted over a hit of meat placed on
dew-like drop of a viscid but the disk. (After Darwin.)
transparent and glittering secretion, and the popular names
(Sundew, French Bossolis, German Sonnenthau) as well as
the Linnsean (from Spocros, dew) have been thus suggested.
The stalk of the tentacle has the essential structure of a leaf.
A small fibro-vascular bundle, consisting mainly of spiral
vessels, runs up through the stalk and is surrounded by a
Fig. 3.—Glands of Sundew magnified. (After Dodel-Port.) A, external aspect
with drop of secretion; it, internal structure.
layer of elongated parenchyma cells lined by a thin layer
of colourless circulating protoplasm, and filled with a homo¬
geneous fluid, tinted purple by a modification of chlorophyll
(erythrophyll, Sorby). The epidermis bears small multi
cellular prominences. The glandular head of the tentacle
contains a central mass of spirally thickened cells in im¬
mediate contact with the upper end of the fibro-vascular
bundle. Around these (but separated from them by a