ALDROVANDA VESICULOSA.
Captures crustaceans—Structure of the leaves in comparison with those
of Dionaea— Absorption by the glands, by the quadrifid processes, and
points on the infolded margins— Aldrovanda vesiculosa, var.
australis—Captures prey—Absorption of animal matter—Aldrovanda
vesiculosa, var. verticillata—Concluding remarks.
This plant may be called a miniature aquatic Dionaea. Stein discovered
in 1873 that the bilobed leaves, which are generally found closed in
Europe, open under a sufficiently high temperature, and, when touched,
suddenly close.* They re-expand in from 24 to 36 hours, but only, as it
appears, when inorganic objects are enclosed. The leaves sometimes
contain bubbles of air, and were formerly supposed to be bladders;
hence the specific name of vesiculosa. Stein observed that
water-insects were sometimes caught, and Prof. Cohn has recently found
within the leaves of naturally growing plants many kinds of crustaceans
and larvæ.** Plants which had been kept in filtered water were placed
by him in a vessel con-
* Since his original publication, Stein has found out that the
irritability of the leaves was observed by De Sassus, as recorded in
‘Bull. Bot. Soc. de France,’ in 1861. Delpino states in a paper
published in 1871 (‘Nuovo Giornale Bot. Ital.’ vol. iii. p. 174) that
“una quantit di chioccioline e di altri animalcoli acquatici” are
caught and suffocated by the leaves. I presume that chioccioline are
fresh-water molluscs. It would be interesting to know whether their
shells are at all corroded by the acid of the digestive secretion.
** I am greatly indebted to this distinguished naturalist for having
sent me a copy of his memoir on Aldrovanda, before its publication in
his ‘Beiträge zur Biologie der Pflanzen,’ drittes Heft, 1875, page 71.
[page 322]
taining numerous crustaceans of the genus Cypris, and next morning many
were found imprisoned and alive, still swimming about within the closed
leaves, but doomed to certain death.
Directly after reading Prof. Cohn’s memoir, I received through the
kindness of Dr. Hooker living plants from Germany. As I can add nothing
to Prof. Cohn’s excellent description, I will give only two
illustrations, one of a whorl of leaves copied from his work, and the
other of a leaf pressed flat open, drawn by my son Francis. I will,
however, append a few remarks on the differences between this plant and
Dionaea.
Aldrovanda is destitute of roots and floats freely in the water. The
leaves are arranged in whorls round the stem. Their broad petioles
terminate in from four to six rigid projections,* each tipped with a
stiff, short bristle. The bilobed leaf, with the midrib likewise tipped
with a bristle, stands in the midst of these projections, and is
evidently defended by them. The lobes are formed of very delicate
tissue, so as to be translucent; they open, according to Cohn, about as
much as the two valves of a living mussel-shell, therefore even less
than the lobes of Dionaea; and this must make the capture of aquatic
animals more easy. The outside of the leaves and the petioles are
covered with minute two-armed papillae, evidently answering to the
eight-rayed papillae of Dionaea.
Each lobe rather exceeds a semi-circle in convexity, and consists of
two very different concentric portions; the inner and lesser portion,
or that next to the midrib,
* There has been much discussion by botanists on the homological nature
of these projections. Dr. Nitschke (‘Bot. Zeitung,’ 1861, p. 146)
believes that they correspond with the fimbriated scale-like bodies
found at the bases of the petioles of Drosera. [page 323]
is slightly concave, and is formed, according to Cohn, of three layers
of cells. Its upper surface is studded with colourless glands like, but
more simple than, those of Dionaea; they are supported on distinct
footstalks, consisting of two rows of cells. The outer
FIG. 13. (Aldrovanda vesiculosa.) Upper figure, whorl of leaves (from
Prof. Cohn). Lower figure, leaf pressed flat open and greatly enlarged.
and broader portion of the lobe is flat and very thin, being formed of
only two layers of cells. Its upper surface does not bear any glands,
but, in their place, small quadrifid processes, each consisting of four
tapering projections, which rise from a common [page 324] prominence.
These processes are formed of very delicate membrane lined with a layer
of protoplasm; and they sometimes contain aggregated globules of
hyaline matter. Two of the slightly diverging arms are directed towards
the circumference, and two towards the midrib, forming together a sort
of Greek cross. Occasionally two of the arms are replaced by one, and
then the projection is trifid. We shall see in a future chapter that
these projections curiously resemble those found within the bladders of
Utricularia, more especially of Utricularia montana, although this
genus is not related to Aldrovanda.
A narrow rim of the broad flat exterior part of each lobe is turned
inwards, so that, when the lobes are closed, the exterior surfaces of
the infolded portions come into contact. The edge itself bears a row of
conical, flattened, transparent points with broad bases, like the
prickles on the stem of a bramble or Rubus. As the rim is infolded,
these points are directed towards the midrib, and they appear at first
as if they were adapted to prevent the escape of prey; but this can
hardly be their chief function, for they are composed of very delicate
and highly flexible membrane, which can be easily bent or quite doubled
back without being cracked. Nevertheless, the infolded rims, together
with the points, must somewhat interfere with the retrograde movement
of any small creature, as soon as the lobes begin to close. The
circumferential part of the leaf of Aldrovanda thus differs greatly
from that of Dionaea; nor can the points on the rim be considered as
homologous with the spikes round the leaves of Dionaea, as these latter
are prolongations of the blade, and not mere epidermic productions.
They appear also to serve for a widely different purpose. [page 325]
On the concave gland-bearing portion of the lobes, and especially on
the midrib, there are numerous, long, finely pointed hairs, which, as
Prof. Cohn remarks, there can be little doubt are sensitive to a touch,
and, when touched, cause the leaf to close. They are formed of two rows
of cells, or, according to Cohn, sometimes of four, and do not include
any vascular tissue. They differ also from the six sensitive filaments
of Dionaea in being colourless, and in having a medial as well as a
basal articulation. No doubt it is owing to these two articulations
that, notwithstanding their length, they escape being broken when the
lobes close.
The plants which I received during the early part of October from Kew
never opened their leaves, though subjected to a high temperature.
After examining the structure of some of them, I experimented on only
two, as I hoped that the plants would grow; and I now regret that I did
not sacrifice a greater number.
A leaf was cut open along the midrib, and the glands examined under a
high power. It was then placed in a few drops of an infusion of raw
meat. After 3 hrs. 20 m. there was no change, but when next examined
after 23 hrs. 20 m., the outer cells of the glands contained, instead
of limpid fluid, spherical masses of a granular substance, showing that
matter had been absorbed from the infusion. That these glands secrete a
fluid which dissolves or digests animal matter out of the bodies of the
creatures which the leaves capture, is also highly probable from the
analogy of Dionaea. If we may trust to the same analogy, the concave
and inner portions of the two lobes probably close together by a slow
movement, as soon as the glands have absorbed a slight amount of [page
326] already soluble animal matter. The included water would thus be
pressed out, and the secretion consequently not be too much diluted to
act. With respect to the quadrifid processes on the outer parts of the
lobes, I was not able to decide whether they had been acted on by the
infusion; for the lining of protoplasm was somewhat shrunk before they
were immersed. Many of the points on the infolded rims also had their
lining of protoplasm similarly shrunk, and contained spherical granules
of hyaline matter.
A solution of urea was next employed. This substance was chosen partly
because it is absorbed by the quadrifid processes and more especially
by the glands of Utricularia—a plant which, as we shall hereafter see,
feeds on decayed animal matter. As urea is one of the last products of
the chemical changes going on in the living body, it seems fitted to
represent the early stages of the decay of the dead body. I was also
led to try urea from a curious little fact mentioned by Prof. Cohn,
namely that when rather large crustaceans are caught between the
closing lobes, they are pressed so hard whilst making their escape that
they often void their sausage-shaped masses of excrement, which were
found within most of the leaves. These masses, no doubt, contain urea.
They would be left either on the broad outer surfaces of the lobes
where the quadrifids are situated, or within the closed concavity. In
the latter case, water charged with excrementitious and decaying matter
would be slowly forced outwards, and would bathe the quadrifids, if I
am right in believing that the concave lobes contract after a time like
those of Dionaea. Foul water would also be apt to ooze out at all
times, especially when bubbles of air were generated within the
concavity.
A leaf was cut open and examined, and the outer [page 327] cells of the
glands were found to contain only limpid fluid. Some of the quadrifids
included a few spherical granules, but several were transparent and
empty, and their positions were marked. This leaf was now immersed in a
little solution of one part of urea to 146 of water, or three grains to
the ounce. After 3 hrs. 40 m. there was no change either in the glands
or quadrifids; nor was there any certain change in the glands after 24
hrs.; so that, as far as one trial goes, urea does not act on them in
the same manner as an infusion of raw meat. It was different with the
quadrifids; for the lining of protoplasm, instead of presenting a
uniform texture, was now slightly shrunk, and exhibited in many places
minute, thickened, irregular, yellowish specks and ridges, exactly like
those which appear within the quadrifids of Utricularia when treated
with this same solution. Moreover, several of the quadrifids, which
were before empty, now contained moderately sized or very small, more
or less aggregated, globules of yellowish matter, as likewise occurs
under the same circumstances with Utricularia. Some of the points on
the infolded margins of the lobes were similarly affected; for their
lining of protoplasm was a little shrunk and included yellowish specks;
and those which were before empty now contained small spheres and
irregular masses of hyaline matter, more or less aggregated; so that
both the points on the margins and the quadrifids had absorbed matter
from the solution in the course of 24 hrs.; but to this subject I shall
recur. In another rather old leaf, to which nothing had been given, but
which had been kept in foul water, some of the quadrifids contained
aggregated translucent globules. These were not acted on by a solution
of one part of carbonate of ammonia to 218 of water; and this negative
result [page 328] agrees with what I have observed under similar
circumstances with Utricularia.
Aldrovanda vesiculosa, var. australis.—Dried leaves of this plant from
Queensland in Australia were sent me by Prof. Oliver from the herbarium
at Kew. Whether it ought to be considered as a distinct species or a
variety, cannot be told until the flowers are examined by a botanist.
The projections at the upper end of the petiole (from four to six in
number) are considerably longer relatively to the blade, and much more
attenuated than those of the European form. They are thickly covered
for a considerable space near their extremities with the upcurved
prickles, which are quite absent in the latter form; and they generally
bear on their tips two or three straight prickles instead of one. The
bilobed leaf appears also to be rather larger and somewhat broader,
with the pedicel by which it is attached to the upper end of the
petiole a little longer. The points on the infolded margins likewise
differ; they have narrower bases, and are more pointed; long and short
points also alternate with much more regularity than in the European
form. The glands and sensitive hairs are similar in the two forms. No
quadrifid processes could be seen on several of the leaves, but I do
not doubt that they were present, though indistinguishable from their
delicacy and from having shrivelled; for they were quite distinct on
one leaf under circumstances presently to be mentioned.
Some of the closed leaves contained no prey, but in one there was a
rather large beetle, which from its flattened tibiae I suppose was an
aquatic species, but was not allied to Colymbetes. All the softer
tissues of this beetle were completely dissolved, and its chitinous
integuments were as clean as if they had been [page 329] boiled in
caustic potash; so that it must have been enclosed for a considerable
time. The glands were browner and more opaque than those on other
leaves which had caught nothing; and the quadrifid processes, from
being partly filled with brown granular matter, could be plainly
distinguished, which was not the case, as already stated, on the other
leaves. Some of the points on the infolded margins likewise contained
brownish granular matter. We thus gain additional evidence that the
glands, the quadrifid processes, and the marginal points, all have the
power of absorbing matter, though probably of a different nature.
Within another leaf disintegrated remnants of a rather small animal,
not a crustacean, which had simple, strong, opaque mandibles, and a
large unarticulated chitinous coat, were present. Lumps of black
organic matter, possibly of a vegetable nature, were enclosed in two
other leaves; but in one of these there was also a small worm much
decayed. But the nature of partially digested and decayed bodies, which
have been pressed flat, long dried, and then soaked in water, cannot be
recognised easily. All the leaves contained unicellular and other
Algae, still of a greenish colour, which had evidently lived as
intruders, in the same manner as occurs, according to Cohn, within the
leaves of this plant in Germany.
Aldrovanda vesiculosa, var. verticillata.—Dr. King, Superintendent of
the Botanic Gardens, kindly sent me dried specimens collected near
Calcutta. This form was, I believe, considered by Wallich as a distinct
species, under the name of verticillata. It resembles the Australian
form much more nearly than the European; namely in the projections at
the upper end of the petiole being much attenuated and covered with
[page 330] upcurved prickles; they terminate also in two straight
little prickles. The bilobed leaves are, I believe, larger and
certainly broader even than those of the Australian form; so that the
greater convexity of their margins was conspicuous. The length of an
open leaf being taken at 100, the breadth of the Bengal form is nearly
173, of the Australian form 147, and of the German 134. The points on
the infolded margins are like those in the Australian form. Of the few
leaves which were examined, three contained entomostracan crustaceans.
Concluding Remarks.—The leaves of the three foregoing closely allied
species or varieties are manifestly adapted for catching living
creatures. With respect to the functions of the several parts, there
can be little doubt that the long jointed hairs are sensitive, like
those of Dionaea, and that, when touched, they cause the lobes to
close. That the glands secrete a true digestive fluid and afterwards
absorb the digested matter, is highly probable from the analogy of
Dionaea,—from the limpid fluid within their cells being aggregated into
spherical masses, after they had absorbed an infusion of raw meat,—from
their opaque and granular condition in the leaf, which had enclosed a
beetle for a long time,—and from the clean condition of the integuments
of this insect, as well as of crustaceans (as described by Cohn), which
have been long captured. Again, from the effect produced on the
quadrifid processes by an immersion for 24 hrs. in a solution of
urea,—from the presence of brown granular matter within the quadrifids
of the leaf in which the beetle had been caught,—and from the analogy
of Utricularia,—it is probable that these processes absorb
excrementitious and decaying animal matter. It is a more curious fact
that the points on [page 331] the infolded margins apparently serve to
absorb decayed animal matter in the same manner as the quadrifids. We
can thus understand the meaning of the infolded margins of the lobes
furnished with delicate points directed inwards, and of the broad,
flat, outer portions, bearing quadrifid processes; for these surfaces
must be liable to be irrigated by foul water flowing from the concavity
of the leaf when it contains dead animals. This would follow from
various causes,—from the gradual contraction of the concavity,—from
fluid in excess being secreted,—and from the generation of bubbles of
air. More observations are requisite on this head; but if this view is
correct, we have the remarkable case of different parts of the same
leaf serving for very different purposes—one part for true digestion,
and another for the absorption of decayed animal matter. We can thus
also understand how, by the gradual loss of either power, a plant might
be gradually adapted for the one function to the exclusion of the
other; and it will hereafter be shown that two genera, namely
Pinguicula and Utricularia, belonging to the same family, have been
adapted for these two different functions. [page 332]