DIONAEA MUSCIPULA.
Structure of the leaves—Sensitiveness of the filaments—Rapid movement
of the lobes caused by irritation of the filaments—Glands, their power
of secretion—Slow movement caused by the absorption of animal
matter—Evidence of absorption from the aggregated condition of the
glands—Digestive power of the secretion—Action of chloroform, ether,
and hydrocyanic acid—The manner in which insects are captured—Use of
the marginal spikes—Kinds of insects captured—The transmission of the
motor impulse and mechanism of the movements—Re-expansion of the lobes.
This plant, commonly called Venus’ fly-trap, from the rapidity and
force of its movements, is one of the most wonderful in the world.* It
is a member of the small family of the Droseraceae, and is found only
in the eastern part of North Carolina, growing in damp situations. The
roots are small; those of a moderately fine plant which I examined
consisted of two branches about 1 inch in length, springing from a
bulbous enlargement. They probably serve, as in the case of Drosera,
solely for the absorption of water; for a gardener, who has been very
successful in the cultivation of this plant, grows it, like an
epiphytic orchid, in well-drained damp moss without any soil.** The
form of the bilobed leaf, with its foliaceous footstalk, is shown in
the accompanying drawing (fig. 12).
* Dr. Hooker, in his address to the British Association at Belfast,
1874, has given so full an historical account of the observations which
have been published on the habits of this plant, that it would be
superfluous on my part to repeat them.
** ‘Gardener’s Chronicle,’ 1874, p. 464. [page 287]
The two lobes stand at rather less than a right angle to each other.
Three minute pointed processes or filaments, placed triangularly,
project from the upper surfaces of both; but I have seen two leaves
with four filaments on each side, and another with only two. These
filaments are remarkable from their extreme sensitiveness to a touch,
as shown not by their own movement, but by that of the lobes. The
margins of the leaf are prolonged into sharp rigid projections which I
will call spikes, into each of which a bundle
FIG. 12. (Dionaea muscipula.) Leaf viewed laterally in its expanded
state.
of spiral vessels enters. The spikes stand in such a position that,
when the lobes close, they inter-lock like the teeth of a rat-trap. The
midrib of the leaf, on the lower side, is strongly developed and
prominent.
The upper surface of the leaf is thickly covered, excepting towards the
margins, with minute glands of a reddish or purplish colour, the rest
of the leaf being green. There are no glands on the spikes, or on the
foliaceous footstalk, The glands are formed of from [page 288] twenty
to thirty polygonal cells, filled with purple fluid. Their upper
surface is convex. They stand on very short pedicels, into which spiral
vessels do not enter, in which respect they differ from the tentacles
of Drosera. They secrete, but only when excited by the absorption of
certain matters; and they have the power of absorption. Minute
projections, formed of eight divergent arms of a reddish-brown or
orange colour, and appearing under the microscope like elegant little
flowers, are scattered in considerable numbers over the foot-stalk, the
backs of the leaves, and the spikes, with a few on the upper surface of
the lobes. These octofid projections are no doubt homologous with the
papillae on the leaves of Drosera rotundifolia. There are also a few
very minute, simple, pointed hairs, about 7/12000 (.0148 mm.) of an
inch in length on the backs of the leaves.
The sensitive filaments are formed of several rows of elongated cells,
filled with purplish fluid. They are a little above the 1/20 of an inch
in length; are thin and delicate, and taper to a point. I examined the
bases of several, making sections of them, but no trace of the entrance
of any vessel could be seen. The apex is sometimes bifid or even
trifid, owing to a slight separation between the terminal pointed
cells. Towards the base there is constriction, formed of broader cells,
beneath which there is an articulation, supported on an enlarged base,
consisting of differently shaped polygonal cells. As the filaments
project at right angles to the surface of the leaf, they would have
been liable to be broken whenever the lobes closed together, had it not
been for the articulation which allows them to bend flat down.
These filaments, from their tips to their bases, are exquisitely
sensitive to a momentary touch. It is scarcely [page 289] possible to
touch them ever so lightly or quickly with any hard object without
causing the lobes to close. A piece of very delicate human hair, 2 1/2
inches in length, held dangling over a filament, and swayed to and fro
so as to touch it, did not excite any movement. But when a rather thick
cotton thread of the same length was similarly swayed, the lobes
closed. Pinches of fine wheaten flour, dropped from a height, produced
no effect. The above-mentioned hair was then fixed into a handle, and
cut off so that 1 inch projected; this length being sufficiently rigid
to support itself in a nearly horizontal line. The extremity was then
brought by a slow movement laterally into contact with the tip of a
filament, and the leaf instantly closed. On another occasion two or
three touches of the same kind were necessary before any movement
ensued. When we consider how flexible a fine hair is, we may form some
idea how slight must be the touch given by the extremity of a piece, 1
inch in length, moved slowly.
Although these filaments are so sensitive to a momentary and delicate
touch, they are far less sensitive than the glands of Drosera to
prolonged pressure. Several times I succeeded in placing on the tip of
a filament, by the aid of a needle moved with extreme slowness, bits of
rather thick human hair, and these did not excite movement, although
they were more than ten times as long as those which caused the
tentacles of Drosera to bend; and although in this latter case they
were largely supported by the dense secretion. On the other hand, the
glands of Drosera may be struck with a needle or any hard object, once,
twice, or even thrice, with considerable force, and no movement ensues.
This singular difference in the nature of the sensitiveness of the
filaments of Dionaea and of [page 290] the glands of Drosera evidently
stands in relation to the habits of the two plants. If a minute insect
alights with its delicate feet on the glands of Drosera, it is caught
by the viscid secretion, and the slight, though prolonged pressure,
gives notice of the presence of prey, which is secured by the slow
bending of the tentacles. On the other hand, the sensitive filaments of
Dionaea are not viscid, and the capture of insects can be assured only
by their sensitiveness to a momentary touch, followed by the rapid
closure of the lobes.
As just stated, the filaments are not glandular, and do not secrete.
Nor have they the power of absorption, as may be inferred from drops of
a solution of carbonate of ammonia (one part to 146 of water), placed
on two filaments, not producing any effect on the contents of their
cells, nor causing the lobes to close, When, however, a small portion
of a leaf with an attached filament was cut off and immersed in the
same solution, the fluid within the basal cells became almost instantly
aggregated into purplish or colourless, irregularly shaped masses of
matter. The process of aggregation gradually travelled up the filaments
from cell to cell to their extremities, that is in a reverse course to
what occurs in the tentacles of Drosera when their glands have been
excited. Several other filaments were cut off close to their bases, and
left for 1 hr. 30 m. in a weaker solution of one part of the carbonate
to 218 of water, and this caused aggregation in all the cells,
commencing as before at the bases of the filaments.
Long immersion of the filaments in distilled water likewise causes
aggregation. Nor is it rare to find the contents of a few of the
terminal cells in a spontaneously aggregated condition. The aggregated
[page 291] masses undergo incessant slow changes of form, uniting and
again separating; and some of them apparently revolve round their own
axes. A current of colourless granular protoplasm could also be seen
travelling round the walls of the cells. This current ceases to be
visible as soon as the contents are well aggregated; but it probably
still continues, though no longer visible, owing to all the granules in
the flowing layer having become united with the central masses. In all
these respects the filaments of Dionaea behave exactly like the
tentacles of Drosera.
Notwithstanding this similarity there is one remarkable difference. The
tentacles of Drosera, after their glands have been repeatedly touched,
or a particle of any kind has been placed on them, become inflected and
strongly aggregated. No such effect is produced by touching the
filaments of Dionaea; I compared, after an hour or two, some which had
been touched and some which had not, and others after twenty-five
hours, and there was no difference in the contents of the cells. The
leaves were kept open all the time by clips; so that the filaments were
not pressed against the opposite lobe.
Drops of water, or a thin broken stream, falling from a height on the
filaments, did not cause the blades to close; though these filaments
were afterwards proved to be highly sensitive. No doubt, as in the case
of Drosera, the plant is indifferent to the heaviest shower of rain.
Drops of a solution of a half an ounce of sugar to a fluid ounce of
water were repeatedly allowed to fall from a height on the filaments,
but produced no effect, unless they adhered to them. Again, I blew many
times through a fine pointed tube with my utmost force against the
filaments without any effect; such blowing being received [page 292]
with as much indifference as no doubt is a heavy gale of wind. We thus
see that the sensitiveness of the filaments is of a specialised nature,
being related to a momentary touch rather than to prolonged pressure;
and the touch must not be from fluids, such as air or water, but from
some solid object.
Although drops of water and of a moderately strong solution of sugar,
falling on the filaments, does not excite them, yet the immersion of a
leaf in pure water sometimes caused the lobes to close. One leaf was
left immersed for 1 hr. 10 m., and three other leaves for some minutes,
in water at temperatures varying between 59° and 65° (15° to 18°.3
Cent.) without any effect. One, however, of these four leaves, on being
gently withdrawn from the water, closed rather quickly. The three other
leaves were proved to be in good condition, as they closed when their
filaments were touched. Nevertheless two fresh leaves on being dipped
into water at 75° and 62 1/2° (23°.8 and 16°.9 Cent.) instantly closed.
These were then placed with their footstalks in water, and after 23
hrs. partially re-expanded; on touching their filaments one of them
closed. This latter leaf after an additional 24 hrs. again re-expanded,
and now, on the filaments of both leaves being touched, both closed. We
thus see that a short immersion in water does not at all injure the
leaves, but sometimes excites the lobes to close. The movement in the
above cases was evidently not caused by the temperature of the water.
It has been shown that long immersion causes the purple fluid within
the cells of the sensitive filaments to become aggregated; and the
tentacles of Drosera are acted on in the same manner by long immersion,
often being somewhat inflected. In both cases the result is probably
due to a slight degree of exosmose. [page 293]
I am confirmed in this belief by the effects of immersing a leaf of
Dionaea in a moderately strong solution of sugar; the leaf having been
previously left for 1 hr. 10 m. in water without any effect; for now
the lobes closed rather quickly, the tips of the marginal spikes
crossing in 2 m. 30 s., and the leaf being completely shut in 3 m.
Three leaves were then immersed in a solution of half an ounce of sugar
to a fluid ounce of water, and all three leaves closed quickly. As I
was doubtful whether this was due to the cells on the upper surface of
the lobes, or to the sensitive filaments, being acted on by exosmose,
one leaf was first tried by pouring a little of the same solution in
the furrow between the lobes over the midrib, which is the chief seat
of movement. It was left there for some time, but no movement ensued.
The whole upper surface of leaf was then painted (except close round
the bases of the sensitive filaments, which I could not do without risk
of touching them) with the same solution, but no effect was produced.
So that the cells on the upper surface are not thus affected. But when,
after many trials, I succeeded in getting a drop of the solution to
cling to one of the filaments, the leaf quickly closed. Hence we may, I
think, conclude that the solution causes fluid to pass out of the
delicate cells of the filaments by exosmose; and that this sets up some
molecular change in their contents, analogous to that which must be
produced by a touch.
The immersion of leaves in a solution of sugar affects them for a much
longer time than does an immersion in water, or a touch on the
filaments; for in these latter cases the lobes begin to re-expand in
less than a day. On the other hand, of the three leaves which were
immersed for a short time in the solution, and were then washed by
means of a syringe inserted [page 294] between the lobes, one
re-expanded after two days; a second after seven days; and the third
after nine days. The leaf which closed, owing to a drop of the solution
having adhered to one of the filaments, opened after two days.
I was surprised to find on two occasions that the heat from the rays of
the sun, concentrated by a lens on the bases of several filaments, so
that they were scorched and discoloured, did not cause any movement;
though the leaves were active, as they closed, though rather slowly,
when a filament on the opposite side was touched. On a third trial, a
fresh leaf closed after a time, though very slowly; the rate not being
increased by one of the filaments, which had not been injured, being
touched. After a day these three leaves opened, and were fairly
sensitive when the uninjured filaments were touched. The sudden
immersion of a leaf into boiling water does not cause it to close.
Judging from the analogy of Drosera, the heat in these several cases
was too great and too suddenly applied. The surface of the blade is
very slightly sensitive; It may be freely and roughly handled, without
any movement being caused. A leaf was scratched rather hard with a
needle, but did not close; but when the triangular space between the
three filaments on another leaf was similarly scratched, the lobes
closed. They always closed when the blade or midrib was deeply pricked
or cut. Inorganic bodies, even of large size, such as bits of stone,
glass, &c.—or organic bodies not containing soluble nitrogenous matter,
such as bits of wood, cork, moss,—or bodies containing soluble
nitrogenous matter, if perfectly dry, such as bits of meat, albumen,
gelatine, &c., may be long left (and many were tried) on the lobes, and
no movement is excited. The result, however, is widely different, as we
[page 295] shall presently see, if nitrogenous organic bodies which are
at all damp, are left on the lobes; for these then close by a slow and
gradual movement, very different from that caused by touching one of
the sensitive filaments. The footstalk is not in the least sensitive; a
pin may be driven through it, or it may be cut off, and no movement
follows.
The upper surface of the lobes, as already stated, is thickly covered
with small purplish, almost sessile glands. These have the power both
of secretion and absorption; but unlike those of Drosera, they do not
secrete until excited by the absorption of nitrogenous matter. No other
excitement, as far as I have seen, produces this effect. Objects, such
as bits of wood, cork, moss, paper, stone, or glass, may be left for a
length of time on the surface of a leaf, and it remains quite dry. Nor
does it make any difference if the lobes close over such objects. For
instance, some little balls of blotting paper were placed on a leaf,
and a filament was touched; and when after 24 hrs. the lobes began to
re-open, the balls were removed by the aid of thin pincers, and were
found perfectly dry. On the other hand, if a bit of damp meat or a
crushed fly is placed on the surface of an expanded leaf, the glands
after a time secrete freely. In one such case there was a little
secretion directly beneath the meat in 4 hrs.; and after an additional
3 hrs. there was a considerable quantity both under and close round it.
In another case, after 3 hrs. 40 m., the bit of meat was quite wet. But
none of the glands secreted, excepting those which actually touched the
meat or the secretion containing dissolved animal matter.
If, however, the lobes are made to close over a bit of meat or an
insect, the result is different, for the glands over the whole surface
of the leaf now secrete copiously. [page 296] As in this case the
glands on both sides are pressed against the meat or insect, the
secretion from the first is twice as great as when a bit of meat is
laid on the surface of one lobe; and as the two lobes come into almost
close contact, the secretion, containing dissolved animal matter,
spreads by capillary attraction, causing fresh glands on both sides to
begin secreting in a continually widening circle. The secretion is
almost colourless, slightly mucilaginous, and, judging by the manner in
which it coloured litmus paper, more strongly acid than that of
Drosera. It is so copious that on one occasion, when a leaf was cut
open, on which a small cube of albumen had been placed 45 hrs. before,
drops rolled off the leaf. On another occasion, in which a leaf with an
enclosed bit of roast meat spontaneously opened after eight days, there
was so much secretion in the furrow over the midrib that it trickled
down. A large crushed fly (Tipula) was placed on a leaf from which a
small portion at the base of one lobe had previously been cut away, so
that an opening was left; and through this, the secretion continued to
run down the footstalk during nine days,—that is, for as long a time as
it was observed. By forcing up one of the lobes, I was able to see some
distance between them, and all the glands within sight were secreting
freely.
We have seen that inorganic and non-nitrogenous objects placed on the
leaves do not excite any movement; but nitrogenous bodies, if in the
least degree damp, cause after several hours the lobes to close slowly.
Thus bits of quite dry meat and gelatine were placed at opposite ends
of the same leaf, and in the course of 24 hrs. excited neither
secretion nor movement. They were then dipped in water, their surfaces
dried on blotting paper, and replaced on the same [page 297] leaf, the
plant being now covered with a bell-glass. After 24 hrs. the damp meat
had excited some acid secretion, and the lobes at this end of the leaf
were almost shut. At the other end, where the damp gelatine lay, the
leaf was still quite open, nor had any secretion been excited; so that,
as with Drosera, gelatine is not nearly so exciting a substance as
meat. The secretion beneath the meat was tested by pushing a strip of
litmus paper under it (the filaments not being touched), and this
slight stimulus caused the leaf to shut. On the eleventh day it
reopened; but the end where the gelatine lay, expanded several hours
before the opposite end with the meat.
A second bit of roast meat, which appeared dry, though it had not been
purposely dried, was left for 24 hrs. on a leaf, caused neither
movement nor secretion. The plant in its pot was now covered with a
bell-glass, and the meat absorbed some moisture from the air; this
sufficed to excite acid secretion, and by the next morning the leaf was
closely shut. A third bit of meat, dried so as to be quite brittle, was
placed on a leaf under a bell-glass, and this also became in 24 hrs.
slightly damp, and excited some acid secretion, but no movement.
A rather large piece of perfectly dry albumen was left at one end of a
leaf for 24 hrs. without any effect. It was then soaked for a few
minutes in water, rolled about on blotting paper, and replaced on the
leaf; in 9 hrs. some slightly acid secretion was excited, and in 24
hrs. this end of the leaf was partially closed. The bit of albumen,
which was now surrounded by much secretion, was gently removed, and
although no filament was touched, the lobes closed. In this and the
previous case, it appears that the absorption of animal matter by the
glands renders [page 298] the surface of the leaf much more sensitive
to a touch than it is in its ordinary state; and this is a curious
fact. Two days afterwards the end of the leaf where nothing had been
placed began to open, and on the third day was much more open than the
opposite end where the albumen had lain.
Lastly, large drops of a solution of one part of carbonate of ammonia
to 146 of water were placed on some leaves, but no immediate movement
ensued. I did not then know of the slow movement caused by animal
matter, otherwise I should have observed the leaves for a longer time,
and they would probably have been found closed, though the solution
(judging from Drosera) was, perhaps, too strong.
From the foregoing cases it is certain that bits of meat and albumen,
if at all damp, excite not only the glands to secrete, but the lobes to
close. This movement is widely different from the rapid closure caused
by one of the filaments being touched. We shall see its importance when
we treat of the manner in which insects are captured. There is a great
contrast between Drosera and Dionaea in the effects produced by
mechanical irritation on the one hand, and the absorption of animal
matter on the other. Particles of glass placed on the glands of the
exterior tentacles of Drosera excite movement within nearly the same
time, as do particles of meat, the latter being rather the most
efficient; but when the glands of the disc have bits of meat given
them, they transmit a motor impulse to the exterior tentacles much more
quickly than do these glands when bearing inorganic particles, or when
irritated by repeated touches. On the other hand, with Dionaea,
touching the filaments excites incomparably quicker movement than the
absorption of animal matter by the glands. Nevertheless, in [page 299]
certain cases, this latter stimulus is the more powerful of the two. On
three occasions leaves were found which from some cause were torpid, so
that their lobes closed only slightly, however much their filaments
were irritated; but on inserting crushed insects between the lobes,
they became in a day closely shut.
The facts just given plainly show that the glands have the power of
absorption, for otherwise it is impossible that the leaves should be so
differently affected by non-nitrogenous and nitrogenous bodies, and
between these latter in a dry and damp condition. It is surprising how
slightly damp a bit of meat or albumen need be in order to excite
secretion and afterwards slow movement, and equally surprising how
minute a quantity of animal matter, when absorbed, suffices to produce
these two effects. It seems hardly credible, and yet it is certainly a
fact, that a bit of hard-boiled white of egg, first thoroughly dried,
then soaked for some minutes in water and rolled on blotting paper,
should yield in a few hours enough animal matter to the glands to cause
them to secrete, and afterwards the lobes to close. That the glands
have the power of absorption is likewise shown by the very different
lengths of time (as we shall presently see) during which the lobes
remain closed over insects and other bodies yielding soluble
nitrogenous matter, and over such as do not yield any. But there is
direct evidence of absorption in the condition of the glands which have
remained for some time in contact with animal matter. Thus bits of meat
and crushed insects were several times placed on glands, and these were
compared after some hours with other glands from distant parts of the
same leaf. The latter showed not a trace of aggregation, whereas those
which had been in contact with the animal matter were [page 300] well
aggregated. Aggregation may be seen to occur very quickly if a piece of
a leaf is immersed in a weak solution of carbonate of ammonia. Again,
small cubes of albumen and gelatine were left for eight days on a leaf,
which was then cut open. The whole surface was bathed with acid
secretion, and every cell in the many glands which were examined had
its contents aggregated in a beautiful manner into dark or pale purple,
or colourless globular masses of protoplasm. These underwent incessant
slow changes of forms; sometimes separating from one another and then
reuniting, exactly as in the cells of Drosera. Boiling water makes the
contents of the gland-cells white and opaque, but not so purely white
and porcelain-like as in the case of Drosera. How living insects, when
naturally caught, excite the glands to secrete so quickly as they do, I
know not; but I suppose that the great pressure to which they are
subjected forces a little excretion from either extremity of their
bodies, and we have seen that an extremely small amount of nitrogenous
matter is sufficient to excite the glands.
Before passing on to the subject of digestion, I may state that I
endeavoured to discover, with no success, the functions of the minute
octofid processes with which the leaves are studded. From facts
hereafter to be given in the chapters on Aldrovanda and Utricularia, it
seemed probable that they served to absorb decayed matter left by the
captured insects; but their position on the backs of the leaves and on
the footstalks rendered this almost impossible. Nevertheless, leaves
were immersed in a solution of one part of urea to 437 of water, and
after 24 hrs. the orange layer of protoplasm within the arms of these
processes did not appear more aggregated than in other speci- [page
301] mens kept in water, I then tried suspending a leaf in a bottle
over an excessively putrid infusion of raw meat, to see whether they
absorbed the vapour, but their contents were not affected.
Digestive Power of the Secretion.*—When a leaf closes over any object,
it may be said to form itself into a temporary stomach; and if the
object yields ever so little animal matter, this serves, to use
Schiff’s expression, as a peptogene, and the glands on the surface pour
forth their acid secretion, which acts like the gastric juice of
animals. As so many experiments were tried on the digestive power of
Drosera, only a few were made with Dionaea, but they were amply
sufficient to prove that it digests, This plant, moreover, is not so
well fitted as Drosera for observation, as the process goes on within
the closed lobes. Insects, even beetles, after being subjected to the
secretion for several days, are surprisingly softened, though their
chitinous coats are not corroded,
[Experiment 1.—A cube of albumen of 1/10 of an inch (2.540 mm.) was
placed at one end of a leaf, and at the other end an oblong piece of
gelatine, 1/5 of an inch (5.08 mm.) long, and
* Dr. W.M. Canby, of Wilmington, to whom I am much indebted for
information regarding Dionaea in its native home, has published in the
‘Gardener’s Monthly,’ Philadelphia, August 1868, some interesting
observations. He ascertained that the secretion digests animal matter,
such as the contents of insects, bits of meat, &c.; and that the
secretion is reabsorbed. He was also well aware that the lobes remain
closed for a much longer time when in contact with animal matter than
when made to shut by a mere touch, or over objects not yielding soluble
nutriment; and that in these latter cases the glands do not secrete.
The Rev. Dr. Curtis first observed (‘Boston Journal Nat. Hist.’ vol.
i., p. 123) the secretion from the glands. I may here add that a
gardener, Mr. Knight, is said (Kirby and Spencer’s ‘Introduction to
Entomology,’ 1818, vol. i., p. 295) to have found that a plant of the
Dionaea, on the leaves of which “he laid fine filaments of raw beef,
was much more luxuriant in its growth than others not so treated.”
[page 302]
1/10 broad; the leaf was then made to close. It was cut open after 45
hrs. The albumen was hard and compressed, with its angles only a little
rounded; the gelatine was corroded into an oval form; and both were
bathed in so much acid secretion that it dropped off the leaf. The
digestive process apparently is rather slower than in Drosera, and this
agrees with the length of time during which the leaves remain closed
over digestible objects.
Experiment 2.—A bit of albumen 1/10 of an inch square, but only 1/20 in
thickness, and a piece of gelatine of the same size as before, were
placed on a leaf, which eight days afterwards was cut open. The surface
was bathed with slightly adhesive, very acid secretion, and the glands
were all in an aggregated condition. Not a vestige of the albumen or
gelatine was left. Similarly sized pieces were placed at the same time
on wet moss on the same pot, so that they were subjected to nearly
similar conditions; after eight days these were brown, decayed, and
matted with fibres of mould, but had not disappeared.
Experiment 3.—A piece of albumen 3/20 of an inch (3.81 mm.) long, and
1/20 broad and thick, and a piece of gelatine of the same size as
before, were placed on another leaf, which was cut open after seven
days; not a vestige of either substance was left, and only a moderate
amount of secretion on the surface.
Experiment 4.—Pieces of albumen and gelatine, of the same size as in
the last experiment, were placed on a leaf, which spontaneously opened
after twelve days, and here again not a vestige of either was left, and
only a little secretion at one end of the midrib.
Experiment 5.—Pieces of albumen and gelatine of the same size were
placed on another leaf, which after twelve days was still firmly
closed, but had begun to wither; it was cut open, and contained nothing
except a vestige of brown matter where the albumen had lain.
Experiment 6.—A cube of albumen of 1/10 of an inch and a piece of
gelatine of the same size as before were placed on a leaf, which opened
spontaneously after thirteen days, The albumen, which was twice as
thick as in the latter experiments, was too large; for the glands in
contact with it were injured and were dropping off; a film also of
albumen of a brown colour, matted with mould, was left. All the
gelatine was absorbed, and there was only a little acid secretion left
on the midrib.
Experiment 7.—A bit of half roasted meat (not measured) and a bit of
gelatine were placed on the two ends of a leaf, which [page 303] opened
spontaneously after eleven days; a vestige of the meat was left, and
the surface of the leaf was here blackened; the gelatine had all
disappeared.
Experiment 8.—A bit of half roasted meat (not measured) was placed on a
leaf which was forcibly kept open by a clip, so that it was moistened
with the secretion (very acid) only on its lower surface. Nevertheless,
after only 22 1/2 hrs. it was surprisingly softened, when compared with
another bit of the same meat which had been kept damp.
Experiment 9.—A cube of 1/10 of an inch of very compact roasted beef
was placed on a leaf, which opened spontaneously after twelve days; so
much feebly acid secretion was left on the leaf that it trickled off.
The meat was completely disintegrated, but not all dissolved; there was
no mould. The little mass was placed under the microscope; some of the
fibrillae in the middle still exhibited transverse striae; others
showed not a vestige of striae; and every gradation could be traced
between these two states. Globules, apparently of fat, and some
undigested fibro-elastic tissue remained. The meat was thus in the same
state as that formerly described, which was half digested by Drosera.
Here, again, as in the case of albumen, the digestive process seems
slower than in Drosera. At the opposite end of the same leaf, a firmly
compressed pellet of bread had been placed; this was completely
disintegrated, I suppose, owing to the digestion of the gluten, but
seemed very little reduced in bulk.
Experiment 10.—A cube of 1/20 of an inch of cheese and another of
albumen were placed at opposite ends of the same leaf. After nine days
the lobes opened spontaneously a little at the end enclosing the
cheese, but hardly any or none was dissolved, though it was softened
and surrounded by secretion. Two days subsequently the end with the
albumen also opened spontaneously (i.e. eleven days after it was put
on), a mere trace in a blackened and dry condition being left.
Experiment 11.—The same experiment with cheese and albumen repeated on
another and rather torpid leaf. The lobes at the end with the cheese,
after an interval of six days, opened spontaneously a little; the cube
of cheese was much softened, but not dissolved, and but little, if at
all, reduced in size. Twelve hours afterwards the end with the albumen
opened, which now consisted of a large drop of transparent, not acid,
viscid fluid.
Experiment 12.—Same experiment as the two last, and here again the leaf
at the end enclosing the cheese opened before the [page 304] opposite
end with the albumen; but no further observations were made.
Experiment 13.—A globule of chemically prepared casein, about 1/10 of
an inch in diameter, was placed on a leaf, which spontaneously opened
after eight days. The casein now consisted of a soft sticky mass, very
little, if at all, reduced in size, but bathed in acid secretion.]
These experiments are sufficient to show that the secretion from the
glands of Dionaea dissolves albumen, gelatine, and meat, if too large
pieces are not given. Globules of fat and fibro-elastic tissue are not
digested. The secretion, with its dissolved matter, if not in excess,
is subsequently absorbed. On the other hand, although chemically
prepared casein and cheese (as in the case of Drosera) excite much acid
secretion, owing, I presume, to the absorption of some included
albuminous matter, these substances are not digested, and are not
appreciably, if at all, reduced in bulk.
[Effects of the Vapours of Chloroform, Sulphuric Ether, and Hydrocyanic
Acid.—A plant bearing one leaf was introduced into a large bottle with
a drachm (3.549 ml.) of chloroform, the mouth being imperfectly closed
with cotton-wool. The vapour caused in 1 m. the lobes to begin moving
at an imperceptibly slow rate; but in 3 m. the spikes crossed, and the
leaf was soon completely shut. The dose, however, was much too large,
for in between 2 and 3 hrs. the leaf appeared as if burnt, and soon
died.
Two leaves were exposed for 30 m. in a 2-oz: vessel to the vapour of 30
minims (1.774 ml.) of sulphuric ether. One leaf closed after a time, as
did the other whilst being removed from the vessel without being
touched. Both leaves were greatly injured. Another leaf, exposed for 20
m. to 15 minims of ether, closed its lobes to a certain extent, and the
sensitive filaments were now quite insensible. After 24 hrs. this leaf
recovered its sensibility, but was still rather torpid. A leaf exposed
in a large bottle for only 3 m. to ten drops was rendered insensible.
After 52 m. it recovered its sensibility, and when one of the filaments
was touched, the lobes closed. It began [page 305] to reopen after 20
hrs. Lastly another leaf was exposed for 4 m. to only four drops of the
ether; it was rendered insensible, and did not close when its filaments
were repeatedly touched, but closed when the end of the open leaf was
cut off. This shows either that the internal parts had not been
rendered insensible, or that an incision is a more powerful stimulus
than repeated touches on the filaments. Whether the larger doses of
chloroform and ether, which caused the leaves to close slowly, acted on
the sensitive filaments or on the leaf itself, I do not know.
Cyanide of potassium, when left in a bottle, generates prussic or
hydrocyanic acid. A leaf was exposed for 1 hr. 35 m. to the vapour thus
formed; and the glands became within this time so colourless and
shrunken as to be scarcely visible, and I at first thought that they
had all dropped off. The leaf was not rendered insensible; for as soon
as one of the filaments was touched it closed. It had, however,
suffered, for it did not reopen until nearly two days had passed, and
was not even then in the least sensitive. After an additional day it
recovered its powers, and closed on being touched and subsequently
reopened. Another leaf behaved in nearly the same manner after a
shorter exposure to this vapour.]
On the Manner in which Insects are caught.—We will now consider the
action of the leaves when insects happen to touch one of the sensitive
filaments. This often occurred in my greenhouse, but I do not know
whether insects are attracted in any special way by the leaves. They
are caught in large numbers by the plant in its native country. As soon
as a filament is touched, both lobes close with astonishing quickness;
and as they stand at less than a right angle to each other, they have a
good chance of catching any intruder. The angle between the blade and
footstalk does not change when the lobes close. The chief seat of
movement is near the midrib, but is not confined to this part; for, as
the lobes come together, each curves inwards across its whole breadth;
the marginal spikes however, not becoming curved. This move- [page 306]
ment of the whole lobe was well seen in a leaf to which a large fly had
been given, and from which a large portion had been cut off the end of
one lobe; so that the opposite lobe, meeting with no resistance in this
part, went on curving inwards much beyond the medial line. The whole of
the lobe, from which a portion had been cut, was afterwards removed,
and the opposite lobe now curled completely over, passing through an
angle of from 120o to 130o, so as to occupy a position almost at right
angles to that which it would have held had the opposite lobe been
present.
From the curving inwards of the two lobes, as they move towards each
other, the straight marginal spikes intercross by their tips at first,
and ultimately by their bases. The leaf is then completely shut and
encloses a shallow cavity. If it has been made to shut merely by one of
the sensitive filaments having been touched, or if it includes an
object not yielding soluble nitrogenous matter, the two lobes retain
their inwardly concave form until they re-expand. The re-expansion
under these circumstances—that is when no organic matter is
enclosed—was observed in ten cases. In all of these, the leaves
re-expanded to about two-thirds of the full extent in 24 hrs. from the
time of closure. Even the leaf from which a portion of one lobe had
been cut off opened to a slight degree within this same time. In one
case a leaf re-expanded to about two-thirds of the full extent in 7
hrs., and completely in 32 hrs.; but one of its filaments had been
touched merely with a hair just enough to cause the leaf to close. Of
these ten leaves only a few re-expanded completely in less than two
days, and two or three required even a little longer time. Before,
however, they fully re-expand, they are ready to close [page 307]
instantly if their sensitive filaments are touched. How many times a
leaf is capable of shutting and opening if no animal matter is left
enclosed, I do not know; but one leaf was made to close four times,
reopening afterwards, within six days, On the last occasion it caught a
fly, and then remained closed for many days.
This power of reopening quickly after the filaments have been
accidentally touched by blades of grass, or by objects blown on the
leaf by the wind, as occasionally happens in its native place,* must be
of some importance to the plant; for as long as a leaf remains closed,
it cannot of course capture an insect.
When the filaments are irritated and a leaf is made to shut over an
insect, a bit of meat, albumen, gelatine, casein, and, no doubt, any
other substance containing soluble nitrogenous matter, the lobes,
instead of remaining concave, thus including a concavity, slowly press
closely together throughout their whole breadth. As this takes place,
the margins gradually become a little everted, so that the spikes,
which at first intercrossed, at last project in two parallel rows. The
lobes press against each other with such force that I have seen a cube
of albumen much flattened, with distinct impressions of the little
prominent glands; but this latter circumstance may have been partly
caused by the corroding action of the secretion. So firmly do they
become pressed together that, if any large insect or other object has
been caught, a corresponding projection on the outside of the leaf is
distinctly visible. When the two lobes are thus completely shut, they
* According to Dr. Curtis, in ‘Boston Journal of Nat. Hist,’ vol. i
1837, p. 123. [page 308]
resist being opened, as by a thin wedge driven between them, with
astonishing force, and are generally ruptured rather than yield. If not
ruptured, they close again, as Dr. Canby informs me in a letter, “with
quite a loud flap.” But if the end of a leaf is held firmly between the
thumb and finger, or by a clip, so that the lobes cannot begin to
close, they exert, whilst in this position, very little force.
I thought at first that the gradual pressing together of the lobes was
caused exclusively by captured insects crawling over and repeatedly
irritating the sensitive filaments; and this view seemed the more
probable when I learnt from Dr. Burdon Sanderson that whenever the
filaments of a closed leaf are irritated, the normal electric current
is disturbed. Nevertheless, such irritation is by no means necessary,
for a dead insect, or a bit of meat, or of albumen, all act equally
well; proving that in these cases it is the absorption of animal matter
which excites the lobes slowly to press close together. We have seen
that the absorption of an extremely small quantity of such matter also
causes a fully expanded leaf to close slowly; and this movement is
clearly analogous to the slow pressing together of the concave lobes.
This latter action is of high functional importance to the plant, for
the glands on both sides are thus brought into contact with a captured
insect, and consequently secrete. The secretion with animal matter in
solution is then drawn by capillary attraction over the whole surface
of the leaf, causing all the glands to secrete and allowing them to
absorb the diffused animal matter. The movement, excited by the
absorption of such matter, though slow, suffices for its final purpose,
whilst the movement excited by one of the sensitive filaments being
touched is rapid, and this is indis- [page 309] pensable for the
capturing of insects. These two movements, excited by two such widely
different means, are thus both well adapted, like all the other
functions of the plant, for the purposes which they subserve.
There is another wide difference in the action of leaves which enclose
objects, such as bits of wood, cork, balls of paper, or which have had
their filaments merely touched, and those which enclose organic bodies
yielding soluble nitrogenous matter. In the former case the leaves, as
we have seen, open in under 24 hrs. and are then ready, even before
being fully-expanded, to shut again. But if they have closed over
nitrogen-yielding bodies, they remain closely shut for many days; and
after re-expanding are torpid, and never act again, or only after a
considerable interval of time. In four instances, leaves after catching
insects never reopened, but began to wither, remaining closed—in one
case for fifteen days over a fly; in a second, for twenty-four days,
though the fly was small; in a third for twenty-four days over a
woodlouse; and in a fourth, for thirty-five days over a large Tipula.
In two other cases leaves remained closed for at least nine days over
flies, and for how many more I do not know. It should, however, be
added that in two instances in which very small insects had been
naturally caught the leaf opened as quickly as if nothing had been
caught; and I suppose that this was due to such small insects not
having been crushed or not having excreted any animal matter, so that
the glands were not excited. Small angular bits of albumen and gelatine
were placed at both ends of three leaves, two of which remained closed
for thirteen and the other for twelve days. Two other leaves remained
closed over bits of [page 310] meat for eleven days, a third leaf for
eight days, and a fourth (but this had been cracked and injured) for
only six days. Bits of cheese, or casein, were placed at one end and
albumen at the other end of three leaves; and the ends with the former
opened after six, eight, and nine days, whilst the opposite ends opened
a little later. None of the above bits of meat, albumen, &c., exceeded
a cube of 1/10 of an inch (2.54 mm.) in size, and were sometimes
smaller; yet these small portions sufficed to keep the leaves closed
for many days. Dr. Canby informs me that leaves remain shut for a
longer time over insects than over meat; and from what I have seen, I
can well believe that this is the case, especially if the insects are
large.
In all the above cases, and in many others in which leaves remained
closed for a long but unknown period over insects naturally caught,
they were more or less torpid when they reopened. Generally they were
so torpid during many succeeding days that no excitement of the
filaments caused the least movement. In one instance, however, on the
day after a leaf opened which had clasped a fly, it closed with extreme
slowness when one of its filaments was touched; and although no object
was left enclosed, it was so torpid that it did not re-open for the
second time until 44 hrs. had elapsed. In a second case, a leaf which
had expanded after remaining closed for at least nine days over a fly,
when greatly irritated, moved one alone of its two lobes, and retained
this unusual position for the next two days. A third case offers the
strongest exception which I have observed; a leaf, after remaining
clasped for an unknown time over a fly, opened, and when one of its
filaments was touched, closed, though rather slowly. Dr. Canby, [page
311] who observed in the United States a large number of plants which,
although not in their native site, were probably more vigorous than my
plants, informs me that he has “several times known vigorous leaves to
devour their prey several times; but ordinarily twice, or, quite often,
once was enough to render them unserviceable.” Mrs. Treat, who
cultivated many plants in New Jersey, also informs me that “several
leaves caught successively three insects each, but most of them were
not able to digest the third fly, but died in the attempt. Five leaves,
however, digested each three flies, and closed over the fourth, but
died soon after the fourth capture. Many leaves did not digest even one
large insect.” It thus appears that the power of digestion is somewhat
limited, and it is certain that leaves always remain clasped for many
days over an insect, and do not recover their power of closing again
for many subsequent days. In this respect Dionaea differs from Drosera,
which catches and digests many insects after shorter intervals of time.
We are now prepared to understand the use of the marginal spikes, which
form so conspicuous a feature in the appearance of the plant (fig. 12,
p. 287), and which at first seemed to me in my ignorance useless
appendages. From the inward curvature of the lobes as they approach
each other, the tips of the marginal spikes first intercross, and
ultimately their bases. Until the edges of the lobes come into contact,
elongated spaces between the spikes, varying from the 1/15 to the 1/10
of an inch (1.693 to 2.54 mm.) in breadth, according to the size of the
leaf, are left open. Thus an insect, if its body is not thicker than
these measurements, can easily escape between the crossed spikes, when
disturbed by the closing lobes and in- [page 312] creasing darkness;
and one of my sons actually saw a small insect thus escaping. A
moderately large insect, on the other hand, if it tries to escape
between the bars will surely be pushed back again into its horrid
prison with closing walls, for the spikes continue to cross more and
more until the edges of the lobes come into contact. A very strong
insect, however, would be able to free itself, and Mrs. Treat saw this
effected by a rose-chafer (Macrodactylus subspinosus) in the United
States. Now it would manifestly be a great disadvantage to the plant to
waste many days in remaining clasped over a minute insect, and several
additional days or weeks in afterwards recovering its sensibility;
inasmuch as a minute insect would afford but little nutriment. It would
be far better for the plant to wait for a time until a moderately large
insect was captured, and to allow all the little ones to escape; and
this advantage is secured by the slowly intercrossing marginal spikes,
which act like the large meshes of a fishing-net, allowing the small
and useless fry to escape.
As I was anxious to know whether this view was correct—and as it seems
a good illustration of how cautious we ought to be in assuming, as I
had done with respect to the marginal spikes, that any fully developed
structure is useless—I applied to Dr. Canby. He visited the native site
of the plant, early in the season, before the leaves had grown to their
full size, and sent me fourteen leaves, containing naturally captured
insects. Four of these had caught rather small insects, viz. three of
them ants, and the fourth a rather small fly, but the other ten had all
caught large insects, namely, five elaters, two chrysomelas, a
curculio, a thick and broad spider, and a scolopendra. Out of these ten
insects, no less than eight [page 313] were beetles,* and out of the
whole fourteen there was only one, viz. a dipterous insect, which could
readily take flight. Drosera, on the other hand, lives chiefly on
insects which are good flyers, especially Diptera, caught by the aid of
its viscid secretion. But what most concerns us is the size of the ten
larger insects. Their average length from head to tail was .256 of an
inch, the lobes of the leaves being on an average .53 of an inch in
length, so that the insects were very nearly half as long as the leaves
within which they were enclosed. Only a few of these leaves, therefore,
had wasted their powers by capturing small prey, though it is probable
that many small insects had crawled over them and been caught, but had
then escaped through the bars.
The Transmission of the Motor Impulse, and Means of Movement.—It is
sufficient to touch any one of the six filaments to cause both lobes to
close, these becoming at the same time incurved throughout their whole
breadth. The stimulus must therefore radiate in all directions from any
one filament. It must also be transmitted with much rapidity across the
leaf, for in all ordinary cases both lobes close simultaneously, as far
as the eye can judge. Most physiologists believe that in irritable
plants the excitement is transmitted along, or in close connection
with, the fibro-vascular bundles. In Dionaea, the course of these
vessels (composed of spiral and ordinary vascular
* Dr. Canby remarks (‘Gardener’s Monthly,’ August 1868), “as a general
thing beetles and insects of that kind, though always killed, seem to
be too hard-shelled to serve as food, and after a short time are
rejected.” I am surprised at this statement, at least with respect to
such beetles as elaters, for the five which I examined were in an
extremely fragile and empty condition, as if all their internal parts
had been partially digested. Mrs. Treat informs me that the plants
which she cultivated in New Jersey chiefly caught Diptera. [page 314]
tissue) seems at first sight to favour this belief; for they run up the
midrib in a great bundle, sending off small bundles almost at right
angles on each side. These bifurcate occasionally as they extend
towards the margin, and close to the margin small branches from
adjoining vessels unite and enter the marginal spikes. At some of these
points of union the vessels form curious loops, like those described
under Drosera. A continuous zigzag line of vessels thus runs round the
whole circumference of the leaf, and in the midrib all the vessels are
in close contact; so that all parts of the leaf seem to be brought into
some degree of communication. Nevertheless, the presence of vessels is
not necessary for the transmission of the motor impulse, for it is
transmitted from the tips of the sensitive filaments (these being about
the 1/20 of an inch in length), into which no vessels enter; and these
could not have been overlooked, as I made thin vertical sections of the
leaf at the bases of the filaments.
On several occasions, slits about the 1/10 of an inch in length were
made with a lancet, close to the bases of the filaments, parallel to
the midrib, and, therefore, directly across the course of the vessels.
These were made sometimes on the inner and sometimes on the outer sides
of the filaments; and after several days, when the leaves had reopened,
these filaments were touched roughly (for they were always rendered in
some degree torpid by the operation), and the lobes then closed in the
ordinary manner, though slowly, and sometimes not until after a
considerable interval of time. These cases show that the motor impulse
is not transmitted along the vessels, and they further show that there
is no necessity for a direct line of communication from the filament
which is [page 315] touched towards the midrib and opposite lobe, or
towards the outer parts of the same lobe.
Two slits near each other, both parallel to the midrib, were next made
in the same manner as before, one on each side of the base of a
filament, on five distinct leaves, so that a little slip bearing a
filament was connected with the rest of the leaf only at its two ends.
These slips were nearly of the same size; one was carefully measured;
it was .12 of an inch (3.048 mm.) in length, and .08 of an inch (2.032
mm.) in breadth; and in the middle stood the filament. Only one of
these slips withered and perished. After the leaf had recovered from
the operation, though the slits were still open, the filaments thus
circumstanced were roughly touched, and both lobes, or one alone,
slowly closed. In two instances touching the filament produced no
effect; but when the point of a needle was driven into the slip at the
base of the filament, the lobes slowly closed. Now in these cases the
impulse must have proceeded along the slip in a line parallel to the
midrib, and then have radiated forth, either from both ends or from one
end alone of the slip, over the whole surface of the two lobes.
Again, two parallel slits, like the former ones, were made, one on each
side of the base of a filament, at right angles to the midrib. After
the leaves (two in number) had recovered, the filaments were roughly
touched, and the lobes slowly closed; and here the impulse must have
travelled for a short distance in a line at right angles to the midrib,
and then have radiated forth on all sides over both lobes. These
several cases prove that the motor impulse travels in all directions
through the cellular tissue, independently of the course of the
vessels.
With Drosera we have seen that the motor impulse [page 316] is
transmitted in like manner in all directions through the cellular
tissue; but that its rate is largely governed by the length of the
cells and the direction of their longer axes. Thin sections of a leaf
of Dionaea were made by my son, and the cells, both those of the
central and of the more superficial layers, were found much elongated,
with their longer axes directed towards the midrib; and it is in this
direction that the motor impulse must be sent with great rapidity from
one lobe to the other, as both close simultaneously. The central
parenchymatous cells are larger, more loosely attached together, and
have more delicate walls than the more superficial cells. A thick mass
of cellular tissue forms the upper surface of the midrib over the great
central bundle of vessels.
When the filaments were roughly touched, at the bases of which slits
had been made, either on both sides or on one side, parallel to the
midrib or at right angles to it, the two lobes, or only one, moved. In
one of these cases, the lobe on the side which bore the filament that
was touched moved, but in three other cases the opposite lobe alone
moved; so that an injury which was sufficient to prevent a lobe moving
did not prevent the transmission from it of a stimulus which excited
the opposite lobe to move. We thus also learn that, although normally
both lobes move together, each has the power of independent movement. A
case, indeed, has already been given of a torpid leaf that had lately
re-opened after catching an insect, of which one lobe alone moved when
irritated. Moreover, one end of the same lobe can close and re- expand,
independently of the other end, as was seen in some of the foregoing
experiments.
When the lobes, which are rather thick, close, no trace of wrinkling
can be seen on any part of their upper [page 317] surfaces, It appears
therefore that the cells must contract. The chief seat of the movement
is evidently in the thick mass of cells which overlies the central
bundle of vessels in the midrib. To ascertain whether this part
contracts, a leaf was fastened on the stage of the microscope in such a
manner that the two lobes could not become quite shut, and having made
two minute black dots on the midrib, in a transverse line and a little
towards one side, they were found by the micrometer to be 17/1000 of an
inch apart. One of the filaments was then touched and the lobes closed;
but as they were prevented from meeting, I could still see the two
dots, which now were 15/1000 of an inch apart, so that a small portion
of the upper surface of the midrib had contracted in a transverse line
2/1000 of an inch (.0508 mm.).
We know that the lobes, whilst closing, become slightly incurved
throughout their whole breadth. This movement appears to be due to the
contraction of the superficial layers of cells over the whole upper
surface. In order to observe their contraction, a narrow strip was cut
out of one lobe at right angles to the midrib, so that the surface of
the opposite lobe could be seen in this part when the leaf was shut.
After the leaf had recovered from the operation and had re-expanded,
three minute black dots were made on the surface opposite to the slit
or window, in a line at right angles to the midrib. The distance
between the dots was found to be 40/1000 of an inch, so that the two
extreme dots were 80/1000 of an inch apart. One of the filaments was
now touched and the leaf closed. On again measuring the distances
between the dots, the two next to the midrib were nearer together by 1
to 2/1000 of an inch, and the two further dots by 3 to 4/1000 of an
inch, than they were before; so that the two extreme [page 318] dots
now stood about 5/1000 of an inch (.127 mm.) nearer together than
before. If we suppose the whole upper surface of the lobe, which was
400/1000 of an inch in breadth, to have contracted in the same
proportion, the total contraction will have amounted to about 25/1000
or 1/40 of an inch (.635 mm.); but whether this is sufficient to
account for the slight inward curvature of the whole lobe, I am unable
to say.
Finally, with respect to the movement of the leaves, the wonderful
discovery made by Dr. Burdon Sanderson* is now universally known;
namely that there exists a normal electrical current in the blade and
footstalk; and that when the leaves are irritated, the current is
disturbed in the same manner as takes place during the contraction of
the muscle of an animal.
The Re-expansion of the Leaves.—This is effected at an insensibly slow
rate, whether or not any object is enclosed.** One lobe can re-expand
by itself, as occurred with the torpid leaf of which one lobe alone had
closed. We have also seen in the experiments with cheese and albumen
that the two ends of the same lobe can re-expand to a certain extent
independently of each other. But in all ordinary cases both lobes open
at the same time. The re-expansion is not determined by the sensitive
filaments; all three filaments on one lobe were cut off close to their
bases; and the three
* Proc. Royal Soc.’ vol. xxi. p. 495; and lecture at the Royal
Institution, June 5, 1874, given in ‘Nature,’ 1874, pp. 105 and 127.
** Nuttall, in his ‘Gen. American Plants,’ p. 277 (note), says that,
whilst collecting this plant in its native home, “I had occasion to
observe that a detached leaf would make repeated efforts towards
disclosing itself to the influence of the sun; these attempts consisted
in an undulatory motion of the marginal ciliae, accompanied by a
partial opening and succeeding collapse of the lamina, which at length
terminated in a complete expansion and in the destruction of
sensibility.” I am indebted to Prof. Oliver for this reference; but I
do not understand what took place. [page 319]
leaves thus treated re-expanded,—one to a partial extent in 24 hrs.,—a
second to the same extent in 48 hrs., and the third, which had been
previously injured, not until the sixth day. These leaves after their
re-expansion closed quickly when the filaments on the other lobe were
irritated. These were then cut off one of the leaves, so that none were
left. This mutilated leaf, notwithstanding the loss of all its
filaments, re-expanded in two days in the usual manner. When the
filaments have been excited by immersion in a solution of sugar, the
lobes do not expand so soon as when the filaments have been merely
touched; and this, I presume, is due to their having been strongly
affected through exosmose, so that they continue for some time to
transmit a motor impulse to the upper surface of the leaf.
The following facts make me believe that the several layers of cells
forming the lower surface of the leaf are always in a state of tension;
and that it is owing to this mechanical state, aided probably by fresh
fluid being attracted into the cells, that the lobes begin to separate
or expand as soon as the contraction of the upper surface diminishes. A
leaf was cut off and suddenly plunged perpendicularly into boiling
water: I expected that the lobes would have closed, but instead of
doing so, they diverged a little. I then took another fine leaf, with
the lobes standing at an angle of nearly 80o to each other; and on
immersing it as before, the angle suddenly increased to 90o. A third
leaf was torpid from having recently re-expanded after having caught a
fly, so that repeated touches of the filaments caused not the least
movement; nevertheless, when similarly immersed, the lobes separated a
little. As these leaves were inserted perpendicularly into the boiling
water, both surfaces and the filaments [page 320] must have been
equally affected; and I can understand the divergence of the lobes only
by supposing that the cells on the lower side, owing to their state of
tension, acted mechanically and thus suddenly drew the lobes a little
apart, as soon as the cells on the upper surface were killed and lost
their contractile power. We have seen that boiling water in like manner
causes the tentacles of Drosera to curve backwards; and this is an
analogous movement to the divergence of the lobes of Dionaea.
In some concluding remarks in the fifteenth chapter on the Droseraceae,
the different kinds of irritability possessed by the several genera,
and the different manner in which they capture insects, will be
compared. [page 321]