ON THE STRUCTURE AND MOVEMENTS OF SOME OTHER SPECIES OF DROSERA.
Drosera anglica—Drosera intermedia—Drosera capensis—Drosera
spathulata—Drosera filiformis—Drosera binata—Concluding remarks.
I examined six other species of Drosera, some of them inhabitants of
distant countries, chiefly for the sake of ascertaining whether they
caught insects. This seemed the more necessary as the leaves of some of
the species differ to an extraordinary degree in shape from the rounded
ones of Drosera rotundifolia. In functional powers, however, they
differ very little.
[Drosera anglica (Hudson).*—The leaves of this species, which was sent
to me from Ireland, are much elongated, and gradually widen from the
footstalk to the bluntly pointed apex. They stand almost erect, and
their blades sometimes exceed 1 inch in length, whilst their breadth is
only the 1/5 of an inch. The glands of all the tentacles have the same
structure, so that the extreme marginal ones do not differ from the
others, as in the case of Drosera rotundifolia. When they are irritated
by being roughly touched, or by the pressure of minute inorganic
particles, or by contact with animal matter, or by the absorption of
carbonate of ammonia, the tentacles become inflected; the basal portion
being the chief seat of movement. Cutting or pricking the blade of the
leaf did not excite any movement. They frequently capture insects, and
the glands of the inflected tentacles pour forth much acid secretion.
Bits of roast meat were placed on some glands, and the tentacles began
to move in 1 m. or
* Mrs. Treat has given an excellent account in ‘The American
Naturalist,’ December 1873, p. 705, of Drosera longifolia (which is a
synonym in part of Drosera anglica), of Drosera rotundifolia and
filiformis. [page 279]
1 m. 30 s.; and in 1 hr. 10 m. reached the centre. Two bits of boiled
cork, one of boiled thread, and two of coal-cinders taken from the
fire, were placed, by the aid of an instrument which had been immersed
in boiling water, on five glands; these superfluous precautions having
been taken on account of M. Ziegler’s statements. One of the particles
of cinder caused some inflection in 8 hrs. 45 m., as did after 23 hrs.
the other particle of cinder, the bit of thread, and both bits of cork.
Three glands were touched half a dozen times with a needle; one of the
tentacles became well inflected in 17 m., and re-expanded after 24
hrs.; the two others never moved. The homogeneous fluid within the
cells of the tentacles undergoes aggregation after these have become
inflected; especially if given a solution of carbonate of ammonia; and
I observed the usual movements in the masses of protoplasm. In one
case, aggregation ensued in 1 hr. 10 m. after a tentacle had carried a
bit of meat to the centre. From these facts it is clear that the
tentacles of Drosera anglica behave like those of Drosera rotundifolia.
If an insect is placed on the central glands, or has been naturally
caught there, the apex of the leaf curls inwards. For instance, dead
flies were placed on three leaves near their bases, and after 24 hrs.
the previously straight apices were curled completely over, so as to
embrace and conceal the flies; they had therefore moved through an
angle of 180o. After three days the apex of one leaf, together with the
tentacles, began to re-expand. But as far as I have seen— and I made
many trials—the sides of the leaf are never inflected, and this is the
one functional difference between this species and Drosera
rotundifolia.
Drosera intermedia (Hayne).—This species is quite as common in some
parts of England as Drosera rotundifolia. It differs from Drosera
anglica, as far as the leaves are concerned, only in their smaller
size, and in their tips being generally a little reflexed. They capture
a large number of insects. The tentacles are excited into movement by
all the causes above specified; and aggregation ensues, with movement
of the protoplasmic masses. I have seen, through a lens, a tentacle
beginning to bend in less than a minute after a particle of raw meat
had been placed on the gland. The apex of the leaf curls over an
exciting object as in the case of Drosera anglica. Acid secretion is
copiously poured over captured insects. A leaf which had embraced a fly
with all its tentacles re-expanded after nearly three days.
Drosera capensis.—This species, a native of the Cape of Good Hope, was
sent to me by Dr. Hooker. The leaves are elongated, slightly concave
along the middle and taper towards the apex, [page 280] which is
bluntly pointed and reflexed. They rise from an almost woody axis, and
their greatest peculiarity consists in their foliaceous green
footstalks, which are almost as broad and even longer than the
gland-bearing blade. This species, therefore, probably draws more
nourishment from the air, and less from captured insects, than the
other species of the genus. Nevertheless, the tentacles are crowded
together on the disc, and are extremely numerous; those on the margins
being much longer than the central ones. All the glands have the same
form; their secretion is extremely viscid and acid.
The specimen which I examined had only just recovered from a weak state
of health. This may account for the tentacles moving very slowly when
particles of meat were placed on the glands, and perhaps for my never
succeeding in causing any movement by repeatedly touching them with a
needle. But with all the species of the genus this latter stimulus is
the least effective of any. Particles of glass, cork, and coal-cinders,
were placed on the glands of six tentacles; and one alone moved after
an interval of 2 hrs. 30 m. Nevertheless, two glands were extremely
sensitive to very small doses of the nitrate of ammonia, namely to
about 1/20 of a minim of a solution (one part to 5250 of water),
containing only 1/115200 of a grain (.000562 mg.) of the salt.
Fragments of flies were placed on two leaves near their tips, which
became incurved in 15 hrs. A fly was also placed in the middle of the
leaf; in a few hours the tentacles on each side embraced it, and in 8
hrs. the whole leaf directly beneath the fly was a little bent
transversely. By the next morning, after 23 hrs., the leaf was curled
so completely over that the apex rested on the upper end of the
footstalk. In no case did the sides of the leaves become inflected. A
crushed fly was placed on the foliaceous footstalk, but produced no
effect.
Drosera spathulata (sent to me by Dr. Hooker).—I made only a few
observations on this Australian species, which has long, narrow leaves,
gradually widening towards their tips. The glands of the extreme
marginal tentacles are elongated and differ from the others, as in the
case of Drosera rotundifolia. A fly was placed on a leaf, and in 18
hrs. it was embraced by the adjoining tentacles. Gum-water dropped on
several leaves produced no effect. A fragment of a leaf was immersed in
a few drops of a solution of one part of carbonate of ammonia to 146 of
water; all the glands were instantly blackened; the process of
aggregation could be seen travelling rapidly down the cells of the
tentacles; and the granules of protoplasm soon united into spheres and
variously shaped masses, which displayed the usual move- [page 281]
ments. Half a minim of a solution of one part of nitrate of ammonia to
146 of water was next placed on the centre of a leaf; after 6 hrs. some
marginal tentacles on both sides were inflected, and after 9 hrs. they
met in the centre. The lateral edges of the leaf also became incurved,
so that it formed a half-cylinder; but the apex of the leaf in none of
my few trials was inflected. The above dose of the nitrate (viz. 1/320
of a grain, or .202 mg.) was too powerful, for in the course of 23 hrs.
the leaf died.
Drosera filiformis.—This North American species grows in such abundance
in parts of New Jersey as almost to cover the ground. It catches,
according to Mrs. Treat,* an extraordinary number of small and large
insects, even great flies of the genus Asilus, moths, and butterflies.
The specimen which I examined, sent me by Dr. Hooker, had thread-like
leaves, from 6 to 12 inches in length, with the upper surface convex
and the lower flat and slightly channelled. The whole convex surface,
down to the roots—for there is no distinct footstalk—is covered with
short gland-bearing tentacles, those on the margins being the longest
and reflexed. Bits of meat placed on the glands of some tentacles
caused them to be slightly inflected in 20 m.; but the plant was not in
a vigorous state. After 6 hrs. they moved through an angle of 90o, and
in 24 hrs. reached the centre. The surrounding tentacles by this time
began to curve inwards. Ultimately a large drop of extremely viscid,
slightly acid secretion was poured over the meat from the united
glands. Several other glands were touched with a little saliva, and the
tentacles became incurved in under 1 hr., and re-expanded after 18 hrs.
Particles of glass, cork, cinders, thread, and gold-leaf, were placed
on numerous glands on two leaves; in about 1 hr. four tentacles became
curved, and four others after an additional interval of 2 hrs. 30 m. I
never once succeeded in causing any movement by repeatedly touching the
glands with a needle; and Mrs. Treat made similar trials for me with no
success. Small flies were placed on several leaves near their tips, but
the thread-like blade became only on one occasion very slightly bent,
directly beneath the insect. Perhaps this indicates that the blades of
vigorous plants would bend over captured insects, and Dr. Canby informs
me that this is the case; but the movement cannot be strongly
pronounced, as it was not observed by Mrs. Treat.
Drosera binata (or dichotoma).—I am much indebted to Lady
* ‘American Naturalist,’ December 1873, page 705. [page 282]
Dorothy Nevill for a fine plant of this almost gigantic Australian
species, which differs in some interesting points from those previously
described. In this specimen the rush-like footstalks of the leaves were
20 inches in length. The blade bifurcates at its junction with the
footstalk, and twice or thrice afterwards, curling about in an
irregular manner. It is narrow, being only 3/20 of an inch in breadth.
One blade was 7 1/2 inches long, so that the entire leaf, including the
footstalk, was above 27 inches in length. Both surfaces are slightly
hollowed out. The upper surface is covered with tentacles arranged in
alternate rows; those in the middle being short and crowded together,
those towards the margins longer, even twice or thrice as long as the
blade is broad. The glands of the exterior tentacles are of a much
darker red than those of the central ones. The pedicels of all are
green. The apex of the blade is attenuated, and bears very long
tentacles. Mr. Copland informs me that the leaves of a plant which he
kept for some years were generally covered with captured insects before
they withered.
The leaves do not differ in essential points of structure or of
function from those of the previously described species. Bits of meat
or a little saliva placed on the glands of the exterior tentacles
caused well-marked movement in 3 m., and particles of glass acted in 4
m. The tentacles with the latter particles re-expanded after 22 hrs. A
piece of leaf immersed in a few drops of a solution of one part of
carbonate of ammonia to 437 of water had all the glands blackened and
all the tentacles inflected in 5 m. A bit of raw meat, placed on
several glands in the medial furrow, was well clasped in 2 hrs. 10 m.
by the marginal tentacles on both sides. Bits of roast meat and small
flies did not act quite so quickly; and albumen and fibrin still less
quickly. One of the bits of meat excited so much secretion (which is
always acid) that it flowed some way down the medial furrow, causing
the inflection of the tentacles on both sides as far as it extended.
Particles of glass placed on the glands in the medial furrow did not
stimulate them sufficiently for any motor impulse to be sent to the
outer tentacles. In no case was the blade of the leaf, even the
attenuated apex, at all inflected.
On both the upper and lower surface of the blade there are numerous
minute, almost sessile glands, consisting of four, eight, or twelve
cells. On the lower surface they are pale purple, on the upper
greenish. Nearly similar organs occur on the foot-stalks, but they are
smaller and often in a shrivelled condition. The minute glands on the
blade can absorb rapidly: thus, a piece of leaf was immersed in a
solution of one part of carbonate [page 283] of ammonia to 218 of water
(1 gr. to 2 oz.), and in 5 m. they were all so much darkened as to be
almost black, with their contents aggregated. They do not, as far as I
could observe, secrete spontaneously; but in between 2 and 3 hrs. after
a leaf had been rubbed with a bit of raw meat moistened with saliva,
they seemed to be secreting freely; and this conclusion was afterwards
supported by other appearances. They are, therefore, homologous with
the sessile glands hereafter to be described on the leaves of Dionaea
and Drosophyllum. In this latter genus they are associated, as in the
present case, with glands which secrete spontaneously, that is, without
being excited.
Drosera binata presents another and more remarkable peculiarity,
namely, the presence of a few tentacles on the backs of the leaves,
near their margins. These are perfect in structure; spiral vessels run
up their pedicels; their glands are surrounded by drops of viscid
secretion, and they have the power of absorbing. This latter fact was
shown by the glands immediately becoming black, and the protoplasm
aggregated, when a leaf was placed in a little solution of one part of
carbonate of ammonia to 437 of water. These dorsal tentacles are short,
not being nearly so long as the marginal ones on the upper surface;
some of them are so short as almost to graduate into the minute sessile
glands. Their presence, number, and size, vary on different leaves, and
they are arranged rather irregularly. On the back of one leaf I counted
as many as twenty-one along one side.
These dorsal tentacles differ in one important respect from those on
the upper surface, namely, in not possessing any power of movement, in
whatever manner they may be stimulated. Thus, portions of four leaves
were placed at different times in solutions of carbonate of ammonia
(one part to 437 or 218 of water), and all the tentacles on the upper
surface soon became closely inflected; but the dorsal ones did not
move, though the leaves were left in the solution for many hours, and
though their glands from their blackened colour had obviously absorbed
some of the salt. Rather young leaves should be selected for such
trials, for the dorsal tentacles, as they grow old and begin to wither,
often spontaneously incline towards the middle of the leaf. If these
tentacles had possessed the power of movement, they would not have been
thus rendered more serviceable to the plant; for they are not long
enough to bend round the margin of the leaf so as to reach an insect
caught on the upper surface, Nor would it have been of any use if these
tentacles could have [page 284] moved towards the middle of the lower
surface, for there are no viscid glands there by which insects can be
caught. Although they have no power of movement, they are probably of
some use by absorbing animal matter from any minute insect which may be
caught by them, and by absorbing ammonia from the rain-water. But their
varying presence and size, and their irregular position, indicate that
they are not of much service, and that they are tending towards
abortion. In a future chapter we shall see that Drosophyllum, with its
elongated leaves, probably represents the condition of an early
progenitor of the genus Drosera; and none of the tentacles of
Drosophyllum, neither those on the upper nor lower surface of the
leaves, are capable of movement when excited, though they capture
numerous insects, which serve as nutriment. Therefore it seems that
Drosera binata has retained remnants of certain ancestral
characters—namely a few motionless tentacles on the backs of the
leaves, and fairly well developed sessile glands—which have been lost
by most or all of the other species of the genus.]
Concluding Remarks.—From what we have now seen, there can be little
doubt that most or probably all the species of Drosera are adapted for
catching insects by nearly the same means. Besides the two Australian
species above described, it is said* that two other species from this
country, namely Drosera pallida and Drosera sulphurea, “close their
leaves upon insects with great rapidity: and the same phenomenon is
manifested by an Indian species, D. lunata, and by several of those of
the Cape of Good Hope, especially by D. trinervis.” Another Australian
species, Drosera heterophylla (made by Lindley into a distinct genus,
Sondera) is remarkable from its peculiarly shaped leaves, but I know
nothing of its power of catching insects, for I have seen only dried
specimens. The leaves form minute flattened cups, with the footstalks
attached not to one margin, but to the bottom. The
* ‘Gardener’s Chronicle,’ 1874, p. 209. [page 285]
inner surface and the edges of the cups are studded with tentacles,
which include fibro-vascular bundles, rather different from those seen
by me in any other species; for some of the vessels are barred and
punctured, instead of being spiral. The glands secrete copiously,
judging from the quantity of dried secretion adhering to them. [page
286]