CHAPTER II. THE MOVEMENTS OF THE TENTACLES FROM THE CONTACT OF SOLID BODIES.
CHAPTER III. AGGREGATION OF THE PROTOPLASM WITHIN THE CELLS OF THE TENTACLES.
CHAPTER IV. THE EFFECTS OF HEAT ON THE LEAVES.
CHAPTER V. THE EFFECTS OF NON-NITROGENOUS AND NITROGENOUS ORGANIC FLUIDS ON THE LEAVES.
CHAPTER VI. THE DIGESTIVE POWER OF THE SECRETION OF DROSERA.
CHAPTER VII. THE EFFECTS OF SALTS OF AMMONIA.
CHAPTER VIII. THE EFFECTS OF VARIOUS OTHER SALTS AND ACIDS ON THE LEAVES.
CHAPTER IX. THE EFFECTS OF CERTAIN ALKALOID POISONS, OTHER SUBSTANCES AND VAPOURS.
CHAPTER X. ON THE SENSITIVENESS OF THE LEAVES, AND ON THE LINES OF TRANSMISSION OF THE MOTOR IMPULSE.
CHAPTER XI. RECAPITULATION OF THE CHIEF OBSERVATIONS ON DROSERA ROTUNDIFOLIA.
CHAPTER XII. ON THE STRUCTURE AND MOVEMENTS OF SOME OTHER SPECIES OF DROSERA.
CHAPTER XIII. DIONAEA MUSCIPULA.
CHAPTER XIV. ALDROVANDA VESICULOSA.
CHAPTER XV. DROSOPHYLLUM—RORIDULA—BYBLIS—GLANDULAR HAIRS OF OTHER PLANTS—CONCLUDING REMARKS ON THE DROSERACEÆ.
CHAPTER XVI. PINGUICULA.
CHAPTER XVII. UTRICULARIA.
CHAPTER XVIII. UTRICULARIA (continued).
CONCLUSION.
INDEX.
DETAILED TABLE OF CONTENTS.
CHAPTER I.
DROSERA ROTUNDIFOLIA, OR THE COMMON SUN-DEW.
Number of insects captured—Description of the leaves and their
appendages or tentacles— Preliminary sketch of the action of the
various parts, and of the manner in which insects are captured—Duration
of the inflection of the tentacles—Nature of the secretion—Manner in
which insects are carried to the centre of the leaf—Evidence that the
glands have the power of absorption—Small size of the roots.
CHAPTER II.
THE MOVEMENTS OF THE TENTACLES FROM THE CONTACT OF SOLID BODIES.
Inflection of the exterior tentacles owing to the glands of the disc
being excited by repeated touches, or by objects left in contact with
them—Difference in the action of bodies yielding and not yielding
soluble nitrogenous matter—Inflection of the exterior tentacles
directly caused by objects left in contact with their glands—Periods of
commencing inflection and of subsequent re-expansion—Extreme minuteness
of the particles causing inflection—Action under water—Inflection of
the exterior tentacles when their glands are excited by repeated
touches—Falling drops of water do not cause inflection.
CHAPTER III.
AGGREGATION OF THE PROTOPLASM WITHIN THE CELLS OF THE TENTACLES.
Nature of the contents of the cells before aggregation—Various causes
which excite aggregation—The process commences within the glands and
travels down the tentacles— Description of the aggregated masses and of
their spontaneous movements—Currents of protoplasm along the walls of
the cells—Action of carbonate of ammonia—The granules in the protoplasm
which flows along the walls coalesce with the central masses—Minuteness
of the quantity of carbonate of ammonia causing aggregation—Action of
other salts of ammonia—Of other substances, organic fluids, &c.—Of
water—Of heat—Redissolution of the aggregated masses—Proximate causes
of the aggregation of the protoplasm—Summary and concluding
remarks—Supplementary observations on aggregation in the roots of
plants.
CHAPTER IV.
THE EFFECTS OF HEAT ON THE LEAVES.
Nature of the experiments—Effects of boiling water—Warm water causes
rapid inflection— Water at a higher temperature does not cause
immediate inflection, but does not kill the leaves, as shown by their
subsequent re-expansion and by the aggregation of the protoplasm— A
still higher temperature kills the leaves and coagulates the albuminous
contents of the glands.
CHAPTER V.
THE EFFECTS OF NON-NITROGENOUS AND NITROGENOUS ORGANIC FLUIDS ON THE
LEAVES.
Non-nitrogenous fluids—Solutions of gum arabic—Sugar—Starch—Diluted
alcohol—Olive oil— Infusion and decoction of tea—Nitrogenous
fluids—Milk—Urine—Liquid albumen—Infusion of raw meat—Impure
mucus—Saliva—Solution of isinglass—Difference in the action of these
two sets of fluids—Decoction of green peas—Decoction and infusion of
cabbage—Decoction of grass leaves.
CHAPTER VI.
THE DIGESTIVE POWER OF THE SECRETION OF DROSERA.
The secretion rendered acid by the direct and indirect excitement of
the glands—Nature of the acid—Digestible substances—Albumen, its
digestion arrested by alkalies, recommences by the addition of an
acid—Meat—Fibrin—Syntonin—Areolar tissue—Cartilage—Fibro-cartilage—
Bone—Enamel and dentine—Phosphate of lime—Fibrous basis of
bone—Gelatine—Chondrin— Milk, casein and
cheese—Gluten—Legumin—Pollen—Globulin—Haematin—Indigestible
substances—Epidermic productions—Fibro-elastic
tissue—Mucin—Pepsin—Urea—Chitine— Cellulose—Gun-cotton—Chlorophyll—Fat
and oil—Starch—Action of the secretion on living seeds—Summary and
concluding remarks.
CHAPTER VII.
THE EFFECTS OF SALTS OF AMMONIA.
Manner of performing the experiments—Action of distilled water in
comparison with the solutions—Carbonate of ammonia, absorbed by the
roots—The vapour absorbed by the glands—Drops on the disc—Minute drops
applied to separate glands—Leaves immersed in weak solutions—Minuteness
of the doses which induce aggregation of the protoplasm—Nitrate of
ammonia, analogous experiments with—Phosphate of ammonia, analogous
experiments with—Other salts of ammonia—Summary and concluding remarks
on the action of salts of ammonia.
CHAPTER VIII.
THE EFFECTS OF VARIOUS OTHER SALTS, AND ACIDS, ON THE LEAVES.
Salts of sodium, potassium, and other alkaline, earthy, and metallic
salts—Summary on the action of these salts—Various acids—Summary on
their action.
CHAPTER IX.
THE EFFECTS OF CERTAIN ALKALOID POISONS, OTHER SUBSTANCES AND VAPOURS.
Strychnine, salts of—Quinine, sulphate of, does not soon arrest the
movement of the protoplasm—Other salts of
quinine—Digitaline—Nicotine—Atropine—Veratrine— Colchicine—
Theine—Curare—Morphia—Hyoscyamus—Poison of the cobra, apparently
accelerates the movements of the protoplasm—Camphor, a powerful
stimulant, its vapour narcotic—Certain essential oils excite
movement—Glycerine—Water and certain solutions retard or prevent the
subsequent action of phosphate of ammonia—Alcohol innocuous, its vapour
narcotic and poisonous—Chloroform, sulphuric and nitric ether, their
stimulant, poisonous, and narcotic power—Carbonic acid narcotic, not
quickly poisonous—Concluding remarks.
CHAPTER X.
ON THE SENSITIVENESS OF THE LEAVES, AND ON THE LINES OF TRANSMISSION OF
THE MOTOR IMPULSE.
Glands and summits of the tentacles alone sensitive—Transmission of the
motor impulse down the pedicels of the tentacles, and across the blade
of the leaf—Aggregation of the protoplasm, a reflex action—First
discharge of the motor impulse sudden—Direction of the movements of the
tentacles—Motor impulse transmitted through the cellular tissue—
Mechanism of the movements—Nature of the motor impulse—Re-expansion of
the tentacles.
CHAPTER XI.
RECAPITULATION OF THE CHIEF OBSERVATIONS ON DROSERA ROTUNDIFOLIA.
CHAPTER XII.
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.
CHAPTER XIII.
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.
CHAPTER XIV.
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.
CHAPTER XV.
DROSOPHYLLUM—RORIDULA—BYBLIS—GLANDULAR HAIRS OF OTHER PLANTS—
CONCLUDING REMARKS ON THE DROSERACEÆ.
Drosophyllum—Structure of leaves—Nature of the secretion—Manner of
catching insects— Power of absorption—Digestion of animal
substances—Summary on Drosophyllum—Roridula—Byblis—Glandular hairs of
other plants, their power of absorption—Saxifraga—Primula—
Pelargonium—Erica—Mirabilis—Nicotiana—Summary on glandular
hairs—Concluding remarks on the Droseraceae.
CHAPTER XVI.
PINGUICULA.
Pinguicula vulgaris—Structure of leaves—Number of insects and other
objects caught—Movement of the margins of the leaves—Uses of this
movement—Secretion, digestion, and absorption—Action of the secretion
on various animal and vegetable substances—The effects of substances
not containing soluble nitrogenous matter on the glands—Pinguicula
grandiflora—Pinguicula lusitanica, catches insects—Movement of the
leaves, secretion and digestion.
CHAPTER XVII.
UTRICULARIA.
Utricularia neglecta—Structure of the bladder—The uses of the several
parts—Number of imprisoned animals—Manner of capture—The bladders
cannot digest animal matter, but absorb the products of its
decay—Experiments on the absorption of certain fluids by the quadrifid
processes—Absorption by the glands—Summary of the observation on
absorption— Development of the bladders—Utricularia
vulgaris—Utricularia minor—Utricularia clandestina.
CHAPTER XVIII.
UTRICULARIA (continued).
Utricularia montana—Description of the bladders on the subterranean
rhizomes—Prey captured by the bladders of plants under culture and in a
state of nature—Absorption by the quadrifid processes and glands—Tubers
serving as reservoirs for water—Various other species of
Utricularia—Polypompholyx—Genlisea, different nature of the trap for
capturing prey— Diversified methods by which plants are nourished.
INSECTIVOROUS PLANTS.
CHAPTER I.
DROSERA ROTUNDIFOLIA, OR THE COMMON SUN-DEW.
Number of insects captured—Description of the leaves and their
appendages or tentacles— Preliminary sketch of the action of the
various parts, and of the manner in which insects are captured—Duration
of the inflection of the tentacles—Nature of the secretion—Manner in
which insects are carried to the centre of the leaf—Evidence that the
glands have the power of absorption—Small size of the roots.
During the summer of 1860, I was surprised by finding how large a
number of insects were caught by the leaves of the common sun-dew
(Drosera rotundifolia) on a heath in Sussex. I had heard that insects
were thus caught, but knew nothing further on the subject.* I
* As Dr. Nitschke has given (‘Bot. Zeitung,’ 1860, p. 229) the
bibliography of Drosera, I need not here go into details. Most of the
notices published before 1860 are brief and unimportant. The oldest
paper seems to have been one of the most valuable, namely, by Dr. Roth,
in 1782. There is also an interesting though short account of the
habits of Drosera by Dr. Milde, in the ‘Bot. Zeitung,’ 1852, p. 540. In
1855, in the ‘Annales des Sc. nat. bot.’ tom. iii. pp. 297 and 304, MM.
Groenland and Trcul each published papers, with figures, on the
structure of the leaves; but M. Trcul went so far as to doubt whether
they possessed any power of movement. Dr. Nitschke’s papers in the
‘Bot. Zeitung’ for 1860 and 1861 are by far the most important ones
which have been published, both on the habits and structure of this
plant; and I shall frequently have occasion to quote from them. His
discussions on several points, for instance on the transmission of an
excitement from one part of the leaf to another, are excellent. On
December 11, 1862, Mr. J. Scott read a paper before the Botanical
Society of Edinburgh, [[page 2]] which was published in the ‘Gardeners’
Chronicle,’ 1863, p. 30. Mr. Scott shows that gentle irritation of the
hairs, as well as insects placed on the disc of the leaf, cause the
hairs to bend inwards. Mr. A.W. Bennett also gave another interesting
account of the movements of the leaves before the British Association
for 1873. In this same year Dr. Warming published an essay, in which he
describes the structure of the so-called hairs, entitled, “Sur la
Diffrence entre les Trichomes,” &c., extracted from the proceedings of
the Soc. d’Hist. Nat. de Copenhague. I shall also have occasion
hereafter to refer to a paper by Mrs. Treat, of New Jersey, on some
American species of Drosera. Dr. Burdon Sanderson delivered a lecture
on Dionaea, before the Royal Institution published in ‘Nature,’ June
14, 1874, in which a short account of my observations on the power of
true digestion possessed by Drosera and Dionaea first appeared. Prof.
Asa Gray has done good service by calling attention to Drosera, and to
other plants having similar habits, in ‘The Nation’ (1874, pp. 261 and
232), and in other publications. Dr. Hooker, also, in his important
address on Carnivorous Plants (Brit. Assoc., Belfast, 1874), has given
a history of the subject. [page 2]
gathered by chance a dozen plants, bearing fifty-six fully expanded
leaves, and on thirty-one of these dead insects or remnants of them
adhered; and, no doubt, many more would have been caught afterwards by
these same leaves, and still more by those as yet not expanded. On one
plant all six leaves had caught their prey; and on several plants very
many leaves had caught more than a single insect. On one large leaf I
found the remains of thirteen distinct insects. Flies (Diptera) are
captured much oftener than other insects. The largest kind which I have
seen caught was a small butterfly (Caenonympha pamphilus); but the Rev.
H.M. Wilkinson informs me that he found a large living dragon-fly with
its body firmly held by two leaves. As this plant is extremely common
in some districts, the number of insects thus annually slaughtered must
be prodigious. Many plants cause the death of insects, for instance the
sticky buds of the horse-chestnut (Aesculus hippocastanum), without
thereby receiving, as far as we can perceive, any advantage; but it was
soon evident that Drosera was [page 3] excellently adapted for the
special purpose of catching insects, so that the subject seemed well
worthy of investigation.
The results have proved highly remarkable; the more important ones
being—firstly, the extraordinary
FIG. 1.* (Drosera rotundifolia.) Leaf viewed from above; enlarged four
times.
sensitiveness of the glands to slight pressure and to minute doses of
certain nitrogenous fluids, as shown by the movements of the so-called
hairs or tentacles;
* The drawings of Drosera and Dionaea, given in this work, were made
for me by my son George Darwin; those of Aldrovanda, and of the several
species of Utricularia, by my son Francis. They have been excellently
reproduced on wood by Mr. Cooper, 188 Strand. [page 4]
secondly, the power possessed by the leaves of rendering soluble or
digesting nitrogenous substances, and of afterwards absorbing them;
thirdly, the changes which take place within the cells of the
tentacles, when the glands are excited in various ways.
It is necessary, in the first place, to describe briefly the plant. It
bears from two or three to five or six leaves, generally extended more
or less horizontally, but sometimes standing vertically upwards. The
shape and general appearance of a leaf is shown, as seen from above, in
fig. 1, and as seen laterally, in fig. 2. The leaves are commonly a
little broader than long,
FIG. 2. (Drosera rotundifolia.) Old leaf viewed laterally; enlarged
about five times.
but this was not the case in the one here figured. The whole upper
surface is covered with gland-bearing filaments, or tentacles, as I
shall call them, from their manner of acting. The glands were counted
on thirty-one leaves, but many of these were of unusually large size,
and the average number was 192; the greatest number being 260, and the
least 130. The glands are each surrounded by large drops of extremely
viscid secretion, which, glittering in the sun, have given rise to the
plant’s poetical name of the sun-dew.
[The tentacles on the central part of the leaf or disc are short and
stand upright, and their pedicels are green. Towards the margin they
become longer and longer and more inclined [page 5] outwards, with
their pedicels of a purple colour. Those on the extreme margin project
in the same plane with the leaf, or more commonly (see fig. 2) are
considerably reflexed. A few tentacles spring from the base of the
footstalk or petiole, and these are the longest of all, being sometimes
nearly 1/4 of an inch in length. On a leaf bearing altogether 252
tentacles, the short ones on the disc, having green pedicels, were in
number to the longer submarginal and marginal tentacles, having purple
pedicels, as nine to sixteen.
A tentacle consists of a thin, straight, hair-like pedicel, carrying a
gland on the summit. The pedicel is somewhat flattened, and is formed
of several rows of elongated cells, filled with purple fluid or
granular matter.* There is, however, a narrow zone close beneath the
glands of the longer tentacles, and a broader zone near their bases, of
a green tint. Spiral vessels, accompanied by simple vascular tissue,
branch off from the vascular bundles in the blade of the leaf, and run
up all the tentacles into the glands.
Several eminent physiologists have discussed the homological nature of
these appendages or tentacles, that is, whether they ought to be
considered as hairs (trichomes) or prolongations of the leaf. Nitschke
has shown that they include all the elements proper to the blade of a
leaf; and the fact of their including vascular tissue was formerly
thought to prove that they were prolongations of the leaf, but it is
now known that vessels sometimes enter true hairs.** The power of
movement which they possess is a strong argument against their being
viewed as hairs. The conclusion which seems to me the most probable
will be given in Chap. XV., namely that they existed primordially as
glandular hairs, or mere epidermic formations, and that their upper
part should still be so considered; but that their lower
* According to Nitschke (‘Bot. Zeitung,’ 1861, p. 224) the purple fluid
results from the metamorphosis of chlorophyll. Mr. Sorby examined the
colouring matter with the spectroscope, and informs me that it consists
of the commonest species of erythrophyll, “which is often met with in
leaves with low vitality, and in parts, like the petioles, which carry
on leaf-functions in a very imperfect manner. All that can be said,
therefore, is that the hairs (or tentacles) are coloured like parts of
a leaf which do not fulfil their proper office.”
** Dr. Nitschke has discussed this subject in ‘Bot. Zeitung,’ 1861, p.
241 &c. See also Dr. Warming (‘Sur la Diffrence entre les Trichomes’
&c., 1873), who gives references to various publications. See also
Groenland and Trcul ‘Annal. des Sc. nat. bot.’ (4th series), tom. iii.
1855, pp. 297 and 303. [page 6]
part, which alone is capable of movement, consists of a prolongation of
the leaf; the spiral vessels being extended from this to the uppermost
part. We shall hereafter see that the terminal tentacles of the divided
leaves of Roridula are still in an intermediate condition.
The glands, with the exception of those borne by the extreme
FIG. 3. (Drosera rotundifolia.) Longitudinal section of a gland;
greatly magnified. From Dr. Warming.
marginal tentacles, are oval, and of nearly uniform size, viz. about
4/500 of an inch in length. Their structure is remarkable, and their
functions complex, for they secrete, absorb, and are acted on by
various stimulants. They consist of an outer layer of small polygonal
cells, containing purple granular matter or fluid, and with the walls
thicker than those of the pedicels. [page 7] Within this layer of cells
there is an inner one of differently shaped ones, likewise filled with
purple fluid, but of a slightly different tint, and differently
affected by chloride of gold. These two layers are sometimes well seen
when a gland has been crushed or boiled in caustic potash. According to
Dr. Warming, there is still another layer of much more elongated cells,
as shown in the accompanying section (fig. 3) copied from his work; but
these cells were not seen by Nitschke, nor by me. In the centre there
is a group of elongated, cylindrical cells of unequal lengths, bluntly
pointed at their upper ends, truncated or rounded at their lower ends,
closely pressed together, and remarkable from being surrounded by a
spiral line, which can be separated as a distinct fibre.
These latter cells are filled with limpid fluid, which after long
immersion in alcohol deposits much brown matter. I presume that they
are actually connected with the spiral vessels which run up the
tentacles, for on several occasions the latter were seen to divide into
two or three excessively thin branches, which could be traced close up
to the spiriferous cells. Their development has been described by Dr.
Warming. Cells of the same kind have been observed in other plants, as
I hear from Dr. Hooker, and were seen by me in the margins of the
leaves of Pinguicula. Whatever their function may be, they are not
necessary for the secretion of a digestive fluid, or for absorption, or
for the communication of a motor impulse to other parts of the leaf, as
we may infer from the structure of the glands in some other genera of
the Droseraceae.
The extreme marginal tentacles differ slightly from the others. Their
bases are broader, and besides their own vessels, they receive a fine
branch from those which enter the tentacles on each side. Their glands
are much elongated, and lie embedded on the upper surface of the
pedicel, instead of standing at the apex. In other respects they do not
differ essentially from the oval ones, and in one specimen I found
every possible transition between the two states. In another specimen
there were no long-headed glands. These marginal tentacles lose their
irritability earlier than the others; and when a stimulus is applied to
the centre of the leaf, they are excited into action after the others.
When cut-off leaves are immersed in water, they alone often become
inflected.
The purple fluid or granular matter which fills the cells of the glands
differs to a certain extent from that within the cells of the pedicels.
For when a leaf is placed in hot water or in certain acids, the glands
become quite white and opaque, whereas [page 8] the cells of the
pedicels are rendered of a bright red, with the exception of those
close beneath the glands. These latter cells lose their pale red tint;
and the green matter which they, as well as the basal cells, contain,
becomes of a brighter green. The petioles bear many multicellular
hairs, some of which near the blade are surmounted, according to
Nitschke, by a few rounded cells, which appear to be rudimentary
glands. Both surfaces of the leaf, the pedicels of the tentacles,
especially the lower sides of the outer ones, and the petioles, are
studded with minute papillae (hairs or trichomes), having a conical
basis, and bearing on their summits two, and occasionally three or even
four, rounded cells, containing much protoplasm. These papillae are
generally colourless, but sometimes include a little purple fluid. They
vary in development, and graduate, as Nitschke* states, and as I
repeatedly observed, into the long multicellular hairs. The latter, as
well as the papillae, are probably rudiments of formerly existing
tentacles.
I may here add, in order not to recur to the papillae, that they do not
secrete, but are easily permeated by various fluids: thus when living
or dead leaves are immersed in a solution of one part of chloride of
gold, or of nitrate of silver, to 437 of water, they are quickly
blackened, and the discoloration soon spreads to the surrounding
tissue. The long multicellular hairs are not so quickly affected. After
a leaf had been left in a weak infusion of raw meat for 10 hours, the
cells of the papillae had evidently absorbed animal matter, for instead
of limpid fluid they now contained small aggregated masses of
protoplasm, which slowly and incessantly changed their forms. A similar
result followed from an immersion of only 15 minutes in a solution of
one part of carbonate of ammonia to 218 of water, and the adjoining
cells of the tentacles, on which the papillae were seated, now likewise
contained aggregated masses of protoplasm. We may therefore conclude
that when a leaf has closely clasped a captured insect in the manner
immediately to be described, the papillae, which project from the upper
surface of the leaf and of the tentacles, probably absorb some of the
animal matter dissolved in the secretion; but this cannot be the case
with the papillae on the backs of the leaves or on the petioles.]
* Nitschke has elaborately described and figured these papillae, ‘Bot.
Zeitung,’ 1861, pp. 234, 253, 254. [page 9]
_Preliminary Sketch of the Action of the several Parts, and of the
Manner in which Insects are Captured._
If a small organic or inorganic object be placed on the glands in the
centre of a leaf, these transmit a motor impulse to the marginal
tentacles. The nearer ones are first affected and slowly bend towards
the centre, and then those farther off, until at last all become
closely inflected over the object. This takes place in from one hour to
four or five or more hours. The difference in the time required depends
on many circumstances; namely on the size of the object and on its
nature, that is, whether it contains soluble matter of the proper kind;
on the vigour and age of the leaf; whether it has lately been in
action; and, according to Nitschke,* on the temperature of the day, as
likewise seemed to me to be the case. A living insect is a more
efficient object than a dead one, as in struggling it presses against
the glands of many tentacles. An insect, such as a fly, with thin
integuments, through which animal matter in solution can readily pass
into the surrounding dense secretion, is more efficient in causing
prolonged inflection than an insect with a thick coat, such as a
beetle. The inflection of the tentacles takes place indifferently in
the light and darkness; and the plant is not subject to any nocturnal
movement of so-called sleep.
If the glands on the disc are repeatedly touched or brushed, although
no object is left on them, the marginal tentacles curve inwards. So
again, if drops of various fluids, for instance of saliva or of a
solution of any salt of ammonia, are placed on the central glands, the
same result quickly follows, sometimes in under half an hour.
* ‘Bot. Zeitung,’ 1860, p. 246. [page 10]
The tentacles in the act of inflection sweep through a wide space; thus
a marginal tentacle, extended in the same plane with the blade, moves
through an angle of 180o; and I have seen the much reflected tentacles
of a leaf which stood upright move through an angle of not less than
270o. The bending part is almost confined to a short space near the
base; but a rather larger portion of the elongated exterior tentacles
FIG. 4. (Drosera rotundifolia.) Leaf (enlarged) with all the tentacles
closely inflected, from immersion in a solution of phosphate of ammonia
(one part to 87,500 of water.)
FIG. 5. (Drosera rotundifolia.) Leaf (enlarged) with the tentacles on
one side inflected over a bit of meat placed on the disc.
becomes slightly incurved; the distal half in all cases remaining
straight. The short tentacles in the centre of the disc when directly
excited, do not become inflected; but they are capable of inflection if
excited by a motor impulse received from other glands at a distance.
Thus, if a leaf is immersed in an infusion of raw meat, or in a weak
solution of ammonia (if the [page 11] solution is at all strong, the
leaf is paralysed), all the exterior tentacles bend inwards (see fig.
4), excepting those near the centre, which remain upright; but these
bend towards any exciting object placed on one side of the disc, as
shown in fig. 5. The glands in fig. 4 may be seen to form a dark ring
round the centre; and this follows from the exterior tentacles
increasing in length in due proportion, as they stand nearer to the
circumference.
The kind of inflection which the tentacles undergo is best shown when
the gland of one of the long exterior
FIG. 6. (Drosera rotundifolia.) Diagram showing one of the exterior
tentacles closely inflected; the two adjoining ones in their ordinary
position.)
tentacles is in any way excited; for the surrounding ones remain
unaffected. In the accompanying outline (fig. 6) we see one tentacle,
on which a particle of meat had been placed, thus bent towards the
centre of the leaf, with two others retaining their original position.
A gland may be excited by being simply touched three or four times, or
by prolonged contact with organic or inorganic objects, and various
fluids. I have distinctly seen, through a lens, a tentacle beginning to
bend in ten seconds, after an object had been [page 12] placed on its
gland; and I have often seen strongly pronounced inflection in under
one minute. It is surprising how minute a particle of any substance,
such as a bit of thread or hair or splinter of glass, if placed in
actual contact with the surface of a gland, suffices to cause the
tentacle to bend. If the object, which has been carried by this
movement to the centre, be not very small, or if it contains soluble
nitrogenous matter, it acts on the central glands; and these transmit a
motor impulse to the exterior tentacles, causing them to bend inwards.
Not only the tentacles, but the blade of the leaf often, but by no
means always, becomes much incurved, when any strongly exciting
substance or fluid is placed on the disc. Drops of milk and of a
solution of nitrate of ammonia or soda are particularly apt to produce
this effect. The blade is thus converted into a little cup. The manner
in which it bends varies greatly. Sometimes the apex alone, sometimes
one side, and sometimes both sides, become incurved. For instance, I
placed bits of hard-boiled egg on three leaves; one had the apex bent
towards the base; the second had both distal margins much incurved, so
that it became almost triangular in outline, and this perhaps is the
commonest case; whilst the third blade was not at all affected, though
the tentacles were as closely inflected as in the two previous cases.
The whole blade also generally rises or bends upwards, and thus forms a
smaller angle with the footstalk than it did before. This appears at
first sight a distinct kind of movement, but it results from the
incurvation of that part of the margin which is attached to the
footstalk, causing the blade, as a whole, to curve or move upwards.
The length of time during which the tentacles as [page 13] well as the
blade remain inflected over an object placed on the disc, depends on
various circumstances; namely on the vigour and age of the leaf, and,
according to Dr. Nitschke, on the temperature, for during cold weather
when the leaves are inactive, they re-expand at an earlier period than
when the weather is warm. But the nature of the object is by far the
most important circumstance; I have repeatedly found that the tentacles
remain clasped for a much longer average time over objects which yield
soluble nitrogenous matter than over those, whether organic or
inorganic, which yield no such matter. After a period varying from one
to seven days, the tentacles and blade re-expand, and are then ready to
act again. I have seen the same leaf inflected three successive times
over insects placed on the disc; and it would probably have acted a
greater number of times.
The secretion from the glands is extremely viscid, so that it can be
drawn out into long threads. It appears colourless, but stains little
balls of paper pale pink. An object of any kind placed on a gland
always causes it, as I believe, to secrete more freely; but the mere
presence of the object renders this difficult to ascertain. In some
cases, however, the effect was strongly marked, as when particles of
sugar were added; but the result in this case is probably due merely to
exosmose. Particles of carbonate and phosphate of ammonia and of some
other salts, for instance sulphate of zinc, likewise increase the
secretion. Immersion in a solution of one part of chloride of gold, or
of some other salts, to 437 of water, excites the glands to largely
increased secretion; on the other hand, tartrate of antimony produces
no such effect. Immersion in many acids (of the strength of one part to
437 of water) likewise causes a wonderful amount of [page 14]
secretion, so that when the leaves are lifted out, long ropes of
extremely viscid fluid hang from them. Some acids, on the other hand,
do not act in this manner. Increased secretion is not necessarily
dependent on the inflection of the tentacle, for particles of sugar and
of sulphate of zinc cause no movement.
It is a much more remarkable fact that when an object, such as a bit of
meat or an insect, is placed on the disc of a leaf, as soon as the
surrounding tentacles become considerably inflected, their glands pour
forth an increased amount of secretion. I ascertained this by selecting
leaves with equal-sized drops on the two sides, and by placing bits of
meat on one side of the disc; and as soon as the tentacles on this side
became much inflected, but before the glands touched the meat, the
drops of secretion became larger. This was repeatedly observed, but a
record was kept of only thirteen cases, in nine of which increased
secretion was plainly observed; the four failures being due either to
the leaves being rather torpid, or to the bits of meat being too small
to cause much inflection. We must therefore conclude that the central
glands, when strongly excited, transmit some influence to the glands of
the circumferential tentacles, causing them to secrete more copiously.
It is a still more important fact (as we shall see more fully when we
treat of the digestive power of the secretion) that when the tentacles
become inflected, owing to the central glands having been stimulated
mechanically, or by contact with animal matter, the secretion not only
increases in quantity, but changes its nature and becomes acid; and
this occurs before the glands have touched the object on the centre of
the leaf. This acid is of a different nature from that contained in the
tissue of the leaves. As long as the [page 15] tentacles remain closely
inflected, the glands continue to secrete, and the secretion is acid;
so that, if neutralised by carbonate of soda, it again becomes acid
after a few hours. I have observed the same leaf with the tentacles
closely inflected over rather indigestible substances, such as
chemically prepared casein, pouring forth acid secretion for eight
successive days, and over bits of bone for ten successive days.
The secretion seems to possess, like the gastric juice of the higher
animals, some antiseptic power. During very warm weather I placed close
together two equal-sized bits of raw meat, one on a leaf of the
Drosera, and the other surrounded by wet moss. They were thus left for
48 hrs., and then examined. The bit on the moss swarmed with infusoria,
and was so much decayed that the transverse striae on the muscular
fibres could no longer be clearly distinguished; whilst the bit on the
leaf, which was bathed by the secretion, was free from infusoria, and
its striae were perfectly distinct in the central and undissolved
portion. In like manner small cubes of albumen and cheese placed on wet
moss became threaded with filaments of mould, and had their surfaces
slightly discoloured and disintegrated; whilst those on the leaves of
Drosera remained clean, the albumen being changed into transparent
fluid.
As soon as tentacles, which have remained closely inflected during
several days over an object, begin to re-expand, their glands secrete
less freely, or cease to secrete, and are left dry. In this state they
are covered with a film of whitish, semi-fibrous matter, which was held
in solution by the secretion. The drying of the glands during the act
of re-expansion is of some little service to the plant; for I have
often observed that objects adhering to the leaves [page 16] could then
be blown away by a breath of air; the leaves being thus left
unencumbered and free for future action. Nevertheless, it often happens
that all the glands do not become completely dry; and in this case
delicate objects, such as fragile insects, are sometimes torn by the
re-expansion of the tentacles into fragments, which remain scattered
all over the leaf. After the re-expansion is complete, the glands
quickly begin to re-secrete, and as soon as full-sized drops are
formed, the tentacles are ready to clasp a new object.
When an insect alights on the central disc, it is instantly entangled
by the viscid secretion, and the surrounding tentacles after a time
begin to bend, and ultimately clasp it on all sides. Insects are
generally killed, according to Dr. Nitschke, in about a quarter of an
hour, owing to their tracheae being closed by the secretion. If an
insect adheres to only a few of the glands of the exterior tentacles,
these soon become inflected and carry their prey to the tentacles next
succeeding them inwards; these then bend inwards, and so onwards; until
the insect is ultimately carried by a curious sort of rolling movement
to the centre of the leaf. Then, after an interval, the tentacles on
all sides become inflected and bathe their prey with their secretion,
in the same manner as if the insect had first alighted on the central
disc. It is surprising how minute an insect suffices to cause this
action: for instance, I have seen one of the smallest species of gnats
(Culex), which had just settled with its excessively delicate feet on
the glands of the outermost tentacles, and these were already beginning
to curve inwards, though not a single gland had as yet touched the body
of the insect. Had I not interfered, this minute gnat would [page 17]
assuredly have been carried to the centre of the leaf and been securely
clasped on all sides. We shall hereafter see what excessively small
doses of certain organic fluids and saline solutions cause strongly
marked inflection.
Whether insects alight on the leaves by mere chance, as a resting
place, or are attracted by the odour of the secretion, I know not. I
suspect from the number of insects caught by the English species of
Drosera, and from what I have observed with some exotic species kept in
my greenhouse, that the odour is attractive. In this latter case the
leaves may be compared with a baited trap; in the former case with a
trap laid in a run frequented by game, but without any bait.
That the glands possess the power of absorption, is shown by their
almost instantaneously becoming dark-coloured when given a minute
quantity of carbonate of ammonia; the change of colour being chiefly or
exclusively due to the rapid aggregation of their contents. When
certain other fluids are added, they become pale-coloured. Their power
of absorption is, however, best shown by the widely different results
which follow, from placing drops of various nitrogenous and
non-nitrogenous fluids of the same density on the glands of the disc,
or on a single marginal gland; and likewise by the very different
lengths of time during which the tentacles remain inflected over
objects, which yield or do not yield soluble nitrogenous matter. This
same conclusion might indeed have been inferred from the structure and
movements of the leaves, which are so admirably adapted for capturing
insects.
The absorption of animal matter from captured insects explains how
Drosera can flourish in extremely poor peaty soil,—in some cases where
nothing but [page 18] sphagnum moss grows, and mosses depend altogether
on the atmosphere for their nourishment. Although the leaves at a hasty
glance do not appear green, owing to the purple colour of the
tentacles, yet the upper and lower surfaces of the blade, the pedicels
of the central tentacles, and the petioles contain chlorophyll, so
that, no doubt, the plant obtains and assimilates carbonic acid from
the air. Nevertheless, considering the nature of the soil where it
grows, the supply of nitrogen would be extremely limited, or quite
deficient, unless the plant had the power of obtaining this important
element from captured insects. We can thus understand how it is that
the roots are so poorly developed. These usually consist of only two or
three slightly divided branches, from half to one inch in length,
furnished with absorbent hairs. It appears, therefore, that the roots
serve only to imbibe water; though, no doubt, they would absorb
nutritious matter if present in the soil; for as we shall hereafter
see, they absorb a weak solution of carbonate of ammonia. A plant of
Drosera, with the edges of its leaves curled inwards, so as to form a
temporary stomach, with the glands of the closely inflected tentacles
pouring forth their acid secretion, which dissolves animal matter,
afterwards to be absorbed, may be said to feed like an animal. But,
differently from an animal, it drinks by means of its roots; and it
must drink largely, so as to retain many drops of viscid fluid round
the glands, sometimes as many as 260, exposed during the whole day to a
glaring sun. [page 19]