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[page i]







INSECTIVOROUS PLANTS.






[page ii.]



BY THE SAME AUTHOR.

- -

ON THE ORIGIN OF SPECIES BY MEANS OF NATURAL SELECTION; or, THE
PRESERVATION OF FAVORED RACES IN THE STRUGGLE FOR LIFE. New and revised
edition. 12mo. Cloth. 480 pages. Price, $2.00.

THE DESCENT OF MAN, AND SELECTION IN RELATION TO SEX. With
Illustrations. New edition, revised and augmented. 12mo. Cloth. 704
pages. Price, $3.00.

A NATURALIST'S VOYAGE ROUND THE WORLD; or, A JOURNAL OF RESEARCHES INTO
THE NATURAL HISTORY AND GEOLOGY OF THE COUNTRIES visited during the
voyage of H.M.S. Beagle, under the command of Captain Fitz-Roy, R.N.
12mo. Cloth. 530 pages. Price, $2.00.

THE EXPRESSION OF THE EMOTIONS IN MAN AND ANIMALS. With Photographic
and other Illustrations. 12mo. Cloth. Price, $3.00.

- -

THE THEORY OF DESCENT, AND DARWINISM. By Prof. OSCAR SCHMIDT,
University of Strasburg. 12mo. Cloth. Price, $1.50.


[page iii.]




INSECTIVOROUS PLANTS






BY CHARLES DARWIN, M.A., F.R.S.
ETC.




WITH ILLUSTRATIONS.




[page iv.]

[page v.]





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...Pages 1-18


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...19-37 [page vi.]


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...Pages 38-65


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...66-75


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...76-84 [page vii.]


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...Pages
85-135


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...136-173


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...174-198 [page viii.]


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...Pages 199-228


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...229-261


CHAPTER XI.

RECAPITULATION OF THE CHIEF OBSERVATIONS ON DROSERA ROTUNDIFOLIA.

262-277 [page ix.]


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...Pages 278-285


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...286-320


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...321-331


CHAPTER XV.

DROSOPHYLLUM - RORIDULA - BYBLIS - GLANDULAR HAIRS OF OTHER PLANTS -
CONCLUDING REMARKS ON THE DROSERACEAE.

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...332-367 [page x.]


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...Pages 368-394


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...395-430


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...431-453

- - -

INDEX...455-462


[page 1]




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



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