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.
alcohol—Olive oil— Infusion and decoction of tea—Nitrogenous 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 Bone—Enamel and dentine—Phosphate of lime—Fibrous basis of bone—Gelatine—Chondrin— Milk, casein and 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 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.
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.
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 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.
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.
_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.
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