Insectivorous Plants

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 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 yield- ing 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

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 — Descrip- tion 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 observa- tions on aggregation in the roots of plants . . Pages 33-G5

Nature of the experiments — Effects of boiling water — Warm water causes rapid inflection — Water at a higher tempera- ture 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 Non-nitrogen oxis fluids — Solutions of gum arabic — Sugar - Starch — Diluted alcohol — Olive oil — Infusion and decoc- tion 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 76-8-1

The secretion rendered acid by the direct and indirect excite- ment 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 c}ieese _ Gluten — Legumin — Pollen — Globulin — Hsematin — Indigestible substances — Epidermic productions — Fibro- elastic tissue — Mucin — Pepsin — Urea — Chit ine — Gel luloso — Gun-cotton — Chlorophyll — Fat and oil — Starch —Action of the secretion on living seeds — Summary and concluding- remarks Pages 85-13,'

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

Salts of sodium, potassium, and other alkaline, earthy, and metallic salts — Summary on the action of these salts - Various acids — Summary on their action .. .. 174-1'JS arrest the movement of the protoplasm — Other salts oi 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

'Jr lands and summits of the tentacles alone sensitive — Trans- mission of the motor impulse down the pedicels of the tentacles, and across the blade of the leaf — Aggregation oi 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

Drosera anglica — Drosera intermedia — Drosera capensis — Drosera spathulata — Drosera filiformis — Drosera binata — Concluding remarks •• Pages 278-285 Structure of the leaves — Sensitiveness of the filaments — Eapid 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 — Ee-expansion of the lobes .. 286-320 Captures crustaceans — Structure of the leaves in comparison with those of Dionsea — Absorption by the glands, by the quadrifid processes, and points on the infolded margins - Aldrooanila vesiculosa, var. austral is — Captures prey - Absorption of animal matter — Aldrovanda vesiculosa, var. verticillata — Concluding remarks 321-331

Drosophyllum— Structure of leaves— Nature of the secretion- Manner of catching insects— Power of absorption — Digestion of animal substances— Summary on Drosophyllum — Eoridula — Byblis — Glandular hairs of other plants, their power of absorption — Saxifraga — Primula — Pelargonium — Erica— Mirabilis — Nicotiana — Summary on glandular hairs — Con- cluding remarks on the ?>oseracese 332-307 Pinguicula vulgar is — Structure of 1'eaves — 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 con- taining soluble nitrogenous matter on the glands — Pinguicula, grandiflora — Pinguicula lusitanica, catches insects — Move- ment of the leaves, secretion and digestion . . Pages 368-394

Ulriculuria iier/lecfa — 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 tho absorption of certain fluids by the quadrifid processes - Absorption by the glands — Summary of the observation on absorption — Development of the bladders — Utricularin vulgar is — Utricularia minor — Utricularia clandestine!, 395-430

Utricularia mmitana — Description of the bladders on the sub- terranean 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 Number of insects captured — Description of the leaves and theii 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 secre- tion— Manner in which insects are carried to the centre of the leaf — Evidence that the glands have the power of absorption — Small bize of the roots.

DURING the summer of 1860, I was surprised by find- ing 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. Zeituug,' ISO), p. 229) the bibliography of Drosera, I need not here go into details. Most of the notices published before 18(30 are brief and unimportant. The oldest paper seems to have been ; ne of the most valuable, namely, by Dr. Roth, in 1782. There is ako an interesting though short account of the habits of Drosera by Dr. Milde, in the ' Bot. Zeitung,' 1S52, p. 540. In 1S55, in the ' An- nalcs des Sc. nat. bot.' torn. iii. pp. 2i*7 and 301, MM. Greenland and Trecul each published papers, with figures, on the structure of the

leaves; but M. Trtcul went so far as to doubt whether they pos- sessed any power of movement. Dr. Nitschke's papers in the ' Bot. Zeituug' for I860 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 trans- mission of an excitement from one part of the leaf to another, are excellent. On Dec. 11, 1862, Mr. J. Scott read a paper before the Botanical Society of Edinburgh,

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 ex- panded. 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 (Caenonymplia pampliilus) ; but the Kev. 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 (JEsculus hippocastanum], without thereby receiving, as far as we can perceive, any ad- vantage ; but it was soon evident that Drosera was

which was published in the Gar- to a paper by Mrs. Treat, of New ilener's Chronicle,' 1863, p. 80. Jersey, on some American species Mr. Scott shows that gentle irrita- of Drosera. Dr. Bnrdon Sander- tion of the hairs, as well as insects sou delivered a lecture on Dionaea, placed on the disc of the leaf, before the Royal Institution •'pub- cause the hairs to bend in- lished in 'Nature,' June 14, 1874), wards. Mr. A. W. Bennett also in which a short account of my gave another interesting account observations on the power of true of the movements of the leaves digestion possessed by Droser.t brfore the British Association for and Dionsea first appeared. Prof. 1S73. In this same year Dr. Asa Gray has done good service Warming published an essay, in by calling attention to Drosera, which he describes the structure and to other plants having similar of the so-called hairs, entitled, habits, in ' The Nation' (1874, pp. " Sur la Difference entre les Tri- 2(jl and 232), and in other publica- chomes," &c., extracted from the tions. Dr. Hooker, also, in his proceedings of the Soc. d'Hist. important address on Carnivorous NTat. de Copenhague. I shall also Plants (Brit. Assoc., Belfast, 1874), have occasion hereafter to refer has given a history of the subjirt

excellently adapted for the special purpose of catch- ing insects, so that the subject seemed well worthy of investigation. The results have proved highly remarkable ; the more important ones being — firstly, the extraordinary (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 cularia, by my son Francis. They Diona-ft. given in this work, were have been excellently reproduced 1 >nrwin ; those of Aldrovanda, and Stria d. of the several speci.'S of Utri- secondly, the power possessed by the leaves of render- ing 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. (Urosera 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 8hort and .stand upright, and their pedicels are green. Towards the margin they become longer and longer and more inclined 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 i 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, aa nine to sixteen.

A tentacle consists of a thin, straight, hair-like pedicel, carry- ing 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 longsr tentacles, and a broader zone near their bases, of a green tint. Spiral vessels, accom- panied 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 some- times enter true hairs.f 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. Zeitimg,' 1861, p. 224) the purple fluid results from the metamor- phosis of chlorophyll. Mr. Sorl>y examined the colouring matter with the spectroscope, and in- forms me that it consists of the commonest species of erythro- phyll, " which is often met with in leaves with low vitality, and iu 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." part, which alone is capable of movement, consists of a prolon- gation of the leaf; the spiral vessels being extended from this to the uppermost part. We shall hereafter see that the ter- minal tentacles of the divided leaves of Eoriclula are still in an intermediate condition. The glands, with the exception of those borne by the extreme , rottiitdifolia.) Ivmg tudlrul section of a gland ; greatly magnified. From l>r. \Varming.

marginal tentacles, are oval, and of nearly uniform size, viz. about 5-^0 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. 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 bas 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 the 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 Droseracea3.

The extreme marginal tentacles differ slightly from the others. Their bases arc 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 em- bedded 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 exit-off leaves are immersed in water, they alone often become inflected.

The purple fluid or granular matter which fills the cells ot 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 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, espe- cially 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 occasion- ally three or even four, rounded cells, containing much proto- plasm. 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 resiilt 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 proto- plasm. 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.