Insectivorous Plants

In Hie ninth chapter the effects of the absorption of various alkaloids and certain other substances were described. Although some of these are poisonous, yet as several, which act powerfully on the nervous system of animals, produce no effect on Drosera, we may infer that the extreme sensibility of the glands, and their power of transmitting an influence to other parts of the leaf, causing movement, or modified secretion, or aggregation, does not depend on the presence of a diffused element, allied to nerve-tissue. One of the most remarkable facts is that long immersion in the poison of the cobra-snake does not in the least check, but rather stimulates, the spontaneous move- ments of the protoplasm in the cells of the tentacles. Solutions of various salts and acids behave very dif- ferently in delaying or in quite arresting the sub- sequent action of a solution of phosphate of ammonia. Camphor dissolved in water acts as a stimulant, as do small doses of certain essential oils, for they cause rapid and strong inflection. Alcohol is not a stimu- lant. The vapours of camphor, alcohol, chloroform, sulphuric and nitric ether, are poisonous in moderately large doses, but in small doses serve as narcotics or aneesthetics, greatly delaying the subsequent action of meat. But some of these vapours also act as stimu- lants, exciting rapid, almost spasmodic movements in the tentacles. Carbonic acid is likewise a narcotic, and retards the aggregation of the protoplasm when carbonate of ammonia is subsequently given. The first access of air to plants which have, been immersed in this gas sometimes acts as a stimulant and induces movement. But, as before remarked, a special pharma- copoeia would be necessary to describe the diversified effects of various substances on the leaves of Drosera'. In the tenth chapter it was shown that fhe sensitive-

ness of the leaves appears to be wholly confined to the glands and to the immediately underlying cells. It was further shown that the motor impulse and other forces or influences, proceeding from the glands when excited, pass through the cellular tissue, and not along the fibro-vascular bundles. A gland sends its motor impulse with great rapidity down the pedicel of the same tentacle to the basal part which alone bends. The impulse, then passing onwards, spreads on all sides to the surrounding tentacles, first affecting those which stand nearest and then those farther off. But by being thus spread out, and from the cells of the disc not being so much elongated as those of the tentacles, it loses force, and here travels much more slowly than down the pedicels. Owing also to the direction and form of the cells, it passes with greater ease and cele- rity in a longitudinal than in a transverse line across the disc. The impulse proceeding from the glands of the extreme marginal tentacles does not seem to have force enough to affect the adjoining tentacles ; and this may be in part due to their length. The impulse from the glands of the next few inner rows spreads chiefly to the tentacles on each side and towards the centre of the leaf ; but that proceeding from the glands of the shorter tentacles on the disc radiates almost equally on all sides.

When a gland is strongly excited by the quantity or quality of the substance placed on it, the motor impulse travels farther than from one slightly excited ; and if several glands are simultaneously excited, the impulses from all unite and spread still farther. As Boon as a gland is excited, it discharges an impulse which extends to a considerable distance ; but after- wards, whilst the gland is secreting and absorbing, the impulse suffices only to keep the same tentacle

If the bending place of a tentacle receives an impulse from its own gland, the movement is always towards the centre of the leaf; and so it is with all the tentacles, when their glands are excited by immer- sion in a proper fluid. The short ones in the middle part of the disc must be excepted, as these do not bend at all when thus excited. On the other hand, when the motor impulse comes from one side of the disc, the surrounding tentacles, including the short ones in the middle of the disc, all bend with pre- cision towards the point of excitement, wherever this may be seated. This is in every way a remarkable phenomenon ; for the leaf falsely appears as if en- dowed with the senses of an animal. It is all the more remarkable, as when the motor impulse strikes the base of a tentacle obliquely with respect to its flattened surface, the contraction of the cells must be confined to one, two, or a very few rows at one end. And different sides of the surrounding tentacles must be acted on, in order that all should bend with pre- cision to the point of excitement.

The motor impulse, as it spreads from one or more glands across the disc, enters the bases of the sur- rounding tentacles, and immediately acts on the bend- ing place. It does not in the first place proceed up the tentacles to the glands, exciting them to reflect back an impulse to their bases. Nevertheless, some influence is sent up to the glands, as their secre- tion is soon increased and rendered acid; and then the glands, being thus excited, send back some other influence (not dependent on increased secretion, nor on the inflection of the tentacles), causing the proto- plasm to aggregate in cell beneath cell. This may

be called a reflex action, though probably very dif- ferent from that proceeding from the nerve-ganglion of an animal ; and it is the only known case of reflex action in the vegetable kingdom. About the mechanism of the movements and the nature of the motor impulse we know very little. During the act of inflection fluid certainly travels from one part to another of the tentacles. But the hypo- thesis which agrees best with the observed facts is that the motor impulse is allied in nature to the aggregating process ; and that this causes the mole- cules of the cell-walls to approach each other, in the same manner as do the molecules of the protoplasm within the cells ; so that the cell-walls contract. But some strong objections may be urged against this view. The re-expansion of the tentacles is largely clue to the elasticity of their outer cells, which comes into play as soon as those on the inner side cease con- tracting with prepotent force ; but we have reason to suspect that fluid is continually and slowly attracted into the outer cells during the act of re-expansion, thus increasing their tension.

I have now given a brief recapitulation of the chief points observed by me, with respect to the struc- ture, movements, constitution, and habits of Drosera rotundifolia ; and we see how little has been made out in comparison with what remains unexplained and unknown. Drosera anglica — Drosera intermedia — Drosera capensis — Drosera spathulata — Drosera filiformis — Drosera binata — Concluding remarks. I EXAMINED six other species of Drosera, some of them inhabitants of distant countries, chiefly for the sake of ascertaining whether they caught insects. This seemed the more necessary as the leaves of some of the species differ to an extraordinary degree in shape from the rounded ones of Drosera rotundifolia. In functional powers, however, they differ very littlo.

Drosera am/li'ca (Hudson).* — The leaves of this species, which was sent to me from Ireland, are much elongated, and gradually widen from the footstalk to the bluntly pointed apex. They stand almost erect, and their blades sometimes exceed 1 inch in length, whilst their breadth is only the | of an inch. The glands of all the tentacles have the same structure, so that the extreme marginal ones do not differ from the others, as in the case of Drosera rotundifolia. When they are irritated by being roughly touched, or by the pressure of minute inorganic par- ticles, or by contact with animal matter, or by the absorption of carbonate of ammonia, the tentacles become inflected ; the basal portion being the chief seat of movement. Cutting or pricking the blade of the leaf did not excite any movement. They fre- quently capture insects, and the glands of the inflected tentacles pour forth much acid secretion. Bits of roast meat were placed ou some glands, and the tentacles began to move in 1 m. or

* Mrs. Treat has given an ex- synonym in part of Drosera «•»- c( Ment account in ' The American olica), of Drosera rotundifolia uiid 1 m. 80 s. ; and in 1 hr. 10 m. reached the centre. Two bits of boiled cork, one of boiled thread, and two of coal-cinders taken from the fire, were placed, by the aid of an instrument which had been immersed in boiling water, on five glands ; these super- fluous precautions having been taken on account of M. Ziegler's statements. One of the particles of cinder caused some inflection in 8 hrs. 45 m., as did after 23 hrs. the other particle of cinder, the bit of thread, and both bits of cork. Three glands were touched half a dozen times with a needle ; one of the tentacles became well inflected in 17 m., and re -expanded after 24 hrs.; the two others never moved. The homogeneous fluid within the cells of the tentacles undergoes aggregation after these have become inflected ; especially if given a solution of carbonate of ammonia ; and I observed the usual movements in the masses of proto- plasm. In one case, aggregation ensued in 1 hr. 10 m. after a tentacle had carried a bit of meat to the centre. From these facts it is clear that the tentacles of Drosera anylica behave like those of Drosera rotundifolia.

If an insect is placed on the central glands, or has been naturally caught there, the apex of the leaf curls inwards. For instance, dead flies were placed on three leaves near their bases, and after 24 hrs. the previously straight apices were curled completely over, so as to embrace and conceal the flies ; they had therefore moved through an angle of 180°. After three days the apex of one leaf, together with the tentacles, began to re-expand. But as far as I have seen — and I made many trials — the sides of the leaf are never inflected, and this is the one functional differ- ence between this species and Drosera rotundifolia.

Drosera intermedia (Hayne). — This species is quite as common in some parts of England as Drosera rotundifolia. It differs from Drosera anglica, as far as the leaves are concerned, only in their smaller size, and in their tips being generally a little reflexed. They capture a large number of insects. The tentacles are excited into movement by all the causes above specified ; and aggregation ensues, with movement of the protoplasmic masses. I have seen, through a lens, a tentacle beginning to bend in less than a minute after a particle of raw meat had been placed on the gland. The apex of the leaf curls over an exciting object as in the case of Drosera anylica. Acid secretion is copiously poured over captured insects. A leaf which had embraced a fly with all its tentacles re- expanded after nearly three days.

Drosera cupensis. — This species, a native of the Cape of Good Hope, was sent to me by Dr. Hooker. The leaves are elongated, slightly concave along the middle and taper towards the apex, which is bluntly pointed and reflexed. They rise from an almost woody axis, and their greatest peculiarity consists in their foliaceous green footstalks, T\nich are almost as broad and even longer than the gland-bearing blade. This species, therefore, probably draws more nourishment from the air, and less from captured insects, than the other species of the genus. Never- theless, the tentacles are crowded together on the disc, and are extremely numerous ; those on the margins being much longer than the central ones. All the glands have the same form ; their secretion is extremely viscid and acid.

The specimen which I examined had only just recovered from a weak state of health. This may account for the tentacles moving very slowly when particles of meat were placed on the glands, and perhaps for my never succeeding in causing any movement by repeatedly touching them with a needle. But with all the species of the genus this latter stimulus is the least effective of any. Particles of glass, cork, and coal-cinders, were placed on the glands of six tentacles ; and one alone moved after an interval of 2 hrs. 30 m. Nevertheless, two glands were ex- tremely sensitive to very small doses of the nitrate of ammonia, namely to about ^ of a minim of a solution (one part to 525U of water), containing only TTgVoo of a grain ('000562 mg.) of the salt. Fragments of flies were placed on two leaves near their tips, which became incurved in 15 hrs. A fly was also placed in the middle of the leaf ; in a few hours the tentacles on each side embraced it, and in 8 hrs. the whole leaf directly beneath the fly was a little bent transversely. By the next morning, after 23 hrs., the leaf was curled so completely over that the apex rested on the upper end of the footstalk. In no case did the sides of the leaves become inflected. A crushed fly was placed on the foliaceous footstalk, but produced no effect.

Drosera spatlmlata (sent to me by Dr. Hooker). — I made only a few observations on this Australian species, which has long, narrow leaves, gradually widening towards their tips. The glands of the extreme marginal tentacles are elongated and differ from the others, as in the case of Drosera rotundifoiia. A fly was placed on a leaf, and in 18 hrs. it was embraced by the adjoining tentacles. Gum-water dropped on several leaves produced no effect. A fragment of a leaf was immersed in a few drops of a solution of one part of carbonate of ammonia to 146 of water ; all the glands were instantly blackened ; the process of aggrega- tion could be seen travelling rapidly down the cells of the ten- tacles ; and the granules of protoplasm soon united into spheres and variously shaped masses, which displayed the usual move-

ments. Half a minim of a solution of one part of nitrate of ammonia to 146 of water was next placed on the centre of a leaf ; after G hrs. some marginal tentacles on both sides were inflected, and after 9 hrs. they met in the centre. The lateral edges of the leaf also became incurved, so that it formed a half-cylinder ; but the apex of the leaf in none of my few trials was inflected. The above dose of the nitrate (viz. -g^ of a grain, or '202 mg.) was too powerful, for in the course of 23 hrs. the leaf died.

Dyoseni filiformis. — This 'North American species grows in siif ) abundance in parts of New Jersey as almost to cover the ground. It catches, according to Mrs. Treat,* an extraordinary number of small and large insects,— even great flies of the genus Asilus, moths, and butterflies. The specimen which I examined, sent me by Dr. Hooker, had thread-like leaves, from 6 to 12 inches in length, with the upper surface convex and the lower flat and slightly channelled. The whole convex surface, down to the roots — for there is no distinct footstalk— is covered with short gland-bearing tentacles, those on the margins being the longest and reflexed. Bits of meat placed on the glands of some tentacles caused them to be slightly inflected in 20 m. ; but the plant was not in a vigorous state. After 6 hrs. they moved through an angle of 90°, and in 24 hrs. reached the centre. The surrounding tentacles by this time began to curve inwards. Ultimately a large drop of extremely viscid, slightly acid secretion was poured over the meat from the united glands. Several other glands were touched with a little saliva, and the tentacles became incurved in under 1 hr., and re-expanded after 18 hrs. Particles of glass, cork, cinders, thread, and gold-leaf, were placed on numerous glands on two leaves ; in about 1 hr. four tentacles became curved, and four others after an additional interval of 2 hrs. 30 m. I never once succeeded in causing any movement by repeatedly touching the glands with a needle ; and Mrs. Treat made similar trials for me with no success. (Small flies were placed on several leaves near their tips, but the thread-like blade became only on one occasion vf>ry slightly bent, directly beneath the insect. Perhaps this indicates that the blades of vigorous plants would bend ovei captured insects, and Dr.

Cauby informs me that this is the case ; but the movement cannot be strongly pronounced, as it was not observed by Mrs. Treat. JJrosera binata (or dichotoma). — I am much indebted to Ladj Dorothy Nevill for a fine plant of this almost gigantic Australian species, which differs in some interesting points from those pre- viously described. In this specimen the rush-like footstalks of the leaves were 20 inches in length. The blade bifurcates at its junction with the footstalk, and twice or thrice afterwards, curl- ing about in an irregular manner. It is narrow, being only -^ of an inch in breadth. One blade was 1\ inches long, so that the entire leaf, including the footstalk, was above 27 inches in length. Both surfaces are slightly hollowed out. The upper surface is covered with tentacles arranged in alternate rows ; those in the middle being short and crowded together, those towards the margins longer, even twice or thrice as long as the blade is broad. The glands of the exterior tentacles are of a much darker red than those of the central ones. The pedicels of all are green. The apex of the blade is attenuated, and bears very long tentacles. Mr. Copland informs me that the leaves of a plant which he kept for some years were generally covered with captured insects before they withered.

The leaves do not differ in essential points of structure or ol function from those of the previously described species. Bits of meat or a little saliva placed on the glands of the exterior tentacles caused well-marked movement in 3 m., and particles of glass acted in 4 m. The tentacles with the latter particles re-expanded after 22 hrs. A piece of leaf immersed in a few drops of a solution of one part of carbonate of ammonia to 437 of water had all the glands blackened and all the tentacles inflected in 5 m. A bit of raw meat, placed on several glands in the medial furrow, was well clasped in 2 hrs. 10 m. by the mar- ginal tentacles on both sides. Bits of roast meat and small flies did not act quite so quickly ; and albumen and iibrin still less quickly. One of the bits of meat excited so much secretion (which is always acid) that it flowed some way down the medial furrow, causing the inflection of the tentacles on both sides as far as it extended. Particles of glass placed on the glands in the medial furrow did not stimulate them sufficiently for any motor impulse to be sent to the outer tentacles. In no case was the blade of the leaf, even the attenuated apex, at all inflected.

On both the upper and lower surface of the blade there are numerous minute, almost sessile glands, consisting of four, eight, or twelve cells. On the lower surface they are pale purple, on the tapper greenish. Nearly similar organs occur on the foot- stalks, but they are smaller and often in a shrivelled condition. The minute glands on the blade can absorb rapidly: thus, a piece of leaf was immersed in a solution of one part of carbonate of ammonia to 218 of water (1 gr. to 2 oz.), and in 5 m. they were all so much darkened as to be almost black, with their contents aggregated. They do not, as far as I could observe, secrete spontaneously; but in between 2 and 3 hrs. after a leaf had been rubbed with a bit of raw meat moistened with saliva, they seemed to be secreting freely ; and this conclusion was afterwards supported by other appearances. They are, therefore, homologous with the sessile glands hereafter to be described on the leaves of Dionsea and Drosophyllum. In this latter genus they are associated, as in the present case, with glands which secrete spontaneously, that is, without being excited.

Drosera Tjinata presents another and more remarkable pecu- liarity, namely, the presence of a few tentacles on the backs of the leaves, near their margins. These are perfect in structure ; spiral vessels run up their pedicels; their glands are sur- rounded by drops of viscid secretion, and they have the power of absorbing. This latter fact was shown by the glands imme- diately becoming black, and the protoplasm aggregated, when a leaf was placed in a little solution of one part of carbonate of ammonia to 437 of water. These dorsal tentacles are short, not being nearly so long as the marginal ones on the upper surface ; some of them are so short as almost to graduate into the minute sessile glands. Their presence, number, and size, vary on different leaves, and they are arranged rather irre- gularly. On the back of one leaf I counted as many as twenty- one along one side.

These dorsal tentacles differ in one important respect from those on the upper surface, namely, in not possessing any power of movement, in whatever manner they may be stimulated. Thus, portions of four leaves were placed at different times in solutions of carbonate of ammonia (one part to 437 or 218 of water), and all the tentacles on the upper surface soon became closely inflected ; but the dorsal ones did not move, though the leaves were left in the solution for many hours, and though their glands from their blackened colour had obviously absorbed some of the salt. Rather young leaves should be selected for such trials, for the dorsal tentacles, as they grow old and begin to wither, often spontaneously incline towards the middle of the leaf. If these tentacles had possessed the power of movement, they would not have been thus rendered more serviceable to the plant ; for they are not long enough to bend round the margin of the leaf so as to reach an insect caught on the upper surface, Nor would it have been of any use if these tentacles could have

moved towards the middle of the lower surface, for there aro no viscid glands there by which insects can be caught. Although they have no power of movement, they are probably of some use by absorbing animal matter from any minute insect which may be caught by them, and by absorbing ammonia from the rain-water. But their varying presence and size, and their irregular position, indicate that they are not of much service, and that they are tending towards abortion. In a future chap- ter we shall see that Drosophyllum, with its elongated leaves, probably represents the condition of an early progenitor of the genus Drosera; and none of the tentacles of Drosophyllum, neither those on the upper nor lower surface of the leaves, are capable of movement when excited, though they capture numerous insects, which serve as nutriment. Therefore it seems that Dmsent, binata has retained remnants of certain ancestral characters — namely a few motionless tentacles on the backs of the leaves, and fairly well developed sessile glands — which have been lost by most or all of the other species of the genus.

Concluding Remarks. — From what we have now seen, there can be little doubt that most or probably all the species of Drosera are adapted for catching insects by nearly the same means. Besides the two Australian species above described, it is said* that two other species from this country, namely Drosera pallida and Drosera sulpliurea, " close their leaves upon insects with " great rapidity : and the same phenomenon is mani- " fested by an Indian species, D. lunata, and by several " of those of the Cape of Good Hope, especially by " D. trinervis" Another Australian species, Drosera lieteropliylla (made by Lindley into a distinct genus, Sondera) is remarkable from its peculiarly shaped leaves, but I know nothing of its power of catching insects, for I have seen only dried specimens. The leaves form minute flattened cups, with the footstalks attached not to one margin, but to the bottom. The

inner surface and the edges of the cups are studded with tentacles, which include nbro-vascular bundles, rather different from those seen by me in any other species ; for some of the vessels are barred and punc- tured, instead of being spiral. The glands secrete copiously, judging from the quantity of dried secretion adhering to them. Structure of the leaves — Sensitiveness of the filaments — Kapid 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 secre- tion— 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.

THIS plant, commonly called Venus' fly-trap, from the rapidity and force of its movements, is one of the most wonderful in the world.* It is a member of the small family of the Droseraceae, and is found only in the eastern part of North Carolina, growing in damp situations. The roots are small ; those of a mo- derately fine plant which I examined consisted of two branches about 1 inch in length, springing from a bulbous enlargement. They probably serve, as in the case of Drosera, solely for the absorption of water ; for a gardener, who has been very successful in the cultivation of this plant, grows it, like an epiphytic orchid, in well-drained damp moss without any soil.t The form of the bilobed leaf, with its foliaceous foot- stalk, is shown in the accompanying drawing (fig. 12).

* Dr. Hooker, in his address to the habits of this plant, that it the British Association at Belfast, would be superfluous on my part 1874, has given so full an histori- to repeat them, cal account of the observations f 'Gardener's Chronicle,' 1874, The two lobes stand at rather less than a right angle to each other. Three minute pointed processes or filaments, placed triangularly, project from the upper surfaces of both ; but I have seen two leaves with four filaments on each side, and another with only two. These filaments are remarkable from their extreme sensitiveness to a touch, as shown not by their own movement, but by that of the lobes. The margins of the leaf are prolonged into sharp rigid projections which I will call spikes, into each of which a bundle

(IHonaa musciputn.^ Leaf viewed laterally in its expands,' state. of spiral vessels enters. The spikes stand in such a position that, when the lobes close, they inter-lock like the teeth of a rat-trap. The midrib of the leaf, on the lower side, is strongly developed and prominent. The upper surface of the leaf is thickly covered, excepting towards the margins, with minute glands of a reddish or purplish colour, the rest of the leaf being green. There are no glands on the spikes, or on the foliaceous footstalk. The glands are formed of from

twenty to thirty polygonal cells, filled with purpJ.9 fluid. Their upper surface is convex. They stand on very short pedicels, into which spiral vessels do not enter, in which respect they differ from the tentacles of Drosera. They secrete, but only when excited by the absorption of certain matters ; and they have the power of absorption. Minute projections, formed of eight divergent arms of a reddish-brown or orange colour, and appearing under the microscope like elegant little flowers, are scattered in considerable numbers over the foot-stalk, the backs of the leaves, and the spikes, with a few on the upper surface of the lobes. These octofid projections are no doubt homologous with the papillae on the leaves of Drosera rotundifolia. There are also a few very minute, simple, pointed hairs, about , 8%ou ('0148 mm.) of an inch in length on the backs of the leaves.

The sensitive filaments are formed of several rows of elongated cells, filled with purplish fluid. They are a little above the -^ of an inch in length ; are thin and delicate, and taper to a point. I examined the bases of several, making sections of them, but no trace of the entrance of any vessel could be seen. The apex is sometimes bifid or even trifid, owing to a slight separation between the terminal pointed cells. Towards the base there is constriction, formed of broader cells, beneath which there is an articulation, supported on an enlarged base, consisting of differently shaped poly- gonal cells. As the filaments project at right angles to the surface of the leaf, they would have been liable to be broken whenever the lobes closed together, had it not been for the articulation which allows them to bend flat down.

These filaments, from their tips to their bases, are ex- quisitely sensitive to a momentary touch. It is scarcely possible to touch them ever so lightly or quickly with any hard object without causing the lobes to close. A piece of very delicate human hair, 2^ inches in length, held dangling over a filament, and swayed to and fro so as to touch it, did not excite any move- ment. But when a rather thick cotton thread of the same length was similarly swayed, the lobes closed. Pinches of fine wheaten flour, dropped from a height, produced no effect. The above-mentioned hair was then fixed into a handle, and cut off so that 1 inch projected ; this length being sufficiently rigid to sup- port itself in a nearly horizontal line. The extremity was then brought by a slow movement laterally into contact with the tip of a filament, and the leaf instantly closed. On another occasion two or three touches of the same kind were necessary before any movement ensued. When we consider how flexible a fine hair is, we may form some idea how slight must be the touch given by the extremity of a piece, 1 inch in length, moved slowly.