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Chapter 12

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.


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 little.

[Drosera anglica (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 1/5 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 particles, 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 frequently capture insects, and the glands of the inflected tentacles pour forth much acid secretion. Bits of roast meat were placed on some glands, and the tentacles began to move in 1 m. or

* Mrs. Treat has given an excellent account in 'The American Naturalist,' December 1873, p. 705, of Drosera longifolia (which is a synonym in part of Drosera anglica), of Drosera rotundifolia and filiformis.

1 m. 30 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 superfluous 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 protoplasm. 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 anglica 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 180o. 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 difference 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 anglica. 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 capensis.--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, which 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. Nevertheless, 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 extremely sensitive to very small doses of the nitrate of ammonia, namely to about 1/20 of a minim of a solution (one part to 5250 of water), containing only 1/115200 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 spathulata (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 rotundifolia. 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 aggregation could be seen travelling rapidly down the cells of the tentacles; and the granules of protoplasm soon united into spheres and variously shaped masses, which displayed the usual movements. 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 6 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. 1/320 of a grain, or .202 mg.) was too powerful, for in the course of 23 hrs. the leaf died.

Drosera filiformis.--This North American species grows in such 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 90o, 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 very slightly bent, directly beneath the insect. Perhaps this indicates that the blades of vigorous plants would bend over captured insects, and Dr. Canby informs me that this is the case; but the movement cannot be strongly pronounced, as it was not observed by Mrs. Treat.

Drosera binata (or dichotoma).--I am much indebted to Lady

* 'American Naturalist,' December 1873, page 705.

Dorothy Nevill for a fine plant of this almost gigantic Australian species, which differs in some interesting points from those previously 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, curling about in an irregular manner. It is narrow, being only 3/20 of an inch in breadth. One blade was 7 1/2 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 of 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 marginal tentacles on both sides. Bits of roast meat and small flies did not act quite so quickly; and albumen and fibrin 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 upper 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 Dionaea 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 binata presents another and more remarkable peculiarity, 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 surrounded by drops of viscid secretion, and they have the power of absorbing. This latter fact was shown by the glands immediately 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 irregularly. 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 are 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 chapter 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 Drosera 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 sulphurea, "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 heterophylla (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

* 'Gardener's Chronicle,' 1874, p. 209.

inner surface and the edges of the cups are studded with tentacles, which include fibro-vascular bundles, rather different from those seen by me in any other species; for some of the vessels are barred and punctured, instead of being spiral. The glands secrete copiously, judging from the quantity of dried secretion adhering to them.


Charles Darwin

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