Insects are undoubtedly caught in these pitchers, and their destiny is summed up in a few words by Dr. Tait: "In two pitchers I found insects bathed in fluid with a strongly acid reaction, and this fluid digested shreds of albumen exactly as I found the fluid of Nepenthes pitchers did. I conclude, therefore, that a true digestion of its victims is carried on by the Cephalotus pitchers."
The reader will experience no difficulty in coming to conclusions upon the facts we have submitted. This chapter may conclude with a reference to two plants mentioned by Knapp, but which have not received special attention since the subject of carnivorous plants has been fairly under investigation. "The different manner," he says, "in which vegetables exert their organic powers to effect the destruction of insects is not, perhaps, unworthy of a brief notice: some accomplish it by means of elastic or irritable actions, adhesive substances, and so forth; but we have another plant in our green-houses, the glaucous birthwort (Aristolochia glauca), that effects these purposes without any of these means, but principally by conformation. The whole internal surface of the tubular flower is beset with minute strong spines, pointing downwards; these present no impediment to the descent of the animal which may seek for the sweet liquor lodged upon the nectary at the base of the blossom, nor is there any obstruction provided for its return by means of valves or contractions, the tube remaining open; but the creature cannot crawl up by reason of the inverted spines; and, to prevent its escape by flying up the tube, the flower makes an extraordinary curve, bending up like a horn, so that any winged creature must be beaten back by striking against the roof of this neck as often as it attempts to mount, and, falling back to the bulbous prison at the base of the flower, dies by confinement and starvation, and there we find it: a certain number of these perishing, the blossom fades and drops off."
Of the other plant he writes: "It is a perplexing matter to reconcile our feelings to the rigour and our reason to the necessity of some plants being made the instrument of destruction to the insect world. Of British plants we have only a few so constructed, which, having clammy joints and calices, entangle them to death. The sundew (Drosera) destroys in a different manner, yet kills them without torture. But we have one plant in our gardens, a native of North America, than which none can be more cruelly destructive of animal life (Apocynum androsaemifolium), which is generally conducive to the death of every fly that settles upon it. Allured by the honey on the nectary of the expanded blossom, the instant the trunk is protruded to feed on it the filaments close, and, catching the poor fly by the extremity of its proboscis, detain the poor prisoner writhing in protracted struggles till released by death—a death apparently occasioned by exhaustion alone; the filaments then relax and the body falls to the ground. The plant will at times be dusky from the numbers of imprisoned wretches. This elastic action of the filaments may be conducive to the fertilising of the seed by scattering the pollen from the anthers, as is the case with the berberry; but we are not sensible that the destruction of the creatures which excite the action is in any way essential to the wants or perfection of the plant, and our ignorance favours the idea of a wanton cruelty in the herb; but how little of the causes and motives of action of created things do we know? and it must be unlimitable arrogance alone that could question the wisdom of the mechanism of Him 'that judgeth rightly'; the operations of a simple plant confound and humble us, and, like the handwriting on the wall, though seen by many, can be explained but by One."
Twenty-five years ago Mr. P. H. Gosse wrote thus, in reference to this very subject:—"The curious fact stated by Dr. Lindley, that specimens of the fly-trap fed with atoms of chopped meat have evidently thriven under the stimulating diet, and become more vigorous than others left to the resources of the soil and air, tends to confirm this supposition (of carnivorous habits), however strange it may appear. The frequent occurrence of startling facts, facts at variance with pre-established theories, forbids the philosophic naturalist to speak of any statement, professing to rest on observation, as impossible, merely because it has not been hitherto recognised, or cannot be reconciled with existing knowledge. It was the remark of a sage 'The more I learn, the more do I become convinced that I know nothing.'"
NOTE.—In Gerarde's "Herbal" (p. 601) it is written of the limewort, or viscaria: "The whole plant, as well leaves and stalks, as also the floures are here and there covered over with a most thicke and clammy matter, like unto bird-lime, which if you take in your hands, the sliminesse is such that your fingers will sticke and cleave together, as if your hand touched bird-lime: and furthermore, if flies doe light upon the same, they will be so entangled with the liminesse, that they cannot flie away; insomuch that in some hot day or other you shall see many flies caught by that means. Whereupon I have called it Catch Flie or Limewort."
CHAPTER VI
OTHER CARNIVOROUS PLANTS.
IT is by no means unusual for phenomena of the kind to which the preceding chapters have been devoted, to manifest themselves in a modified form in other organisms. These collateral, or supplementary instances, although of but little moment of themselves, are valuable when taken in their relationship to more complete and perfect examples. In plants to which we shall have occasion to refer, some of the phenomena already described, in association with insectivorous plants, will be repeated, but less intensified; so that such examples will hold an intermediate position between the sundews and plants in which no carnivorous propensities have been traced. As might be expected, some of these supplementary plants approach so closely that they cannot be separated from veritable carnivorous plants, whilst others recede so much, perhaps, as only to be strongly suspected of such proclivities.
The Butterworts are little bog-loving plants, mostly in hilly or mountainous districts, with pale green leaves spreading out like a rosette, and lying flat on the ground. The flowers rise from the centre of the rosette singly, on erect footstalks five or six inches in length. The common butterwort (Pinguicula vulgaris) has about eight oblong, thickish leaves in a rosette. They are not more than an inch and a half in length, with a very short footstalk. When fully expanded they fall outwards and lie with their under-surface closely pressed to the surface of the soil. The margins of the leaves have long been known to curve inwards in a peculiar manner, but the reason for this curving had not until recently been at all satisfactorily explained. That they did so was an acknowledged fact, but why they did so was hardly suspected. The surface of the leaves is clammy, and small objects are apt to adhere to them. It having been observed that insects were often found sticking to the leaves led to an investigation of the habits of the plant and its structure. Here, again, we are indebted to Mr. Darwin for a solution of the mystery, and must appeal to his work for the details of his discovery.
Image
Fig. 17.—Butterwort (Pinguicula lusitanica).
Thirty-nine leaves with objects adhering to them were sent from North Wales. Thirty-two of these had caught no less than 142 insects. Subsequently nine plants, bearing 74 leaves, were forwarded, and all of these latter, except three young leaves, had insects adhering to them. Another consignment of plants from Ireland had insects on 70 out of 157 leaves. Most of the insects were diptera, or two-winged flies. Circumstances such as these, presented in such a forcible manner to an investigator like Mr. Darwin, would naturally suggest to his mind the inquiry, "why, and how, are these insects caught?"
An examination of the structure of the leaves showed that the upper surface was thickly covered with glands of two kinds. (1) Large circular glands divided into sixteen cells, and supported upon elongated peduncles, or footstalks; and (2) smaller glands of a similar character on shorter peduncles. All the glands secrete a colourless sticky fluid, so viscid that it may be drawn out in threads for a foot or eighteen inches. The edges of the leaves are devoid of glands. Here, then, is sufficient cause for insects and other objects being found adhering to the leaves, as flies may be seen sticking to the old fashioned fly-papers. So that the plant deserves to be considered a "fly-catcher."
We have said that the margins of the leaves have long been known to curve inwards in a peculiar manner. A number of experiments were therefore instituted in order to determine the cause of this curvature, the result being that the movement was produced by irritation of a particular kind. Touching, pricking, or scratching, produced no effect, neither did drops of water, but the continued pressure of such inanimate objects as fragments of glass, or objects furnishing soluble matter, and infusion of meat, caused the leaves to curl. The shortest period at which a decided curvature was observed was about two hours and a quarter. After bending inwards, and remaining so for a short period, the margins open and expand again, say in about twenty-four hours, but are not stimulated to close again for some time. The curvature only takes place in one direction, that is, longitudinally, at either margin, but the apex does not become inflected. When flies or other small objects are placed near the margin of a leaf they are enclosed, or partly so, by the curvature. The glands thus brought into contact with such objects pour out their secretion. But why is the period of curvature so short? We must here permit Mr. Darwin to give his principal explanation.
"We have seen that when large bits of meat, &c., were placed on a leaf, the margin was not able to embrace them, but as it became incurved pushed them slowly towards the middle of the leaf. Any object, such as a moderately-sized insect, would thus be brought slowly into contact with a far larger number of glands, inducing much more secretion and absorption, than would otherwise have been the case. That this would be highly serviceable to the plant we may infer from the fact that Drosera has acquired highly-developed powers of movement, merely for the sake of bringing all its glands into contact with captured insects. In the case of Pinguicula, as soon as an insect has been pushed for some little distance towards the midrib, immediate re-expansion would be beneficial, as the margins could not capture fresh prey until they were unfolded." On the whole, the movements are not accounted for so satisfactorily and completely as in Drosera and Dionæa.
Image
Fig. 18.—Leaf of Butterwort with the edges curved inwards.
In all the British butterworts, and most others, the margins of the leaves naturally curve a little inwards, and this not only serves to prevent insects being washed away by the rain, but also to retain the secretion. When a number of glands have been excited, the secretion is so profuse that it trickles down towards the incurved edges, but by the curvature is prevented flowing off (fig. 18).
The viscid fluid thrown off by the glands when not excited by contact with insects, or other nitrogenous substances, or fluids, is not in the least acid, but when in contact with such substances is invariably acid. This is an important fact to be borne in mind. Flies caused the glands to secrete freely, the secretion being acid. Small bits of roast meat always caused much acid secretion in a few hours. Sugar and starch caused considerable secretion, but it was not acid. Bits of glass excited little or no secretion, that small quantity not being acid. The secretion, when containing animal matter in solution was soon absorbed, causing the glands, which previously were greenish and limpid, to become brownish, and contain aggregated animal matter. No such effect was produced by other substances. The consequence of this acidulation of the secretion, and the interpretation to be attached to the change of colour and aggregation of protoplasm in the glands, have already been explained in the chapter relating to the sundews, and therefore needs not to be repeated here. Suffice it to say that they indicate a process analogous to digestion.