Darwinism stated by Darwin himself / Characteristic passages from the writings of Charles Darwin

by Charles Darwin

Ordinary plants of the higher classes procure the requisite inorganic elements from the soil by means of their roots, and absorb carbonic acid from the atmosphere by means of their leaves and stems. But we have seen in a previous part of this work that there is a class of plants which digest and afterward absorb animal matter, namely, all the Droseraceæ, Pinguicula, and, as discovered by Dr. Hooker, Nepenthes, and to this class other species will almost certainly soon be added. These plants can dissolve matter out of certain vegetable substances, such as pollen, seeds, and bits of leaves. No doubt their glands likewise absorb the salts of ammonia brought to them by the rain. It has also been shown that some other plants can absorb ammonia by their glandular hairs; and these will profit by that brought to them by the rain. There is a second class of plants which, as we have just seen, can not digest, but absorb, the products of the decay of the animals which they capture, namely, Utricularia and its close allies; and, from the excellent observations of Dr. Mellichamp and Dr. Canby, there can scarcely be a doubt that Sarracenia and Darlingtonia may be added to this class, though the fact can hardly be considered as yet fully proved. There is a third class of plants which feed, as is now generally admitted, on the products of the decay of vegetable matter, such as the bird’s-nest orchid (Neottia), etc. Lastly, there is the well-known fourth class of parasites (such as the mistletoe), which are nourished by the juices of living plants. Most, however, of the plants belonging to these four classes obtain part of their carbon, like ordinary species, from the atmosphere. Such are the diversified means, as far as at present known, by which higher plants gain their subsistence.

HOW A PLANT PREYS UPON ANIMALS.

The genus described is Genlisea ornata.

Insectivorous Plants,
page 446.

The utricle is formed by a slight enlargement of the narrow blade of the leaf. A hollow neck, no less than fifteen times as long as the utricle itself, forms a passage from the transverse slit-like orifice into the cavity of the utricle. A utricle which measured 1/36 of an inch (·795 millimetre) in its longer diameter had a neck 15/36 (10·583 millimetres) in length, and 1/100 of an inch (·254 millimetre) in breadth. On each side of the orifice there is a long spiral arm, or tube; the structure of which will be best understood by the following illustration: Take a narrow ribbon and wind it spirally round a thin cylinder, so that the edges come into contact along its whole length; then pinch up the two edges so as to form a little crest, which will, of course, wind spirally round the cylinder, like a thread round a screw. If the cylinder is now removed, we shall have a tube like one of the spiral arms. The two projecting edges are not actually united, and a needle can be pushed in easily between them. They are indeed in many places a little separated, forming narrow entrances into the tube; but this may be the result of the drying of the specimens. The lamina of which the tube is formed seems to be a lateral prolongation of the lip of the orifice; and the spiral line between the two projecting edges is continuous with the corner of the orifice. If a fine bristle is pushed down one of the arms, it passes into the top of the hollow neck. Whether the arms are open or closed at their extremities could not be determined, as all the specimens were broken; nor does it appear that Dr. Warming ascertained this point.

So much for the external structure. Internally the lower part of the utricle is covered with spherical papillæ, formed of four cells (sometimes eight, according to Dr. Warming), which evidently answer to the quadrifid processes within the bladders of Utricularia. These papillæ extend a little way up the dorsal and ventral surfaces of the utricle; and a few, according to Warming may be found in the upper part. This upper region is covered by many transverse rows, one above the other, of short, closely approximate hairs, pointing downward. These hairs have broad bases, and their tips are formed by a separate cell. They are absent in the lower part of the utricle where the papillæ abound. The neck is likewise lined throughout its whole length with transverse rows of long, thin, transparent hairs, having broad bulbous bases, with similarly constructed sharp points. They arise from little projecting ridges, formed of rectangular epidermic cells. The hairs vary a little in length, but their points generally extend down to the row next below; so that, if the neck is split open and laid flat, the inner surface resembles a paper of pins—the hairs representing the pins, and the little transverse ridges representing the folds of paper through which the pins are thrust. These rows of hairs are indicated in the previous figure by numerous transverse lines crossing the neck. The inside of the neck is also studded with papillæ; those in the lower part are spherical and formed of four cells, as in the lower part of the utricle; those in the upper part are formed of two cells, which are much elongated downward beneath their points of attachment. These two-celled papillæ apparently correspond with the bifid process in the upper part of the bladders of Utricularia. The narrow transverse orifice is situated between the bases of the two spiral arms. No valve could be detected here, nor was any such structure seen by Dr. Warming. The lips of the orifice are armed with many short, thick, sharply pointed, somewhat incurved hairs or teeth.

The two projecting edges of the spirally-wound lamina, forming the arms, are provided with short incurved hairs or teeth, exactly like those on the lips. These project inward at right angles to the spiral line of junction between the two edges. The inner surface of the lamina supports two-celled, elongated papillæ, resembling those in the upper part of the neck, but differing slightly from them, according to Warming, in their footstalks being formed by prolongations of large epidermic cells; whereas the papillæ within the neck rest on small cells sunk amid the larger ones. These spiral arms form a conspicuous difference between the present genus and Utricularia.

Lastly, there is a bundle of spiral vessels which, running up the lower part of the linear leaf, divides close beneath the utricle. One branch extends up the dorsal and the other up the ventral side of both the utricle and neck. Of these two branches, one enters one spiral arm, and the other branch the other arm.

The utricles contained much débris, or dirty matter, which seemed organic, though no distinct organisms could be recognized. It is, indeed, scarcely possible that any object could enter the small orifice and pass down the long, narrow neck, except a living creature. Within the necks, however, of some specimens, a worm, with retracted horny jaws, the abdomen of some articulate animal, and specks of dirt, probably the remnants of other minute creatures, were found. Many of the papillæ within both the utricles and necks were discolored, as if they had absorbed matter.

From this description it is sufficiently obvious how genlisea secures its prey. Small animals entering the narrow orifice—but what induces them to enter is not known any more than in the case of Utricularia—would find their egress rendered difficult by the sharp incurved hairs on the lips, and, as soon as they passed some way down the neck, it would be scarcely possible for them to return, owing to the many transverse rows of long, straight, downward-pointing hairs, together with the ridges from which these project. Such creatures would, therefore, perish either within the neck or utricle; and the quadrifid and bifid papillæ would absorb matter from their decayed remains. The transverse rows of hairs are so numerous that they seem superfluous merely for the sake of preventing the escape of prey, and, as they are thin and delicate, they probably serve as additional absorbents, in the same manner as the flexible bristles on the infolded margins of the leaves of aldrovanda. The spiral arms, no doubt, act as accessory traps. Until fresh leaves are examined, it can not be told whether the line of junction of the spirally-wound lamina is a little open along its whole course or only in parts, but a small creature which forced its way into the tube at any point would be prevented from escaping by the incurved hairs, and would find an open path down the tube into the neck, and so into the utricle. If the creature perished within the spiral arms, its decaying remains would be absorbed and utilized by the bifid papillæ. We thus see that animals are captured by genlisea, not by means of an elastic valve, as with the foregoing species, but by a contrivance resembling an eel-trap, though more complex.