Chapter XIII: Insectivorous Plants

The statement in our previous chapter that the leaf has no power to absorb nitrogen, has to be received with a certain exceptions. These exceptions are discovered in a large group of plants, having little or no botanical relationship, and widely separated as regards their geographical distribution and habit of growth. The term insectivorous (insect-eating) has been applied to these by eminent botanists who have studied their habits and mode of growth. We may, as a preliminary to our study, summarise the main features of these interesting plants, because I wish my readers to see in them an extension and elaboration of the various processes we have tried to investigate in plant-life, and not a mere description of a few vegetable wonders. Rather would I point out that in studying these deviations from the ordinary type, as elsewhere, the young botanist should try to arrive at some explanation of these peculiarities, bearing always in mind that every part of the plant is created for some special purpose. This train of thought, if brought to bear upon our botanical study will prevent our regarding the contrivances of these insectivorous plants as mere freaks of nature, which appears to me to be a low and unworthy view to take of such delicate and wonderful structures.

Occasionally, it is true, we meet with monstrosities, in the formation of which we fail to see any hidden purpose; but even here by careful observation we shall probably be able to perceive that it is the result of some injury or the accompaniment of disease from which plant-life is no more free than animal-life is.

Let us now trace the features that are common to the plants which form the subject of this chapter.

Perhaps their most interesting function is that of catching and retaining insects. This is accomplished in various ways, by viscid fluids which imprison small flies, as in the leaves of the sundew and other plants; by movements in the leaves, as in the Venus fly-trap; by a combination of both, as in the butterwort; or by special pitfalls and traps, as in the pitcher plants, sarracenias, bladderwort, and cephalotus. Having caught their prey, these plants dissolve it by means of an acid secretion; the dissolved animal-life is then absorbed and appropriated for the purposes of vegetable growth. Not all these processes are carried on by insect-eating plants. In some, for example, the secretion of dissolving acid is not very apparent, in others the absorbing glands are not fully developed; but, briefly, the above features are those possessed by this singular class of plants, and there is every reason to believe that powers of this kind are more widely spread than is usually supposed.

We will now notice a few types in detail.

The sundew (_Drosera rotundifolia_) is the pretty and poetic name of a plant which may often be found on boggy moors. It is barely an inch in height, a mere rosette of leaves shaped like a battledore, radiating from a very short root stock, and bearing, in early summer, a central flower-stalk from four to six inches high, furnished with a few tiny white flowers. The whole plant lies close to the ground, and is often embedded in bogmoss, and, were it not for the bright colour of the leaves[27] and their sparkling dewy effect, it would be a difficult plant to find. With the naked eye we can see that the leaves are covered with hairs, and a lens will show still more plainly that these hairs have each a club-like end bearing a gummy fluid, in appearance not unlike glycerine. These globules of fluid sparkle in the sun; hence the name of sundew and the botanical name of _drosera_, from the Greek “_aroseros_,” or dewy.

[27] On sunny heaths they are often of a rich crimson tint.

Leaves with glandular hairs are not rare amongst our wild plants, and if this was the only character that the sundew possessed it would not be specially noticeable. It is, however, the unusual structure and behaviour of these hairs that claims our notice. The term tentacle is a not inappropriate one to apply to these “hairs.” A leaf of sundew, with all its tentacles standing out at different angles from the surface of the leaf, and each point armed with a drop of viscid fluid, is an effective arrangement for catching insects. The bright glistening drops are a fatal attraction to flies, gnats, and other small insects. When they alight upon the points of the tentacles they soon find that they are held prisoners. In their efforts to get free they entangle themselves more and more on the slimy points of the treacherous hairs. If we watch the tentacles after a fly has been caught, it will soon be seen that the hairs are bending over and closely pressing down the wretched captive. This folding over occupies four or five hours from the time the capture is made. The glands also begin to give out an increased amount of gummy secretion, and this flow kills the insect by stopping up its breathing pores, so that literally it dies of suffocation. The fluid not only increases in quantity, but becomes acid, and its effect is to dissolve the insect and render it soluble; the dissolved parts are then absorbed by the glands and digested. This interesting process can be watched quite easily by carefully taking up a few plants of sundew with some of the bog-soil and moss in which they were growing and placing them in a glass dish, where they will continue for months in perfect health if kept very wet and covered with a bell glass.

I once lighted on some magnificent sundew growing on boggy land near Woolmer Forest. Whilst taking up some roots of it I was persistently attacked by a stinging fly, and, my hands being occupied, I could not well defend myself. Happily the sundew acted a friendly part! I was carrying a tuft of it in my hand when, looking down, I saw my tormenting fly was securely caught upon its leaves. Somehow one always feels compassion for the unfortunate, and I confess I tried to rescue the captive, but the creature’s wings and legs were already so glued together by the viscid dew that it was impossible to release it, and I realised more than ever how effective the sundew is as a fly-trap.

In transplanting specimens of drosera great care should be taken that the leaves are untouched, else, being sticky, they will cling together and lose their delicate beauty. Every few days the plants may be fed, and happily they are quite willing to accept very minute pieces of raw beef, so that flies need not be sacrificed in the cause of science. The little “beafeater” must not be fed a second time until the hairs have uncurled and the leaf has fully expanded, showing that the last meal has been digested. I have kept a large pan of sundew in great beauty for about four months in summer, and when the glass was taken off and bright sunshine lit up the jewelled leaves the effect was lovely, and a magnifying glass showed the structure of the leaves and the prismatic colouring of the dew-tipped hairs.

The Venus fly-trap is an exotic member of the insectivorous family. Its leaves are remarkably like an ordinary spring rat-trap. A glance at the drawing will show its formation. On the two lobes of the leaf are a row of stiff bristles occupying the precise position of the teeth of a rat-trap. The inner surface of the leaves is of a reddish colour, due to its being thickly covered with minute red glands; on each lobe there are three stiff hairs. If a fly alighted on the leaf and walked across its surface, it would touch one of these hairs, and no matter how light the touch might be, the hairs are so sensitive they would convey the signal to the hinge of the lobes, and they would instantly rise up and clasp the fly, eventually crushing it to death. Then would follow, as in the case of the sundew, the emission of acrid secretion and the absorption and digestion of the insect.

Insect-destroying plants are numerous in the vegetable world. They may be roughly divided into three groups, although there is no strict line of demarcation between them. First, those like the red lychnis and others, which, by means of sticky hairs, catch and kill small insects, an operation that, so far as we know, results in no special good to the plant. Then there are those, like the sundew, which catch, kill, and digest the insect for food; whilst the third group consists of plants which catch and kill insects, but have no digestive process. Decomposition of the captured insects takes place, but the absorption which goes on is simply that of the liquid products of decomposition, the latter process resulting from the insects being immersed in fluid. To this latter group belong the pitcher plants (_Nepenthes_) and sarracenias. These last are North American plants of peculiar structure and appearance. The leaf is folded and modified into a tunnel-shaped tube differing in form in the various species. In all there is a kind of cap or lid to the tube, so that rain is kept out. In one or two species the lid is so arranged that the mouth is exposed. In the bottom of these tubes there is usually a quantity of somewhat slimy fluid. The inner face of the lid and surface just inside the rim of the tube is smooth, usually of a bright shining colour and covered with minute honey-secreting glands, a most attractive lure for insects. Below this honeyed surface the character of the sides of the tube changes completely; for, down to the fluid, it is covered with stiff hairs all pointing downwards. Now we see how the trap is set. The honey just inside the tube is attractive, and the insect feeding finds it very easy to descend the tube; the smooth surface offers no foothold, and the downward pointed hairs prevent it from returning, until at last the insect becomes engulfed in the pool of water at the bottom of the tube. In this fluid insects generally accumulate, decompose, and become liquid manure.

In Georgia and North Florida these sarracenias are found in the swamps in large quantities attaining one to two feet in height, their great tubes half-filled with insects showing their value in tending to reduce the swarms of flies which abound in such localities. We can see from these characteristics of the sarracenia a link between the insect-eating plants which have a true digestive process and ordinary plants that obtain their food in part direct from the soil. The sarracenia is simply making an attempt to collect nitrogenous food by the aid of its form and sweet secretions; thus it lures on flies and other insects to their doom, which to the plant means an increased supply of liquid manure for its nourishment.

Between the two types of insectivorous plants and ordinary plants there are endless varieties. The largest known species of “fly-catcher” is the _Roridula dentata_ of South Africa, which attains a height of six feet, with leaves similar to the sundew in character. So efficient are these leaves in catching flies that the Boers hang up branches in their rooms as fly-traps.

The smallest insect-eating plant is probably the bladderwort (_Utricularia vulgaris_), a rootless water plant with minute bladders on small thread-like leaves. The bladders only open inwards, so that when an insect pushes against the opening or valve it easily enters, and cannot get out again. The bladder contains water, but the insect quickly consumes the oxygen in it, and consequently dies, and when decayed its substance is absorbed by glands on the inner surface of the bladder.

PITCHER OF NEPENTHES RAFFLESIANA. ]

Perhaps the most attractive of the group of plants we are considering is the pitcher plant or Nepenthes. It grows commonly in Borneo and Ceylon. The pitcher is a direct development of the midrib of the leaf. It varies in size from the little thimble-like pitcher of _Nepenthes gracilis_ to the large jug-like receptacles of _Nepenthes Rafflesiana_[28] and others, each capable of holding nearly a pint of fluid. The pitchers are furnished with a lid overhanging the mouth of the receptacle, this is kept open by a thick rim. This rim and the under-surface of the lid both secrete a sweet fluid which is attractive to insects, and from the rim and opening of the mouth a smooth surface directs the ill-fated flies to the sweet sticky fluid always found at the bottom of the pitcher, out of which they rarely come alive.

[28] See Frontispiece.

Another of our native plants exhibiting these insectivorous habits is the butterwort (_Pinguicula_). Like the sundew it is a mere rosette of radical leaves, having upturned margins and a very succulent pellucid appearance. These leaves are covered with glands which exude a viscid kind of fluid like that on the tentacles of the sundew. This natural birdlime catches and holds small flies, midges, and other tiny flying creatures, as well as crawling insects. The presence of these insects on the leaf appears to stimulate it to further secretion which must, of course, lessen the chances of the insect’s escape, and as a further barrier to prevent its creeping away, the edges of the leaf begin slowly to curve inwards, so that the caught insect is imprisoned in the folds of the leaf. The acid secretion which now exudes from the glands soon dissolves all the nitrogenous and soft parts of the insect, which are taken up by the absorptive glands of the leaf. There are many other plants, of which I have not space to make mention, although they are full of interest, as owing to their curious structure, it is probable that insectivorous habits might also be ascribed to them. The field of study is a wide one, and throws much light upon the physiology of plants as well as the relationship between the plant and animal world. I would suggest to my young readers, as a practical means of knowing more of this subject, to try and grow for themselves the sundew, pinguicula, and sarracenia.

The two first can be found, as I have already said, on boggy moors in England, and the latter plant can be obtained from any florist. All can be successfully grown in a greenhouse or garden frame, and studying their growth and habits in this way will teach the young botanist far more agreeably than learning only from books.

At Kew there is always a fine collection of these insectivorous plants to be seen in vigorous growth, whilst at the South Kensington Natural History Museum (Botanical Department) there are some highly interesting cases illustrating the life history of these remarkable plants.

CHAPTER XIV

_HABIT OF GROWTH IN PLANTS_

“Some clothe the soil that feeds them, far diffused
And lowly creeping, modest and yet fair,
Like virtue, thriving most where little seen;
Some, more aspiring, catch the neighbour shrub
With clasping tendrils, and invest his branch,
Else unadorn’d, with many a gay festoon
And fragrant chaplet, recompensing well
The strength they borrow with the grace they lend.”

+Cowper.+