This afternoon, strolling through the under-cliff, I have come across two quaint and rather uncommon flowers among the straggling brushwood. One of them is growing like a creeper around the branches of this overblown gorse-bush. It is the lesser dodder, a pretty clustering mass of tiny pale pink convolvulus blossoms. The stem consists of a long red thread, twining round and round the gorse, and bursting out here and there into thick bundles of beautiful bell-shaped flowers. But where are the leaves? You may trace the red threads through their labyrinthine windings up and down the supporting gorse-branches all in vain: there is not a leaf to be seen. As a matter of fact, the dodder has none. It is one of the most thorough-going parasites in all nature. Ordinary green-leaved plants live by making starches for themselves out of the carbonic acid in the air, under the influence of sunlight; but the dodder simply fastens itself on to another plant, sends down rootlets or suckers into its veins, and drinks up sap stored with ready-made starches or other foodstuffs, originally destined by its host for the supply of its own growing leaves, branches, and blossoms. It lives upon the gorse just as parasitically as the little green aphides live upon our rose-bushes. The material which it uses up in pushing forth its long thread-like stem and clustered bells is so much dead loss to the unfortunate plant on which it has fixed itself.
Old-fashioned books tell us that the mistletoe is a perfect parasite, while the dodder is an imperfect one; and I believe almost all botanists will still repeat the foolish saying to the present day. But it really shows considerable haziness as to what a true parasite is. The mistletoe is a plant which has taken, it is true, to growing upon other trees. Its very viscid berries are useful for attaching the seeds to the trunk of the oak or the apple; and there it roots itself into the body of its host. But it soon produces real green leaves of its own, which contain the ordinary chlorophyll found in other leaves, and help it to manufacture starch, under the influence of sunlight, on its own account. It is not, therefore, a complete drag upon the tree which it infests; for though it takes sap and mineral food from the host, it supplies itself with carbon, which is after all the important thing for plant-life. Dodder, however, is a parasite pure and simple. Its seeds fall originally upon the ground, and there root themselves at first like those of any other plant. But, as it grows, its long twining stem begins to curl for support round some other and stouter stalk. If it stopped there, and then produced leaves of its own, like the honeysuckle and the clematis, there would be no great harm done: and the dodder would be but another climbing plant the more in our flora. However, it soon insidiously repays the support given it by sending down little bud-like suckers, through which it draws up nourishment from the gorse or clover on which it lives. Thus it has no need to develop leaves of its own; and it accordingly employs all its stolen material in sending forth matted thread-like stems and bunch after bunch of bright flowers. As these increase and multiply, they at last succeed in drawing away all the nutriment from the supporting plant, which finally dies under the constant drain, just as a horse might die under the attacks of a host of leeches. But this matters little to the dodder, which has had time to be visited and fertilised by insects, and to set and ripen its numerous seeds. One species, the greater dodder, is thus parasitic upon hops and nettles; a second kind twines round flax; and the third, which I have here under my eyes, mainly confines its dangerous attentions to gorse, clover, and thyme. All of them are, of course, deadly enemies to the plants they infest.
How the dodder acquired this curious mode of life it is not difficult to see. By descent it is a bind-weed, or wild convolvulus, and its blossoms are in the main miniature convolvulus blossoms still. Now, all bind-weeds, as everybody knows, are climbing plants, which twine themselves round stouter stems for mere physical support This is in itself a half-parasitic habit, because it enables the plant to dispense with the trouble of making a thick and solid stem for its own use. But just suppose that any bind-weed, instead of merely twining, were to put forth here and there little tendrils, something like those of the ivy, which managed somehow to grow into the bark of the host, and so naturally graft themselves to its tissues. In that case the plant would derive nutriment from the stouter stem with no expense to itself, and it might naturally be expected to grow strong and healthy, and hand down its peculiarities to its descendants. As the leaves would thus be rendered needless, they would first become very much reduced in size, and would finally disappear altogether, according to the universal custom of unnecessary organs. So we should get at length a leafless plant, with numerous flowers and seeds, just like the dodder. Parasites, in fact, whether animal or vegetable, always end by becoming mere reproductive sacs, mechanisms for the simple elaboration of eggs or seeds. This is just what has happened to the dodder before me.
The other queer plant here is a broomrape. It consists of a tall, somewhat faded-looking stem, upright instead of climbing, and covered with brown or purplish scales in the place of leaves. Its flowers resemble the scales in colour, and the dead-nettle in shape. It is, in fact, a parasitic dead-nettle, a trifle less degenerate as yet than the dodder. This broomrape has acquired somewhat the same habits as the other plant, only that it fixes itself on the roots of clover or broom, from which it sucks nutriment by its own root, as the dodder does by its stem-suckers. Of course it still retains in most particulars its original characteristics as a dead-nettle; it grows with their upright stem and their curiously shaped flowers, so specially adapted for fertilisation by insect visitors. But it has naturally lost its leaves, for which it has no further use, and it possesses no chlorophyll, as the mistletoe does. Yet it has not probably been parasitic for as long a time as the dodder, since it still retains a dwindling trace of its leaves in the shape of dry purply scales, something like those of young asparagus shoots. These leaves are now, in all likelihood, actually undergoing a gradual atrophy, and we may fairly expect that in the course of a few thousand years they will disappear altogether. At present, however, they remain very conspicuous by their colour, which is not green, owing to the absence of chlorophyll, but is due to the same pigment as that of the blossoms. This generally happens with parasites, or with that other curious sort of plants known as saprophytes, which live upon decaying living matter in the mould of forests. As they need no green leaves, but have often inherited leafy structures of some sort, in a more or less degenerate condition, from their self-supporting ancestors, they usually display most beautiful colours in their stems and scales, and several of them rank amongst our handsomest hot-house plants. Even the dodder has red stalks. Their only work in life being to elaborate the materials stolen from their host into the brilliant pigments used in the petals for attracting insect fertilisers, they pour this same dye into the stems and scales, which thus render them still more conspicuous to the insects’ eyes. Moreover, as they use their whole material in producing flowers, many of these are very large and handsome; one huge Sumatran species has a blossom which measures three feet across. On the other hand, their seeds are usually small and very numerous. Thousands of seeds must fall on unsuitable places, spring up, and waste all their tiny store of nourishment, find no host at hand on which to fasten themselves, and so die down for want of food. It is only by producing a few thousand young plants for every one destined ultimately to survive that dodders and broomrapes manage to preserve their types at all.