A parasite of a quite different sort may be studied in the familiar Mistletoe (_Viscum album_). It is the only parasitic native plant which is shrubby, or which perches itself on trees (the seeds being spread by birds, which devour the white berries). It is not, like some parasites, particular as to the species upon which it grows, flourishing equally upon a number of hosts, and even capable of living upon its own species. It differs from those parasites which we have been considering in possessing an abundance of green leaves, and being therefore capable of manufacturing its own food. At the same time, it has no roots which can penetrate the soil, and is incapable of an independent existence. It seems probable that its relations with its host are to some extent symbiotic--that is, each giving to the other--rather than purely parasitic, where the benefit is entirely on one side. The Mistletoe, retaining its leaves and manufacturing food throughout the year, is clearly capable of aiding its host, which loses its leaves in autumn, and cannot form fresh nourishment until spring is well advanced.
Before leaving this question of abnormal methods of procuring food as found among the higher plants, we may return for a few moments to the consideration of carnivorous plants, to which reference was made in Chapter IV. Of these the Sundews (_Drosera_), Butterworts (_Pinguicula_), and Bladderworts (_Utricularia_) supply very interesting examples within our own flora, which anyone may study on a holiday spent on the moors or mountains. The Sundews are familiar to all plant lovers--little plants of the bogland, usually growing among Sphagnum, and well distinguished by their leaves decked with spreading red hairs, each of which is tipped with a little drop of sparkling sticky fluid. It is these hairs or tentacles and their movements which place the Sundews among the most interesting of all plants. It is important to note that they are not hairs in the ordinary sense, which are organs of very simple structure arising from the epidermis or skin of the leaf. The tentacles of _Drosera_ have a complicated structure resembling that of leaves, and the tip is occupied by a gland which produces the sticky secretion already mentioned. These glands are exceedingly sensitive, and, moreover, sensitive in a selective way. They are unaffected by the drops of rain which frequently fall on them, but the touch of any solid body, especially of organic material, immediately affects them; most of all nitrogenous substances of any kind. Darwin found that a morsel of human hair weighing only 1/78,740 of a grain was sufficient to set the machinery of _Drosera_ in motion, and that immersion of a leaf in a solution of phosphate of ammonium so weak that each tentacle could absorb only 1/20,000,000 of a grain acted as a strong stimulus. In nature the stimulus is usually given by some unwary insect--a midge or other small flying creature--which, attracted by the bright colour or by the odour of the leaf, ventures too close, and becomes entangled among the sticky hairs. Then a most interesting series of events takes place. Almost at once the tentacles--first the ones actually touched, and then the adjoining ones--bend towards the point of disturbance, closing down one by one on the unfortunate victim till the leaf resembles a closed fist. At the same time the production of secretion increases, so as further to entangle the victim. When it is firmly secured, the secretion changes in character. Digestive ferments, closely resembling those by which animals digest their food, are poured out. These dissolve the animal’s body, all except the horny parts; the digested materials are then absorbed into the plant, which, as experiments show, benefits considerably by the addition to its diet of this animal food. When digestion is completed, the tentacles open again and prepare for a fresh victim. While the details of this remarkable process have been worked out only by careful and minute research in the laboratory, the main movements may be watched by anyone on any British moorland; or, bringing home a few plants in the damp moss in which they grow, we may amuse ourselves by experiments in feeding them.