Parasitic Plants

Parasitic Plants are those which, unable to nourish themselves, prey upon other plants or animals; becoming attached, they gain access to the tissues of their host and feed upon its juices. They are more or less degenerate, according to the extent of their parasitism. Any climbing plant is so far a parasite, but, not drawing any nourishment from its host, merely using it as a support, it can live without it, and is perfect in all its parts. Many parasites have probably developed from such plants. The mistletoe, on the other hand, has no roots in the ground; its seed is left by a bird upon an apple or an oak tree, to which, when it begins to grow, it becomes attached by means of special organs called haustoria, which act as roots and enable it to draw crude sap, water, and salts from its host, and having green leaves it can absorb carbonic acid from the air, and elaborate food for its tissues. In the case of the dodder, again, which begins life as an independent plant, the seed germinates underground; when the young plant reaches the surface it fastens upon some host, twining round it, sending its haustoria deep into the tissues, and drawing all its nourishment from them; it bears no green leaves, but only flowers, while the part in the ground dies. In the Rafflesiaceæ, a foreign order, remarkable for the size of the flowers of some of its genera, the degradation has gone still further, and the whole plant consists of haustoria, a knob-like mass of tissue half formed by the host, and the flowers. There are some parasites which are attached to the roots instead of the stems of their hosts—e.g. Yellow Rattle, Cow-wheat, Eyebright. The attachment by the haustoria is always remarkably intimate; their tissues are always joined to the corresponding ones of the host, often in such a way that it is difficult to say to which plant they belong. The ovules of many parasites are rudimentary, the embryo is small and without chlorophyll; in cases of advanced parasitism it may even produce no leaves. There are parasitic genera in many orders—e.g. Corallorhiza in the Orchidææ, Cuscuta in Convolvulaceæ, Orobranche in Labiati floræ, Monotropa in Pyrolaceæ. The Loranthaceæ, of which is Viscum the mistletoe, the Balanophorææ, and the Santalaceæ are families of doubtful affinity. Nearly all these parasites have a marked preference for a particular species of host, and they are all flowering plants. But there are many others; two whole classes, the Bacteria and the Fungi, are either parasitic or, what is much the same thing, saprophytic—i.e. dependent upon decaying organic matter for food. The Bacteria have animals as their hosts, and cause in them many diseases, the species being often recognised by the disease. When they are saprophytic they cause fermentation and putrefaction. The Fungi are many of them a trouble in agriculture, causing corn, hop, and vine mildew, potato disease (see PLANTS, DISEASES OF), and also salmon disease; others, like the mushroom, are saprophytes. Allied to parasitism is Symbiosis (q.v.), a sort of mutually arranged parasitism for the benefit of both parties; as in the case of the Lichens, which consist of Algæ and Fungi in partnership.