Parasitism

Parasitism, Vegetable, the growth of a plant attached to another living organism, and depending upon it wholly or in part for its nutriment. In other words, parasitism is that form of symbiosis (q.v.) in which the physiological benefit is entirely obtained by one of the associated organisms, the parasite, the other being termed the host. Every gradation from partial to complete parasitism exists, and some plants combine the character of saprophytes (q.v.), living, that is, upon dead organic matter, with parasitism. Parasitism is generally accompanied by the partial or complete loss of chlorophyll, and a reduction in leaves or other assimilating organs, they being rendered unnecessary by this habit. At the same time the uncertainty of the seedling finding a suitable host leads often to increase in the number of seeds. All true parasitic plants seem to be either fungi (q.v.) or dicotyledons, the latter belonging either to the Gamopetalas (q.v.) or to the Incompletae (q.v.). Only in the case of fungi do vegetable parasites attack animals. Such are the salmon-disease (Saprolegnia ferax), Empusa. and Cordyceps, which attack insects, and the fungi producing favus, ringworm and thrush in human beings. All fungi are either saprophytes or parasites, and in none is any chlorophyll present. Most of the parasitic forms are entophytic, growing, that is, mainly within the tissues of tire host, producing copious mycelium (q.v.), and sending out haustoria, or sucker-like branches, through the cell-walls.