Pythium

Pythium, Cystopus, and the potato-disease (Phytophthora) are comparatively simple moulds. The "rust" of wheat (Puccinia graminis) is a typical example of hetercecism, which only occurs in fungal parasites; whilst Agaricus melleus, by means of its subterranean "rhizomorphs," and the various species of Polyporus, are examples of parasitic Hynienomycetes (q.v.). Flowering parasites mostly germinate in the ground, many of them retaining some roots of their own, though becoming attached to the underground portion of the host. The eye-bright, cow-wheat, yellow and red rattle, among Scrophulariaoeae (q.v.) are only partly parasitic and have small green leaves. The broom-rapes (q.v.), though with some roots, have no chlorophyll, and are doubtless more completely so. The dodder (q.v.) after attaching itself to the stem of the host, round which it twines, by haustoria, which are probably modified adventitious roots, dies at its own root, thus losing any connection with the ground. The mistletoe (q.v.), though germinating on the branch of the host tree, has chlorophyll, and is, therefore, not entirely dependent. The allied sandal-wood (q.v.), though a root parasite, resembles the mistletoe in this respect. No flowering-plant is more completely parasitic than Rafflesia (q.v.), which consists mainly of one huge flower on a short root penetrating the stem of its host.