Closely related to the species above described is another one (_C. candidus_), which attacks shepherd's-purse, radish, and others of the mustard family, upon which it forms chalky white blotches, and distorts the diseased parts of the plant very greatly. For some reasons this is the best species for study, longitudinal sections through the stem showing very beautifully the structure of the fungus, and the penetration of the cells of the host[4] by the suckers (Fig. 34). [4] "Host," the plant or animal upon which a parasite lives. [Illustration: FIG. 35.--Non-sexual spores of the vine mildew (_Peronospora viticola_), × 150.] Very similar to the white rusts in most respects, but differing in the arrangement of the non-sexual spores, are the mildews (_Peronospora_, _Phytophthora_). These plants form mouldy-looking patches on the leaves and stems of many plants, and are often very destructive. Among them are the vine mildew (_Peronospora viticola_) (Fig. 35), the potato fungus (_Phytophthora infestans_), and many others. ORDER III.--_Saprolegniaceæ_ (WATER MOULDS). These plants resemble quite closely the white rusts, and are probably related to them. They grow on decaying organic matter in water, or sometimes on living water animals, fish, crustaceans, etc. They may usually be had for study by throwing into water taken from a stagnant pond or aquarium, a dead fly or some other insect. After a few days it will probably be found covered with a dense growth of fine, white filaments, standing out from it in all directions (Fig. 36, _A_). Somewhat later, if carefully examined with a lens, little round, white bodies may be seen scattered among the filaments. [Illustration: FIG. 36.--_A_, an insect that has decayed in water, and become attacked by a water mould (_Saprolegnia_), natural size. _B_, a ripe zoösporangium, × 100. _C_, the same discharging the spores. _D_, active. _E_, germinating zoöspores, × 300. _F_, a second sporangium forming below the empty one. _G_ i-iv, development of the oögonium, × 100. _H_, ripe oögonium filled with resting spores, × 100.] On carefully removing a bit of the younger growth and examining it microscopically, it is found to consist of long filaments much like those of _Vaucheria_, but entirely destitute of chlorophyll. In places these filaments are filled with densely granular protoplasm, which when highly magnified exhibits streaming movements. The protoplasm contains a large amount of oil in the form of small, shining drops.