It is by no means impossible that sometimes the germ cell may develop into a spore without being fertilized, as is the case in many of the water moulds.
Fig. 35.—Non-sexual spores of the vine mildew (Peronospora viticola), × 150.
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]).
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.