312 



PART IV. — VEGETABLE TAXONOMY. 



usually a tube from it penetrates to the oosphere and fertilizes 

 it. In most cases the oospores rest for a considerable period 

 before germinating, and then, in some cases, develop a mycelium 

 directly, in others they first produce a germ-tube which develops 

 one or more sporangia which give rise to zoospores, and these, 

 in turn, to a vegetative mycelium. 



To the Peronosporese belong Phytophthora infestans, a mould 

 which produces potato disease ; Peronospora viticola, the Grape- 

 mildew, a most destructive Fungus, and 

 \qI Cystopus candidus, a mildew found on 



the Shepherd's-purse and some othej 

 Cruciferous plants. 



Some idea of the modes of repro- 

 duction in this group may be obtained 

 from Figs. 510 and 511. 



Fig. 510. 



Fig. 511. 



Fig. 510. — Portion of epidermis of Potato, showing three stomata with hyphae of Phy- 

 tophthora infestans issuing from them and bearing sporangia, a, one of the sporangia. 

 Magnification about ioo diameters. 



Fig. 511. — Stages in reproduction of Peronospora Alsinearum. A, earlier stage, fertili- 

 zation-tube of antheridium, a, not yet fully formed ; B, the tube fully formed and the 

 fertilization of the oogonium, o, completed. Magnified about 350 diameters. After DeBary. 



The Saprolegnieae are aquatic plants which form a dense 

 mycelium on the bodies of animals that decay in water, and 

 sometimes also upon submerged vegetable matter. They resem- 

 ble the Peronosporeas in many respects, but differ from them in 

 the fact that the sporangia produce ciliated swarm-spores, and in 

 the fact that the fertilized oogonia produce several oospores 

 instead of one. 



A curious and suggestive fact connected with some of the spe- 

 cies of this group is, that while oogonia and oospores are produced 



