82 



EVOLUTION OF PLANTS 



the fungi known as white-rusts and mildews. Of the 

 latter the potato-fungus, Phytophthora infestans, the 

 cause of the destructive " potato-rot," is one of the most 

 familiar. 



The water-moulds (Saprolegniacese), (Fig. 21, A, D) 

 are aquatic fungi, either saprophytes on the dead bodies 



an 



FIG. 21 (Phycomycetes). A, a dead fly covered with a growth of water- 

 mould (Saprolegnia) ; B, a sporangium of Saprolegnia about to open; 

 C, a single zoospore ; D, part of a plant of Saprolegnia with two young 

 oogonia, og; E, a filament of white-rust (Cystopus candidus) within 

 the tissues of the host-plant, showing the suckers or haustoria (h) by 

 which it absorbs its food from the cells of the host ; F, conidia, or non- 

 sexual spores of Cystopus being cut off from the ends of the filaments ; 

 G, two germinating conidia; H, a free zoospore which has escaped 

 from the conidium ; I, the oogonium, or/, with the egg, o, in process of 

 fertilization by the tube sent into the oogouium from the antheridium, 

 an. 



of insects and crustaceans, or in a few cases, like Sapro- 

 legnia ferax, which is a very destructive enemy of young 

 fish, they are true parasites. The latter species often 

 causes great damage to young fish in hatcheries. 



These water-moulds, and their immediate allies, closely 

 resemble in general structure such siphoneous algse as 

 Vaucheria, being, like the latter, made up of branching 

 filaments which show no division walls, but the proto- 

 plasm lining the wall of the tubular filament having 



