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 (Saprolegniacee), (Fig. 21, A, D) 
are aquatic fungi, either saprophytes on the dead bodies 
Fic. 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 zodspore; 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 (2) 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 zodspore which has escaped 
from the conidium ; I, the odgonium, og, with the egg, o, in process of 
fertilization by the tube sent into the odgonium 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 alge 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 
