236 PYTHIUM AND SAPROLEGNIA 



hypocotyls of the diseased seedlings (Fig. 124, B), which soon give 

 way at this point and collapse. The rounded gonidia, which are 

 borne on simple or forked hyphse (Fig. 124, C), usually give rise to 

 zoospores without becoming detached, so that the disease rapidly 

 spreads from one seedling to another. The sexual reproduction 

 of Pythium and Phytophthora is practically identical with that 

 of Cystopus. 



Owing to the ease with which the Fungi just considered 

 produce zoospores, which of course require a film of moisture 

 in which to swim, spells of damp, warm weather are particularly 

 favourable to their spread and development. Indeed, the 

 damping-off of seedlings through attacks of Pythium only occurs 

 in conditions of excessive humidity due to overwatering or 

 overcrowding. 



In the aquatic Phycomycetes reproduction by zoospores is 

 naturally the rule. The well-known Salmon disease is caused 

 by one of these Fungi (Saprolegnia ferox), which grows on the 

 gills of the fish, where its wefts cause asphyxiation. The biciliate 

 zoospores are produced in large numbers in tubular sporangia 

 (Fig. 124, D, E), whilst the only essential difference in the sexual 

 reproduction of this genus lies in the development of several, or 

 even many, eggs in each oogonium. In many of the species of this 

 genus, moreover, the eggs develop into oospores without fertili- 

 sation (so-called apogamy), although functionless antheridia may 

 be formed. 



In contrasting the Fungi hitherto described with the Algse, 

 one of the most striking peculiarities, apart from the absence 

 of chlorophyll, is the non-motile character of the male gamete. 

 Spermatozoids, as a matter of fact, are known to occur only in 

 one small group of the Phycomycetes. This feature may be 

 related to the fact that the Fungi as a whole are a terrestrial 

 group, living under conditions (e.g. in the interior of a host-plant) 

 in which the necessary moisture for the movement of spermato- 

 zoids is not available. 



The saprophytes among Phycomycetes are well exemplified 

 by Mucor, which thrives on all kinds of decaying substrata 

 (especially horse manure), upon which its mycelium forms a 

 white weft (Fig. 125, a). Numerous absorptive branches pene- 

 trate downwards into the source of nourishment, and sooner or 



