TIIALLOPIIYTES: FUNGI 273 



theridium comes in contact with tlie oogonium, puts out a 

 tube which pierces the oogonium wall and enters the egg, 

 into which the contents of the antheridium are discharged, 

 and fertilization is effected. The result is a hcavy-walled 

 oospore. As the oospores are not for immediate germina- 

 tion, they are not brought to the surface of the host and 

 scattered, as are the asexual spores. When they are ready 

 to germinate, the leaves hearing them have perished and 

 the oospores are liberated. 



183. Conclusions. — The coenocytic bodies of the whole group 

 are very suggestive of the Siphon forms among Green Alga^, 

 as is also the method of forming oogouia and antheridia. 



The water-moulds, Saprolegnia and its allies, have re- 

 tained the aquatic habit of the Algaj, and their asexual 

 spores are zoospores. Such forms as Mucor and Perono- 

 cpora, liowever, have adapted themselves to terrestrial con- 

 ditions, zoospores are abandoned, and light spores are de- 

 veloped which can be carried about by currents of air. 



In most of them motile gametes are abandoned. Even 

 in the heterogamous forms sperms are not organized within 

 the antheridium, but the contents of the antheridium are 

 discharged through a tube develojied by the wall and pene- 

 trating the oogonium. It should be said, however, that a 

 few forms in this group develop sperms, which make them 

 all the more alga-like. 



They are both isogamous and heterogamous, both zygotes 

 and oospores being resting sfiores. Taking the characters 

 all together, it seems reasonably clear that the Pliycomycetes 

 are an assemblage of forms derived from Green Alga? (Chlo- 

 rophyceffi) of various kinds. 



3. AscoMYCETES {AficKs- Or Sac-Futif/i) 



184. Mildews. — These are very common jDarasites, growing 

 especially upon leaves of seed plants, the mycelium spread- 

 ing over the surface like a cobweb. A very common mil- 



