THALLOPHYTES: FUNGI 273 



theridium comes in contact with the 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 heavy-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 bearing them have perished and 

 the oospores are liberated. 



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

 are very suggestive of the Siphon forms among Green Algae, 

 as is also the method of forming oogonia and antheridia. 



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

 tained the aquatic habit of the Alga?, and their asexual 

 spores are zoospores. Such forms as Mucor and Perono- 

 spora, however, 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 developed 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 spores. Taking the characters 

 all together, it seems reasonably clear that the Phycomycetes 

 are an assemblage of forms derived from Green Algae (Chlo- 

 rophycese) of various kinds. 



2. Ascomtcetes (Ascus- or Sac-Fiuigi) 



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

 especially upon leaves of seed plants, the mycelium spread- 

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



