FUNGI, WATER MOLDS 255 



411. Asexual reproduction. This takes place by the forma- 

 tion of spore cases. In the common water mold (Saprolegnia}, 

 the spore cases are long and cylindrical. They are formed in the 

 ends of threads or branches by a cross wall which cuts off the pro- 

 toplasm from the rest of the thread. The spore case is usually 

 stouter than the thread which bears it. The spores are oval, with 

 two cilia on the smaller end. The spores swim out of an opening 

 at the end and after passing through a first swarming period 

 round up and pass a resting period. Then they slip out of the 

 thin membrane surrounding the protoplasm, and are bean-shaped, 

 with two cilia on the concave side. They now pass through 

 another swarming period. Then they come to rest and germinate, 

 if they have found a suitable substratum. 



412. Sexual reproduction. Sexual organs, sperm and egg 

 cases (antheridia and oogonia), are formed on the mycelium of the 



Fig. 214. 



Fertilization in Saprolegnia, tube of antheridium carrying in the nucleus of the sperm 

 cell to the egg. In the right-hand figure a smaller sperm nucleus is about to fuse with the 

 nucleus of the egg. (After Humphrey and Trow.) 



water molds, but it is a disputed question if fertilization takes 

 place. The egg case in some species is round, in others elongate. 

 It is formed on a short branch or directly in a thread, and is sepa- 

 rated by a cross wall. In Saprolegnia several eggs are formed in a 

 single egg case. The sperm case is a slender branch which coils 

 partly around the egg case and sends a fertilization tube inside and 

 in contact with the eggs. Some claim that a sperm nucleus from 

 the sperm case unites with the egg nucleus to bring about fertili- 

 zation, while others deny it. It is certain that in some species 

 sperm cases are not formed, and yet the eggs ripen without fertili- 



