FUNGI, WATER MOLDS 281 



424. 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 germi- 

 nate, if they have found a suitable substratum. 



425. Sexual reproduction. Sexual organs, sperm and egg 

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



Fig. 225. 



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.) 



the 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. The union of a sperm nucleus from 

 the sperm case with the egg nucleus brings about fertilization. 

 It is certain that in some species sperm cases are not formed, 



