WATER MOLDS 187 



basally rooted. All its vegetative portion is continuous; 

 the reproductive portions only are separated from the 

 rest of the plant-body by partitions. 



274. The propagation is very much the same as in 

 Green Felt. It may be briefly described as follows for 

 Saprolegnia: The protoplasm in the end of a branch 

 becomes somewhat condensed, a partition forms, cutting 

 off this portion from the remainder of the filament, and 

 the whole of its contents becomes converted by inter- 

 nal cell division into zoospores provided with two cilia. 

 These soon escape from a fissure in the wall and are active 

 for a few minutes, after which they come to rest and their 

 cilia disappear. In one or two hours they germinate by 

 sending out a filament, from which a new plant is quickly 

 produced. 



275. The sexual organs also bear a close resemblance 

 to those of Green Felt. The oogones are spherical, or 

 nearly so (in most of the species), and contain from one 

 to many eggs, which are fertilized by means of antherids, 

 which usually develop as lateral branches just below the 

 oogones. Fertilization takes place by the direct contact 

 of the antherid and the passage of its contents into the 

 oogone by means of a tubular process from the former. 

 In some species there is no transfer of the contents of 

 the antherid, and in others again there are no antherids. 

 These eggs must therefore develop without fertilization, 

 indicating that sexuality is disappearing in these plants. 

 Eventually each egg becomes covered with a wall of 

 cellulose and is thus transformed into a resting spore, 

 which later germinates by sending out a tube, as in 

 Green Felt. 



276. The Downy Mildews {Peronosporaceae) and 

 White Rusts (Alhuginaccae) live parasitically in the 

 tissues of higher plants. They are composed of long 



