114 PLANT MORPHOLOGY 



rence of two types of zoospores is very puzzling and its significance has 

 never been satisfactorily explained. 



Saprolegnia is heterogamoiis, forming sex organs on special branches 

 of the mycelium (Fig. 89B). The oogonium is a spherical cell that pro- 

 duces several eggs, sometimes many, rarely only one. At first they are 

 multinucleate but, by degeneration of the extra nuclei, become uni- 

 nucleate. The antheridium is a slender curved tube that arises just 

 below the oogonium or, in most species, from an adjacent hypha. Each 

 oogonium may be surrounded by several antheridia. Both kinds of sex 

 organs are cut off from the vegetative mycelium by a basal wall. No 

 sperms are organized. Instead, the tip of the antheridium comes in 

 contact with the oogonium and sends into one or more of the eggs a fer- 

 tilization tube through which some of the cytoplasm and a male nucleus 

 pass. This nucleus unites with the egg nucleus, resulting in fertilization. 

 The zygote secretes a heavy wall and usually remains dormant for several 

 months, finally producing a new mycelium. In Achlya it has been shown 

 that the reduction of chromosomes occurs when the zygote germinates. 

 In some species of Saprolegnia the antheridia are nonfunctional, while 

 in others antheridia are not even formed. Nevertheless the eggs become 

 thick-walled and later germinate, thus developing by parthenogenesis. 



In Achlya, which is dioecious, the appearance of sex organs is caused by 

 hormone-like substances. These are secreted into the water by the male 

 and female plants and stimulate production of sex organs of the opposite 

 sex. A hormone produced by the male plants causes oogonia to appear 

 on the female plants, while a hormone produced by the female plants 

 results in the appearance of antheridia on the male plants. 



Summary. The Saprolegniales are chiefly saprophytes. They are 

 aquatic fungi Avith a well-developed mycelium. They produce biciliate 

 zoospores in persistent sporangia. All of them are heterogamous. The 

 oogonium contains one or more eggs that are fertilized by a male nucleus 

 coming from the antheridium through a fertilization tube. In most 

 members the entire oogonial protoplast enters into the formation of eggs. 

 The absence of swimming sperms in an exclusively aquatic order is a 

 noteworthy feature. 



5. Peronosporales 



The Peronosporales, or downy mildews, are mostly parasites that 

 attack various seed plants, the mycelium living within the intercellular 

 spaces of the host. The order includes about 12 genera and 150 species, 

 representative members being Pythium, Albugo, Phytophthora, Plasmo- 

 para, and Peronospora. 



Albugo. This fungus lives as a parasite on a number of different seed 

 plants. A common species, Albugo Candida, attacks various members of 



