THE FUNGI : ARCHIMYCETES AND PHYCOMYCETES 207 



host tissue after the retraction of the flagella and sinks to the bottom of the 

 host cell. 



This body then enlarges in size, forming a resting sporangium (Fig. 198), 

 which is characterized by having a thick wall composed of three layers. The 

 inner two are derived from the tissues of the parasite, but the third is obtained 

 from material originating from the disorganization of the host cell. The 

 contents of the resting sporangium divide up into zoospores, w^hich are finally 

 liberated by the swelling of the outer layer and the cracking of the inner 

 layers of the wall. These zoospores are oval in shape with a basal nucleus 

 and a single long apical flagellum. It is during the formation of these 

 zoospores that meiosis occurs. They germinate to produce sori in the tissues 

 of the host plant. 



In order to distinguish more clearly between the sporangia produced in 

 the sori and those with thick walls produced from the zygotes, the former 

 are sometimes referred to as summer sporangia and the others as the winter 

 sporangia (Fig. 199). 



Wart Formation 



The formation of wart-galls in the host tissue is brought about in the 

 following way. A cell invaded by a zoospore enlarges as the parasite grows, 

 and at the same time the adjacent cells elongate and divide, thus forming a 

 raised rosette of cells around the infected one. Later the cells of the rosette 

 may be secondarily infected by zoospores originating from the sporangia 

 formed in the cell of primary infection. These cells then undergo changes of 

 a similar kind, so that in a short while a mass of gall tissue develops. Cells 

 invaded by the zygotes appear to behave differently. They are stimulated 

 to tangential cell division, and since the zygote lies at the base of the cell the 

 thick-walled sporangium, when formed, is buried in the wart produced. 



Saprolegniales 



The Saprolegniales are Phycomycetes in which the vegetative mycelium 

 is profusely branched and coenocytic, septa occurring rarely, in older branches 

 and at the separation of the reproductive organs. Asexual reproduction is by 

 zoospores, which exhibit a peculiar condition of development. In the simpler 

 genera the zoospores when liberated are pyriform and have two equal apical 

 flagella. They then rest and encyst, emerging again with equal lateral flagella 

 and reniform in shape. In the more advanced members the earlier phase 

 of this condition, which is termed diplanetism, may be missing. Sexual 

 reproduction is oogamous, there being more than one oosphere in each 

 oogonium. The antheridium is a filiform structure and fertilization is by a 

 fertilization tube. 



The species may be monoecious or dioecious ; almost all are saprophytes 

 and are characteristic inhabitants of soils. Many are also found in fresh 

 water. 



We shall consider in detail one example, Saprolegnia. 



