CHAPTER XII 



OOSPORE-PRODUCING ALGAL FUNGI 



ORDER II. OOMYCETALES 



The fungi of this order were derived probably from some ancestor, 

 or ancestors, which through the loss of chlorophyll became dependent 

 on extraneous supplies of organic food. If we look for such an ancestral 

 form among the algae, we find that it must have been related to Vau- 

 cheria, if not identic with that filamentous siphonaceous green alga 

 with reproductive organs, as oogonia and antheridia. Vaucheria is a 

 unicellular filamentous sparingly branched cell with a thin cell wall and 

 multinucleate. Hence it is sometimes called a coenocyte. Similarly, 

 the structural features of the more primitive Oomycetales are like 

 Vaucheria, but the absence of chlorophyll is distinctive. The forma- 

 tion of non-sexual sporangia with the formation of zoospores, or swarm 

 spores, known as zoosporangia is a feature of the fungi of this order. 

 As there is a pronounced difference between the male and female sexual 

 organs, oogamous reproduction is the rule. The oogonium is compara- 

 tively large and contains one or more oospheres, which are fertilized 

 by the sperm cell, which swim to it by cilia, creep to it, or are carried 

 into the oogonium through a fertilization tube. Sexual reproduction in 

 these fungi has been investigated cytologically by a number of students, 

 and they have found that the nuclear changes concomitant with fertili- 

 zation are characteristic. Albugo Candida, A. lepigoni, Peronospora 

 parasitica, Plasmopara, Pythium and Scleras para show a single large cen- 

 tral oosphere with a single nucleus, while the remaining nuclei pass from 

 the gonoplasm into the periplasm. A process is sent into the oogonium 

 from the antheridium and a single male nucleus passes into the oogo- 

 nium. A cell wall is developed about the oospheres and the male and 

 female nuclei unite, while the periplasm is used in the formation of the 

 spore wall (episporium). The ripe oospore has a single nucleus in 

 Peronospora parasitica, while in Albugo, it becomes multinucleate after 

 nuclear division. A central oosphere (gonoplasm) surrounded by peri- 



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