100 THE PHYCOMYCETES 



In some species the sporangia are detachable, whereas in others 

 they remain attached to the mycehum. Sporangial shape is one 

 of the features that has been employed as a basis for generic 

 t>^pes, as is indicated bv the names Sphaerosporangium and Nema- 

 tosporangium, more appropriately regarded as names of sub- 

 genera. 



The development of oogonia and antheridia has been studied 

 repeatedly, and essential features in Fythiinn de baryamim were 

 determined long ago bv Aliyake (1901). The hyphal tips first 

 become inflated, are multinucleate, and are set off by septa. As 

 the oogonium enlarges, the central portion becomes the oosphere; 

 and the peripheral portion, the periplasm. All the nuclei but 

 one migrate into the peripheral laver, where they disintegrate. 

 Meanwhile all but one of the antheridial nuclei disintegrate also, 

 and a wide pore forms at the point of contact of the antheridium 

 with the oogonium. This stage is followed by the migration of 

 the antheridial nucleus and a portion of the cytoplasmic content 

 through the periplasm to contact the centrally located tgg nu- 

 cleus and cytoplasm. These two nuclei then fuse, and a thick 

 wall, a modification of the periplasm, is developed around the 

 zygote. 



In respect to position of antheridia with respect to oogonia, 

 there are two general tvpes of Pythiales: paragvnous and amphi- 

 gynous. In the first type the antheridia are applied at the side 

 of the oogonia; in the second they are appUed as a doughnut- 

 shaped collar around the oogonial stalk. 



The Pythiales grow readily on a wide variety of artificial sub- 

 strata. Alany of them can be induced to form both sporangia 

 and sexual spores on agar, whereas others remain sterile or form 

 sporangia only. Emergence of swarm spores often takes place 

 if fresh water is applied or if the sporangia are placed in fresh 

 water. Boiled hempseeds placed in ^\•ater are very satisfactory 

 in isolating Pythiales from soil samples. 



Pythium. The Genus Pythium is extensively treated by But- 

 ler (1907), Matthews (1931), and Middleton (i943). As mono- 

 graphed by Matthews (1931), it contains approximately 40 

 species, whereas Middleton (1943) recognizes 66 species as valid. 

 Perhaps the best known among them is P. de haryamnn, which 

 causes damping off of seedlings, both herbaceous and woody. It 

 is especially destructive to coniferous seedlings in nurseries and 



