88 COMPARATIVE MORPHOLOGY OF FUNGI 



very significant, as all functional nuclei are fertilized by male nuclei; 

 only as it is a question of a simultaneous process in the same organ, this 

 multiple fertilization, in contrast to the Saprolegniaceae, is consummated 

 by a single sexual act. 



Selection goes still further and removes all but one nucleus from the 

 coenocytic egg into the periplasm. Thus in every oogonium a single 

 uninucleate egg is formed secondarily; this should not be considered 

 homologous to a true gamete as its phylogeny shows it to be a coeno- 

 gamete which has become uninucleate. Thus in the highest Oomycetes, 

 the number of functional female energids sometimes is reduced to one. 

 Similarly also in the antheridia of some, only one nucleus is functional 

 and all the others degenerate. Thus between the contents of two game- 

 tangia only one sexual act and one fertilization occurs and its product is 

 only a single uninucleate zygote, which in Olpidium was the product of 

 two daughter cells of the gametangium but is here the product of two 

 gametangia themselves: as in the development of the zoosporangium to 

 conidium, here the whole organ has assumed the function of a part. 



The Pythieae form the transition from the Leptomitaceae and are 

 marked by the possession of true zoosporangia. Four genera should be 

 mentioned here, Pythium, Phytophthora, Pythiomorpha and Pythiogeton. 

 Pythium, as has already been indicated on page 75, is divided into three 

 subgenera according to the structure of their sporangia: Aphragmium, 

 Nematosporangium and Spkaerosporangium. To Aphragmium belong 

 P. gracile which has been found in the north temperate zone, parasitic 

 on Chlorophyceae and saprophytic in soil, and P. Indigoferae which in 

 British India is epiphytic (?) on leaves of Indigofera arrecia. To Nema- 

 tosporangium belong P. monospermum which appears in water on decaying 

 insects and Lepidium seedlings, and P. aphanidermatum which in the 

 United States is parasitic on sugar-beet seedlings, in Hawaii on sugar 

 cane and in India on ginger, tobacco and Papaya. The subgenus 

 Sphaerosporangiiwi finally is divided into two sections ; Orthosporangium, 

 in which the sporangia remain attached to the mycelium; and M eta- 

 sporangium, in which the conidia fall off. The species of Orthosporangium 

 are mostly aquatic on plant and animal remains and renew their sporangia 

 by proliferation as P. proliferum and P. diacarpum. The species of 

 Metasporangium have passed over to terrestrial habitats and renew their 

 conidia by lateral sprouting of sporiferous hyphae, as P. debaryanum 

 which causes the destruction of young seedlings of crucifers, sugar 

 beets, etc., P. palmivorum which in India causes a heart rot of coconuts, 

 and P. intermedium which in Europe and North America is saprophytic 

 on garden soil and parasitic on fern prothallia. 



These forms are so closely allied to Phytophthora that a border line 

 may not be drawn. In some of its species the conidiophores do not 

 possess an entirely mycelial character, as Phytophthora eryihroseptica 



