BLASTOCLADIALES 605 



sexuality in these forms, this hitherto little understood group has been 

 raised to a place of high biological importance among the Thallophyta. 



Until 1929 no type of sexual reproduction had been convincingly 

 demonstrated in any blastocladiaceous fungus. Hitherto, sexuality, 

 wherever found in any mycelium-forming zoosporic phycomycete, had 

 been clearly oogamous, with the diploid phase represented by a thick- 

 walled resting oospore directly formed from the zygote. It is now 

 apparent that as early as 1919 Weston (see Emerson, 1941) had noted 

 in a species of Allomyces, which he collected in the Philippines, that 

 two types of "sporangia" and "zoospores" were produced. He sus- 

 pected them to be gametangia and gametes, respectively. In 1929 and 

 1930 Hans Kniep published accounts of the morphology and life cycle 

 of A. javanicus in which he showed clearly that there existed in this 

 fungus a type of reproduction and life history previously unknown 

 among any of the fungi. Kniep's fungus produced posteriorly uniflagel- 

 late gametes of two sizes, borne in gametangia of two types on a sexual 

 plant. These free-swimming gametes fused in pairs in the water and 

 the biflagellate zygote instead of becoming a thick-walled resting struc- 

 ture, as in all other known "Oomycetes," germinated at once to form 

 a second thallus. The new plant, though resembling the sexual one, 

 bore (instead of gametangia) thin-walled zoosporangia and thick-walled 

 punctate resting spores. The zoospores upon discharge from these zoo- 

 sporangia formed new thalli like the parent, whereas the resting spores 

 at germination gave rise to swarmers which reproduced only sexual 

 plants. 



Kniep, then, was able to prove that instead of lacking sexuality, the 

 Blastocladiales, or at least one member of it, actually possessed ani- 

 sogamous planogametic sexual reproduction and an alternation of iso- 

 morphic (like) generations. The stimulus provided by his researches 

 had far-reaching results. It led to intensive studies by others of the 

 morphology, sexuality, and life histories of various members of the 

 order. These investigations have been unusually fruitful; they have 

 revealed the existence of isogamous planogametic reproduction (Harder 

 and Sorgel, 1938) and in species of Allomyces, Blastocladiella, and 

 Catenaria of various types of life cycles (Emerson, 1938a, 1939, 1941 ; 

 McCranie, 1942; Couch and Whiffen, 1942; Teter, 1944; Couch, 1945a). 



