CHYTRIDIALES 75 



involved. In these genera two thalli come in contact in early- 

 stages, one functioning as the male gametangium, the other as 

 the female. The male individual, after discharging its contents 

 through a pore connection into the female, remains attached to 

 it, and is present later on the enlarged mature female cell (resting 

 spore) as a small empty companion cell. In some cases more 

 than one companion cell may be present. Those thalli which 

 do not conjugate develop into thin-walled swarmsporangia. A 

 critical cytological study of three species of Olpidiopsis was made 

 by Barrett (1912), and his results indicate strongly that the two 

 thalli which fuse show true sexual differentiation. He regards 

 these fungi as primitive Oomycetes. The absence of a companion 

 cell on the resting spore of Olpidium and Pseudolpidium does 

 not on the other hand of necessity indicate a lack of sexuality 

 in these forms. A fusion of ciliate gametes (resembling zoo- 

 spores) has been observed in several cases and perhaps occurs 

 in many species of these genera. In such cases complete fusion 

 results in a zygote, there being no emptied male cell. Kusano 

 (1912) has described the fusion of such swarmspore-like gametes 

 in 0. viciae, and finds that the zygote develops into a resting 

 sporangium, while zoospores (without fusing) develop into 

 swarmsporangia. It seems probable from the observations of 

 Nemec (1912) that the same type of life history exists in 0. bras- 

 sicae, and careful study will doubtless demonstrate it in other 

 species. It was long ago observed by Fisch (1884: 17) in the 

 two species on which he based the genus Reesia. In this genus 

 ciliate zoospore-like gametes fuse in pairs outside the host. The 

 zygote infects the host, and in it is amoeboid for a considerable 

 period, finally maturing into a resting sporangium. Whether 

 the nuclei fuse is not known. Von Minden (1911 : 238, 243) does 

 not recognize the genus, and incorporates the species under 

 Olpidium. An imperfectly understood species occurring in the 

 leaves of ragweed, and made by Griggs (1910: 50) the type of a 

 new genus, Monochytrium Griggs, also probably belongs here. 

 In this case the stages outside the host are unknown, but in the 

 host cells, uninucleate amoeboid individuals fuse in pairs, the 

 zygote finally being invested in a thick wall. Neither nuclear 

 fusion nor germination of the resting spore was observed. The 

 occurrence of planogamic copulation in these forms is of par- 

 ticular interest to the student of phylogeny since it demonstrates 

 parallelism of the evolution of the sexual process between fungi 



