BUTLER. 265 



The conidia, which were especially formed in cultures on 

 plant tissues, do not differ in any way from the sporangia, except 

 in their mode of germination. Morphologically they, no doubt, 

 represent sporangia functionally changed in colonising the land. 

 Germination occurs by the protrusion of one or more germ-tubes 

 directly from the conidium (Fig. 16). 



The sexual reproduction is well known. Oogonia are formed 

 usually at the ends of lateral branches, but are sometimes also inter- 

 calar. A study of the early stages shows that the antheridia 

 are often formed almost simultaneously with the oogonia, by 

 branching of a single hypha (Fig. 5). The oogonial stalk is fre- 

 quently longer than that of the antheridium, but curved so that 

 the tips of the two hyphae come into contact (Figs. 4, 5). Sometimes 

 the antheridial cell is cut off while the oogonium is still in free com- 

 munication with the stalk hypha (Fig. 2). Usually there is only 

 one antheridium, which arises from the same hypha as the oogonium 

 or, less often, from a neighbouring but distinct hypha. Occasion- 

 ally two antheridia are present and other observers have seen 

 three. Whether all are functional is not known. Sometimes 

 instead of the antheridium appearing on a distinct branch, a part 

 of the stalk just below the oogonium becomes cut off to form an 

 antheridial cell and fertilisation takes place across the basal septum 

 of the oogonium. I have not observed this condition. The cyto- 

 logical details of fertilisation have been fully studied by Mivake. 1 



The following are the measurements of the various reproduc- 

 tive bodies obtained in my cultures, after fixation with osmic acid 

 and mounting in glycerine : — 



Sporangia and conidia : 15 to 26m. (av. 18m). 



Zoospores after coming to rest : 6 to 8m. (av. 7m). 



Oogonia : on ants 15*5 to 21m. (av. 18m.) ; on Uicinus and 



Lepidium : 15 to 26m. (av. 21*2m). 



Oospores : 12 to 20m. (av. 16u). 



1 Miyake, K. The Fertilization of Pythium de Baryamm. Ann. of Bot. XV, 1901, 

 p. 653. 



3 



