202 PHYTOPHTHORA PARASITICA. 



um. They are in such intimate contact over a considerable area 

 that nuclear transfer might take place at any time after the penetra- 

 tion of the antheridium by the oogonial origin. After fertilisation, 

 the oogonial stalk within the antheridium is found in the majority of 

 cases to possess a septum or a thick cellulose plug or both ; in those 

 cases in which neither septa nor cellulose plugs have been observed, 

 they might be present just where the antheridium is pierced by the 

 oogonial stalk and so escape observation. The oogonium forms a 

 permanent coating round the oospore which almost fills it ; even after 

 the formation of the oospore the antheridium is not always com- 

 pletely empty of its protoplasmic contents. 



I cannot definitely say if the oogonium is ever developed ter- 

 minally, but in those few cases in which the hyphse bearing the 

 oogonium could be clearly traced for some distance, it was found that 

 it was on a lateral branch of another hypha (Plate VI, Figs. 10. 

 15 and 20), or occasionally sessile or what may be termed intercalar 

 (Plate VI, Fig. 11). 



Both Jones and Clinton 1 have observed that the oogonial 

 thickening in Ph. infestans is influenced by the medium in which 

 it is grown. A similar difference in the oogonial thickenings of 

 the oospores of Ph. -parasitica is found in French-bean juice agar and 

 oat juice agar. The oogonium in the former medium is slightly 

 thickened, at times very little, smooth and yellow. Within it is 

 seen the oospore, almost filling the oogonial cavity. Very often the 

 enclosed oospore lies in such close contact with some portion of the 

 oogonial walls that it is very difficult to differentiate between their 

 walls at this place. The oogonium measures 15-27 n in diame- 

 ter, as a rule 18-25/*, and the average of 60 measurements gave a 

 diameter of 23'8^. The. oogonium in oat juice agar is thick-crusted 

 on account of secondary thickening on its wall, orange yellow in 

 colour, and without a sharp contour. At times the thick oogonial 

 wall is so opaque that the enclosed oospore which almost fills the 

 oogonium is only seen on crushing it. These thick-walled dark 



I Clinton, G. P. he. cit., 1909-1910, p. 772. 



