584 Scientific Proceedings^ Royal Dublin Society. 



rather obscured by the brown stain which, as previously stated, diffuses from 

 it into the surrounding medium. 



The oospores are spherical bodies (with the exception of a few more 

 or less pear-shaped ones whicli have occasionally been seen) with a thick, 

 colourless, smooth, hyaline wall.' Apparently the whole of the protoplasm in 

 the oogonium does not go to form the oosphere and subsequent oospore, for 

 what appear to be remaining portions of it occur between the wall of the 

 spore and that of the oogonium. The diameter of the oospores varies from 

 28^ to 34^, the average diameter being found to be about 30^<. The thick- 

 ness of the walls of the oospores measures from 2-3^, but is slightly greater 

 (4ju) when antheridia are present. 



The spores are filled with colourless, densely granular protoplasm, 

 especially towards the periphery ; the central portion is much clearer and 

 more transparent, and the contents thus resemble those described and figured 

 by de Bary for the oospores of other members of the Peronosporaceae. 



A few attempts have been made to germinate the oospores in hanging 

 drops ; but so far they have met with no success. 



(3) Production of Oogonia and Oos]}ores in the j^^'esence of Antheridia. — 

 Although the oogonia and oospores described above were produced in 

 abundance and with great regularity for many months, the most careful 

 and prolonged search failed for a long time to disclose the presence of 

 antheridia. 



It was not until a period of some fifteen or sixteen months had elapsed 

 that the presence of antheridia was observed, and even then only in the cultures 

 of one series (M. 7), which had been all along one of the most robust and 

 prolific as regards the production of oogonia and oospores parthenogenetically. 

 Meanwhile, one of us had been studying for a couple of years the development 

 of a new species of Phytophthora (P. erythroseptica), which causes a serious 

 rot of potato tubers, temporarily designated as " doubtful " rot.- Owing to 

 the knowledge gained by the study of this organism, a detailed description of 

 which will be published simultaneously with this paper,^ we were in a position 

 to understand at once by analogy the course of events in the development of 

 the antheridia, oogonia, and oospores of P. infestans. Our attempts to get 

 P. infestans to form its sexual organs on Quaker-Oat agar in cover-glass film 

 cultures in a moist chamber met with no success, and consequently the stages 



1 No spores were observed resembling in any degree the resting spores with protuberances on 

 their walls figured by Jones, and recalling Artotrogus hydnosporus. 



' See Journal Department Agrio, and Tech. Instruction for Ireland, vol. xii, 1912, p. 357. 



^ Pethybridge, G. H,- — "On a form of Rot in the Potato tuber caused by a new species of 

 Phytophthora, \is.ying a meihoi of sexual reproduction hitherto undescribed." Scient. Proc. Eoy. 

 Dublin Soc, N.S., vol. xiii, No. 35. 1913. 



