W. MPRAE 249 



Oooronia are found in tho tissues and submerged in the medium. Thev 

 are as a rule found singly each at the end of a hypha. but in a few cases two 

 originated very close together on very short branches from one hypha. The 

 oogonium is roughly a sphere whose lower part tapers towards the septum 

 which cuts it off from the hypha. Its wall is hyaline or very shghtly tinged 

 with yellow, but it may become slightly brown when the oospore is mature. 

 It is thin, seldom exceeding In in thickness but it may be sometimes up to 

 nearly 5/i in thickness. The septum dividing the oogonium from the hypha 

 is sometimes a little thicker than the oogonial wall, and the wall of the 

 lower tapering part of the oogonium is sometimes also slightly thickened. 

 At first the oogonial wall is smooth and even, but after the oospore is mature 

 it becomes irregularly undulated and crinkled and roughened. Oogonia and 

 antheridia are borne on separate branches. The antheridium is colourless, 

 and thin-walled, its walls being about the same in thickness as the oogonial 

 wall. It lies at the base of the oogonium comp'etely surrounding its lower 

 tapering part as well as part of its stalk so that the cross septum cutting off 

 the oogonium is usually seen through the antheridium. It remains attached 

 even after the oospore is mature. It has thus exactly the same relative 

 position with regard to the oogonium as Pethybridge found to occur in P. 

 erythroseptica and Dastur in P. parasitica. Here, too, the branch that becomes 

 the oogonium grows through the antheridium. The actual process of fertili- 

 zation has not been observed. When mature the oospore is spherical with 

 a smooth surface and a thick wall which is not always of uniform thickness. 

 The contents are deep yellow and granular, sometimes with fairly large oil- 

 drops. Before maturity it is thin-walled and hyaline, and completely fills the 

 oogonium or almost so, but when it is dark-yellow and thick-wallcd it occupies 

 only a part of the oogonial cavity. Usually the diameter of the oospore is 

 about 0-8 that of the oogonium, while in the one with the greatest difference 

 it was 0-52. The wall of a mature oospore is from 2 to 4/ti in thickness. The 

 oospore has not been observed to germinate. 



Oogonia and oospores were first seen on a French-bean-agar culture on 

 4th February, 1916. The original culture was made from sporangia developed 

 on a discoloured spot on a leaf placed in a Petri dish on 5th August. 1915. Next 

 morning some sporangia were plated out on carrot-agar, and on 13th August, 

 1915, a sporangium germinating as a conidium was removed to a French-bean- 

 agar tube. The resulting pure culture was sub-cultured several times on 

 French-bean-agar, and one made on 7th December, 1915, was examined on 4th 

 February, 1916, when oogonia were seen. Another sub-culture of the same 



