586 Scientific Proceedings^ Royal Dublin Society. 



sexually produced spores or oospores. Whether, however, these spores are, 

 strictly speaking, formed sexually or not — that is, whether an actual process of 

 fertilization occurs or not — cannot be decided at present. 



In tlie absence of antheridia, Clinton found that the oogonia did not do 

 more than develop oospheres ; but we have found in at least one-third of the 

 cases examined that under such circumstances, both in Clinton's medium 

 and in our own Quaker-Oat agar, oospores were produced ; and we look upon 

 such spores as having been formed parthenogenetically. These spores resemble 

 those formed when antheridia are present, except that in many cases their 

 walls appear to be slightly less thickened. 



Even when antheridia are present it is difficult to see how the oosphere 

 can be fertilized, for it is completely shut off from the antheridium by the 

 funnel-shaped base of the oogonium, and no signs of a fertilization track 

 have been observed. It is of course possible that a union of the male and 

 female elements may occur soon after the entrance of the oogonial incept 

 into the interior of the antheridium ; but if fertilization occurs at this stage, 

 it occurs before the formation of the oosphere, which would represent an 

 unusual state of affairs. 



Clinton was not able to trace the points of origin of tlie oogonia and 

 antheridia, but states that they seem to arise on separate hyphae. Our 

 observations show that this is actually the case, and moreover they explain 

 Clinton's difficulty in finding antheridia, except such as were in contact with 

 oogonia which were already well on in their development. Clinton states 

 that the antheridia observed by him often show the superimposed " oogonial 

 thread " ; but we find that this structure, which is in reality the lower part of 

 the oogonium itself, is actually within the antheridium, and not superimposed 

 upon it. 



Wliether the fungus produces oospores in the potato-plant or not is a 

 question which will have to be settled by further research. As stated before, 

 we (as well as other workers) have found thick-walled spores in the tissues 

 of various parts of the potato-plant, which have been destroyed by P. infestans 

 which may possibly have been such bodies, although as a rule they appear to 

 be smaller than the spores obtained in pure cultures. Many of them, too, 

 have been seen to be surrounded by a kind of halo of brownish material 

 which may possibly be the remains of the oogonium wall. If such bodies 

 are produced in the potato-plant, they would doubtless find their way 

 ultimately to the soil, and probably play an important part in keeping the 

 fungus alive over the winter, and in causing infection of the potato-crop 

 during the following season. 



