Pethybridge and Murphy — On Phytophthora infestans. 585 



of development were not actually observed in the case of this fungus ; but 

 the final state of affairs which results is as follows :— The antheridia are sub- 

 spherical or oval structures borne on the tips of hyphae, or apparently in some 

 cases as sessile lateral outgrowths on them. The oogonia are pear-shaped struc- 

 tures, and are borne on different hyphae from those carrying the antheridia. 

 The oospore is contained within the spherical portion of the oogonium, the 

 lower, tapering, or funnel-shaped portion of which is actually ivitlmi, and is 

 surrotmded by tJie antkeridium. Hence the oogonium appears at first sight to 

 be a spherical structure, sessile on the summit of the antheridium ; but in reality 

 it is pyriform; and its lower portion, which is within the antheridium, is 

 continued out through the base or side of the latter, at which point it is 

 continuous with the mycelium. As a rule, when an antheridium is present, 

 there appears to be no transverse septum at the base of the oogonium where 

 it joins the hypha which bears it. 



An oogonium, containing an oospore, with its lower portion within the 

 antheridium, is shown in the photograph reproduced in fig. 7, Plate XLV. 

 In this case the hyphae bearing the antheridium and the oogonium respectively 

 were removed during the dissection of the structure from the medium ; but 

 figs. 11 and 12, Plate XLVI, will illustrate more fully the connexion of the 

 oogonia and antheridia, with their respective hyphae. 



The course of events in P. infestans is in all probability similar to that in 

 P. erythroseptica, in which the antheridium is formed first, as a lateral or 

 terminal structure on the younger portions of the mycelium. The incept of 

 the oogonium develops on a separate hypha, and enters the antheridium 

 either at or near its base. This " oogonial incept," the top of which is 

 sometimes slightly swollen, remains within the antheridium possibly 

 for some little time — and perhaps fertilization may occur at this period — but 

 gradually grows up through it, and finally breaks out through the top of the 

 antheridium, when it swells out and produces the oogonium proper (i.e. the 

 portion in which the oosphere is ultimately rounded off) in a comparatively 

 short space of time. An oosphere becomes rounded off from a portion of 

 the protoplasmic contents of the oogonium ; and from it a thick-walled 

 oospore, similar to that formed in other Peronosporaoeae, is developed. 



This method of development of the sexual organs is very unusual, and 

 necessitates a revision of the genus PhytophtJiom. A discussion on this point 

 will be found in the paper on P. erythroseptica just alluded to. 



VI. General Conclusions. 

 The results obtained by us confirm the work of Clinton, and show that in 

 pure cultures in certain artificial media, Phytophthora infestans does form 



