Pethybridge — On the Rotting of Potato Tubers. 553 



that the course followed iu this case is similar to what occurs in P. Cactorum 

 and P. Fagi. 



In the case of these three species, then, the course of development is the 

 one which has usually been associated up to the present with the genus 

 Phytophthora, aud, indeed, witli other genera of the family Peronosporaceae. 

 The-antheridia and oogonia arise close together, usually on short but distinct 

 branches of the same main hypha, and more or less coincidently as to time. 

 Fertilization takes place by the lateral penetration of the oogonium by the 

 neighhouring antheridium and the passage of material from the latter into 

 the former. At no time is tlie oogonium or a part of it within the antheridium. 

 I do not consider it superfluous to publish fresh figures (see Plate XLIV, 

 figs. 25, 26, 27) illustrating clearly the relation of tlie antheridia and oogonia 

 to one another in these three species for the purpose of comparison with 

 what occurs in other species. 



While the investigation of the Pink Eot was being carried on, the growth 

 of Phytophthora iii/estaiis in pure cultures was being studied with the help of 

 my assistant, Mr. P. A. Murphy ; and our results will be found in a paper (23) 

 published simultaneously with the present one. We have been able to 

 corroborate fully Clinton's (4, 5, 6) results as to the production of undoubted 

 oospores by this fungus, and not only so, but we are now able to explain with 

 practical certainty the way in which they develop. 



Unfortunately the sexual organs of P. infestans develop as a rule rather 

 slowly, and to some extent somewhat erratically, and up to the present they 

 have only been found iu growths on a semi-opaque medium, oat-agar. The 

 attempt has been made to observe the development of them in cover-glass 

 films of this medium, but without success. In the main characters of their 

 final stages, however, the sexual organs of P. infestans are essentially identical 

 with those of P. erythro&eptka, and there can be little doubt that the inter- 

 mediate and early stages are also similar. 



Fig. 22, Plate XLIV, shows the sexual organs of P. infestans. It will be 

 observed that the thick-walled oospore is contained within a pear-shaped 

 oogonium tlie funnel-shaped lower portion of which is within the antheridium. 

 The separate hyphae on which the oogonium aud antheridium have respectively 

 been produced are also shown. Further figures will be found in the paper 

 referred to. In many cases in P. infestans the oospores develop, of course, 

 partlieuogenetically ; but when an antheridium is present, it seems clear that 

 it must be penetrated at an early stage by the oogonial incept which grows 

 up through it, aud out at the summit in the manner described for 

 p. erytliroseptica. 



Attention was then directed to P. FhaseoU, and on looking up Clinton's 



SOIENT. PROC. R.D.S., VOL. XHI., NO. XXXV. 4 N 



