Pethybiudge and Murphy — On Phytophthora infestans. 581 



The oogonia are found embedded in the medium mainly along a (J-shaped 

 course following the outline of the surface of the slant, and distant a few 

 millimetres from its edge. The apex of the slant is usually free from oogonia. 

 Since they are closely pressed against the glass, as a rule, they can be seen 

 readily with a pocket-lens. They have also been found elsewhere, such as on 

 the surface of the slant, and even on the aerial mycelium, but in smaller 

 numbers. 



Again and again they have been found in contact with a solid body in 

 the medium, most frequently a small portion of the aleurone layer of the 

 grain which has escaped grinding. This fact, and their abundance in contact 

 with the walls of the tube, seem to suggest that possibly some mechanical 

 obstacle to the forward progress of a hypha may act as a stimulus to oogonia 

 formation ; but this may, perhaps, be only a coincidence. 



Submerged mycelium does not appear to penetrate far into that portion of 

 the medium at the bottom of the tube, which does not constitute part of the 

 actual slant. 



A photograph showing some of the oogonia, as seen under the low 

 power of the microscope, embedded in the medium is reproduced in fig. 3, 

 Plate XLV. 



V. Gj-ENEKAL Account of the Development and Structure of the 



Sexual Organs. 



(1) Influence nf Medium, Time, and Illumination on Development of Oogonia. — 

 From a considerably greater number of preliminary isolations, and after much 

 useful experience had been gained in the successful handling of them, nine 

 pure cultures were selected as starting-points, and from them, by continual 

 transfers at suitable intervals, nine series of cultures were kept going and were 

 submitted to extremely close macro- and microscopical observation for many 

 months. The isolations were all made from Clif den material ; and we have no 

 reason to suppose that the nine series of cultures represent so many different 

 " strains " of the fungus. In all of the nine, oogonia were produced ; and 

 in one series antheridia were associated with many of the oogonia. It 

 seems probable that the formation of antheridia is merely a matter of time ; 

 and that on prolonged culture these bodies will also appear in cultures of 

 the remaining eight series. 



Speaking broadly, we have found that when a culture has once commenced 

 to form sexual organs, it continues to do so in the subsequent transfers 

 without intermission ; and although the relative abundance of these bodies 

 may vary somewhat in the successive cultures, as a rule, the subsequent 



SCIENT. PKOC, R.D.S., VOL. Sill,, NO, XXXVI, 4 K 



