STUDIES IN THE FERTILIZATION OF PHYCO- 



MYCETES. 



CONTRIBUTIONS FROM THE HULL BOTANICAL LABORATORY 



XLIL 



Frank Lincoln Stevei^s. 



(with plate XVII ) 



ScLEROSPORA, a genus of the Peronosporales, stands in a way 

 intermediate between the Peronosporaceae and the Albuginaceae, 

 a position that renders the problem of ganietogenesis and fer- 

 tilization of particular interest. This genus has heretofore 

 received no cytological investigation. 



The oogonia of Sclerospora average "about 45 to 50/iin diam- 

 eter and are irregularly spherical in shape. The oogonial wall, 

 even in young stages, is much thickened, thus forming a marked 

 contrast with either Peronospora or Albugo. This wall aver- 

 ages from 3 to 5/i in thickness, and stains readily with 

 orano;-e G. 



Each oogonium contains approximately forty to fifty nuclei; 

 borne in a cytoplasm differing in no essential particular from that 

 of the oogonium of Albugo [fig, /). The nuclei are relatively 

 large and are rather fewer in number in comparison with the size 

 of the oogonium than is usual with the other Peronosporales. 

 The earliest stages of oogonium formation were not observed, 

 so that it is not known whether the peculiar streaming of cyto- 

 plasm and the distortion of nuclei observed in Albugo (Istvanffi 

 '95, Wager '96, Stevens '99) and in Peronospora (Wager '00) 

 obtains here. As in Albugo, the nuclei in the young oogonium 



this stage on the process 



[fig- 



(\V 



'96, Davds '00, Stevens 'oi) more closely than in other Pero- 

 nosporales, yet with certain marked differences. The mitosis is 

 closely simultaneous, and may proceed until near metaphase 

 without any indication of approaching zonation. When meta- 



420 [DECEMBER 



I 



