/O SEXUAL REPRODUCTION IN CERTAIN MILDEWS. 



nuclei underwent changes which resulted in part of their content being 

 thrown out into the cytoplasm as brown pigment granules. It is inter- 

 esting to note in this connection that the abundant highly stainable gran- 

 ules which I have described as present in the ascus of Ascobolus and 

 Peziza (35, p. 71), both around the nuclei and scattered in the cyto- 

 plasm, originate, according to the recent interesting investigations of 

 Guilliermond (34), in the neighborhood of the nucleus. Guilliermond 

 does not believe that the granules arise directly from the nucleus, but 

 thinks it may play an indirect role in their secretion. It is not impos- 

 sible that the formation of these granules in the ascus may be closely 

 related to the formation of pigment granules described by Hertwig. 

 Ikeno (47) has described a throwing out of chromatin material in the 

 case of the nucleus of the ascus of Taphrina, which may be of a similar 

 nature. 



In the further development of the ascus we are confronted with 

 the peculiar fact that nuclear growth continues and nuclear division is 

 inhibited from a period prior to nuclear fusion in the young ascus till 

 the latter is mature and ready for the formation of spores. This con- 

 dition is in sharp contrast with the fact that in all the vegetative devel- 

 opment of the mildew and the development of the ascocarp, up to the 

 formation of the young ascus, nuclear division has always recurred at 

 intervals such as would prevent the growth of any single nucleus beyond 

 the normal size for either vegetative or reproductive cells. We have 

 concluded above that the rich nutrition of the ascogenous hyphse inhibits 

 cell division and leads to the formation of the young asci with two 

 nuclei. This condition makes possible a considerable growth of the 

 ascus before the condition of nucleo-cytoplasmic equilibrium is reached. 

 Soon, however, the nuclei continue their growth in size, and this process 

 continues, as noted, through the process of fusion and after it till the 

 ascus has reached practically its mature size. The nucleo-cytoplasmic 

 relation is thus maintained by the development of a single large nucleus 

 rather than by the formation of many smaller ones, as is elsewhere so 

 commonly the case in the fungi. 



If we seek, now, the cause of this relatively long inhibition of 

 nuclear division, we may note the interesting fact that the inhibition 

 lasts only until the ascus has reached its maximum size, when we may 

 conclude that the rich supply of food which has been poured into it 

 from the mycelium begins to diminish. With this check in assimilative 

 processes, reproductive activity is at once reinaugurated and the ascus- 

 nucleus divides three times in rapid succession, cell division follows, 

 and the ascospores are formed. It seems justifiable to conclude that 



