8 SEXUAL REPRODUCTION IN CERTAIN MILDEWS. 



DEVELOPMENT OF THE PERITHECIUM. 



The sexual apparatus in Phyllactinia arises in general as I have 

 earlier described for Sphaerotheca (36) and Erysiphe (37). As a rule 

 the ascocarps are more sparsely scattered than in either of the above 

 genera, and the labor of bringing together a series of developmental 

 stages is proportionally increased. As has been many times noted, the 

 perithecia of Phyllactinia are regularly hypophyllous, a fact which has 

 its explanation in the existence, as above noted, of interior mycelial 

 branches which gain access to the mesophyl through the stomata of the 

 host. This habit brings with it further a relatively slight development of 

 the superficial mycelium, and hence the infected spots are, as compared 

 with those of most mildews, extremely inconspicuous until the young 

 fruit bodies have grown large enough to be visible to the naked eye and 

 have taken on the characteristic yellowish color of young, half-devel- 

 oped mildew perithecia. At stages when fertilization is going on, the 

 infected spot on the ash or bittersweet leaf is faintly visible as a slightly 

 whitened region scarcely noticeable unless the leaf is held so that the 

 light is reflected from its surface at a particular angle. In my experi- 

 ence conidiophores are almost or entirely lacking, even in the earliest 

 stages, on the mycelia which produce the most abundant perithecia. It 

 should be said, however, that I have uniformly obtained my best material 

 of this fungus in the autumn, and in regions where the fungus develops 

 richly earlier in the season a greater abundance and prevalence of the 

 conidiophores may be expected. 



It is, of course, well known that not all the perithecia on an infected 

 spot develop simultaneously. In Sphaerotheca, as I have already de- 

 scribed (36), there is a considerable peripheral growth of the mycelium, 

 and the perithecia may be half-grown in the center of a spot while an 

 abundance of the younger stages is still to be found on the periphery. 

 In Phyllactinia, however, this is at least not so notably the case; the 

 mycelium on an infected area seems to get almost its full development 

 before the perithecia begin to appear, and they are then formed scatter- 

 ingly, more or less, over the whole spot ; the later- formed fruits are thus 

 mingled with the earlier. The most abundant fertilization stages in 

 my experience are to be found on spots in which the earliest-formed 

 perithecia appear as mere white specks under the magnifier and none 

 are yet far enough developed to have begun to turn yellow. 



