78 SEXUAL REPRODUCTION IN CERTAIN MILDEWS. 



tetrasporic as distinct from sexual plants unexplained, and their validity 

 is to be settled by the determination of the chromosome number in each 

 case. Mottier's (67) discovery that the number of chromosomes in the 

 first division in the tetrasporange of Dictyota is 16, which is about half 

 that found in the vegetative cells of the plant which bears the tetra- 

 sporanges, and the evidence brought by Williams (98) that the tetra- 

 spores of Dictyota develop into sexual plants, while the eggs develop 

 into tetrasporic plants, lead us to the conception of quite a different set 

 of possible homologies, as will be further noted below. 



Better evidence for a relationship between the red algae and Asco- 

 mycetes is found in the method of fertilization by a trichogyne as found 

 in the lichens, Laboulbeniacese and Pyronema, and in the fact that in 

 both groups the fertilized egg through its further development remains 

 in organic continuity with the plant which bore it, instead of being set 

 free to begin an independent existence. It was on this ground that 

 De Bary proposed the conception of the carpogonium to include the 

 female cell of both the Ascomycetes and the red algae. 



Davis (22), in a critical review of the relationships of the higher 

 fungi and algae, while admitting the force of the evidence in favor of a 

 derivation of the Ascomycetes from the red algae through the Laboul- 

 beniacese, is inclined to the assumption of a relationship between the 

 lower Ascomycetes and the Phycomycetes and to the belief that the 

 Ascomycetes may be a polyphyletic group. Blackman is of the opinion 

 that such a vegetative fertilization as he finds may also be present in the 

 higher Basidiomycetes and presumably also in those rusts which seem 

 to lack an aecidium. Further studies in the rusts in the light of Black- 

 man's and Christman's discoveries may be expected to clear up many 

 difficult points as to relationships among the higher fungi and algae. It 

 is interesting to note that Sappin-Trouffy concluded (84) that in some 

 cases the binucleated condition appears first in the teleutospore sorus. 



Blackman does not believe that Coleosporium sonchi-arvensis, as 

 described by Holden and Harper (46) really lacks a true aecidial stage. 

 The form is a very common and familiar one which I have collected 

 for many years without being able to find any suggestion of an associ- 

 ation with an aecidial stage on a conifer, where one would naturally 

 expect it to occur. It is my opinion that this is a form with reduced 

 life cycle, but I do not hold that this opinion is of any final value in the 

 absence of proper culture experiments on our American forms. Still, 

 it is hardly to be doubted that such types do exist, and it would make 

 no difference with the conclusions suggested whether the binucleated 



