1915] 



ATKINSON PHYLOGENY IN THE ASCOMYCETES 



353 



if present, functionless) and that the ascogonium consists of 



a single coenocytic cell. Conjugate division takes place in 

 the ascus hook, and the subsequently fusing cells, so that in 

 most cases rather distantly related pairs of nuclei form the 

 fusion nucleus in the ascus. In L. chlorocephala (Brown, 

 W. H., '10), it appears that the pair of ascus nuclei are some- 

 times sisters. This would indicate an extreme case in the 

 modification of sexuality, the distance of relationship between 

 the sex nuclei being reduced to the minimum. It recalls the 

 very close relationship of the sex nuclei in many of the lower 

 algae, particularly in certain diatoms 1 (Oltmanns, '04), and 

 in the species of Spirogyra having buckle-joint conjugation 

 (Chodat, '10). In the case of Spirogyra it is not known 

 whether the pair of sex nuclei in this type of conjugation 

 are cousins or sisters, or whether now one and then another 

 of these possibilities exists. Such species of Spirogyra in 

 which certain threads present scalariform as well as buckle- 

 joint conjugation offer an interesting parallel to the variation 

 in distant relationship of the fusing nuclei in the young ascus. 



In some other species where the antheridium is function- 

 less or wanting, sex differentiation is said to take place among 

 the nuclei in the ascogonium. This indicates a sex differentia- 

 tion much earlier than that which is supposed to occur in the 

 species just cited. This differentiation in sex nuclei has been 

 described in Humaria granulata (Blackman and Fraser, '06). 



Another species in which similar phenomena are described 

 is Lachnea stercorea (Fraser, '07). Here the archicarp con- 

 sists of several coenocytic large cells and the terminal tricho- 

 gyne of 4-6 smaller coenocytic cells. The unicellular coenocytic 

 antheridium fuses with the terminal cell of the trichogyne, but 

 its nuclei do not reach the single-celled ascogonium, among 

 whose nuclei sex differentiation is said to take place. 



For a number of years Polystigma rubrum, a parasite on 

 cherry leaves, as the result of studies by Fisch ( '82) was re- 

 garded as an example of fertilization of an ascogone coil by 



1 In Achnanthes suhsessilis, the protoplast divides into two parts along with 

 nuclear division. The two uninucleate protoplasts now immediately unite in 

 auxospore formation. 



