SPH.-EROTHECA. Ill 



in the penultimate cell of the row, while all other cells of the ascogo- 

 nium are uninuclear (Fig. 40, G). This penultimate cell becomes 

 the ascus ; it is not to be regarded as the exact equivalent of any other 

 cell of the ascogonium, and its two nuclei are not necessarily sister 

 nuclei, for befoi'e the last cross-wall is formed in the ascogonium the 

 distal cell may contain three nuclei, and of these any pair may remain 

 in the penultimate cell. With further development these two nuclei 

 fuse (Fig. 40, G, H). This fusion is comparable to the nuclear 

 fusion occurring generally in young asci, and consequently it has not 

 the significance of fecundation, but represents merely a vegetative 

 union. In this connection it may be mentioned that the objections which 

 Dangeard ('97) has raised against the true sexual process described 

 by Harper do not seem to me to merit any serious consideration. 



Sphcerotheca represents one of the simplest and perhaps the most 

 primitive forms of the true Ascomycetes, especially as regards the 

 development of the ascogonium. In Erysiphe and Ascobolus a 

 greater complexity in the development of the ascogonium obtains, but 

 there can be no doubt as to the nature of their sexual organs and the 

 fusion of their true sexual nuclei, especially in Erysiphe,^ 



PYRONEMA. 



In Pyronema we have a form which possesses for us a twofold 

 interest. I refer to the trichogyne-like organ borne by the oogonium 

 and the multiple fecundation, or the fusion in pairs of two or more 

 male with two or more female nuclei in the oogonium. 



The development of the sexual organs is briefly as follows : The 

 cells of the mycelium from which these organs are developed are 

 multinucleate. Both oogonia and antheridia arise from the apical 

 cells of thick hyphal branches, standing vertical to the substratum. 

 The young oogonium is more spherical and can be distinguished 

 from the young club-shaped antheridium standing by its side. Soon 

 a small papilla appears at the apex of the oogonium, which event- 

 ually becomes the conjugating-tube or trichogyne (Fig. 41, A. B). 

 Both organs are multinucleate from the start, the number of nuclei 

 increasing by division as the cells grow in size. "The nuclear multi- 

 plication, however, is out of proportion to the vegetative growth, 

 so that when the sexual cells are mature they contain relatively to 

 their size more nuclei than do the ordinary vegetative mycelial cells " 

 (Harper, 1900, p. 341). A broad stalk-cell is cut off from the 

 base of the oogonium at a relatively late stage in its development, 



1 For a detailed discussion of these processes and the phylogenetic significance of the ascus fruit, 

 the reader is referred to the original papers of Professor Harper ('95, "96, 1900). 



