54 A Monograph of the Erysiphaceae 



Thalictruvi simplex (348), Tkelespcrma fiUfolium {2%%), Trifoliiim 

 (204) (209), Urtica (177), Vitis vinifcra (22). 



. Burrill (61) was the first to point out that the old " Sphacro- 

 theca Castagiiei Lev/' was an aggregate species, comprising two 

 sets of forms on different host plants which show constant differ- 

 ences in the size of the cells of the outer perithetical wall. Rely- 

 ing on this character, and on others mentioned below, Burrill 

 divided '* 5. Castagnet Lev." Into two species. S. luunnii and 

 S, Castagnci. These two plants I have described above under 

 the names of 5. hianuli and its variety fitliginca, as from the 

 study of a large amount of material I feel convinced that the two 

 are not specifically distinct. 



To separate the two forms as species, Burrill relied on differ- 

 ences in these characters : the size of the cells of the outer peri- 

 theclal wall, the nature of the appendages, and the size of the 

 spores. S. humiili is stated to have cells usually less than i 5 /^ 

 wide, appendages slender, 3 or more times as long as the diameter 

 of the perithecium, usually colored throughout w'hen mature, 

 mostly free from the mycelium, and spores averaging 20 fi long ; 

 5". Castagnei, cells 20-30/^ wide, appendages long, stout, usually 



colored throughout, but sometimes colorless, flexuous, somewhat 

 uneven in width, more or less interwoven with the mycelium, and 

 spores small, about 15 a long. 



The difference in the average size of the cells of the outer 

 wall of the perithecium, here pointed out, is certainly found, but, 

 at the same time, is not quite so sharply defined. In 5. hujiiuli 

 I have found the cells to vary from 10-20 p. in width, occasionally 

 a single cell may measure 25-28 /^ but the average width may be 

 stated as about 15/^; In the van fuliginea the cells are much more 



Irregular in shape and measure, from 20-30 /i wide, occasionally 

 reaching to 40 /i (it may be noted that the larger size and the 

 irregularity in shape of each cell is often seen to be caused by the 

 absence of a wall, which is sometimes clearly indicated) across 

 the middle. Occasionally, the cells in the var. /////V/y/^v? are not 

 wider than 20-25 fi, and very rarely, indeed, as, e. g., in the speci- 

 men inSacc. Myc. Ven. 1376, we find here and there a few cells 

 as narrow as 15// — in fact, in this example, the cells have a 

 range of 15-25, or rarely 30// in width. On the other hand, in 



