xx.] GRASS MILDEW. 131 



different sizes, some very small, and others comparatively 

 long. The largest of the mature perithecia are, however, 

 so small that it would require more than 100,000 to 

 cover a square inch. To understand the structure of 

 these minute perithecia and their contents it is necessary 

 to cut them in two both horizontally and vertically. Two 

 sections of this nature are shown at Figs. 58 and 59, 

 enlarged 200 and 100 diameters. By referring to the 

 horizontal section, enlarged 200 diameters, at Fig. 58, it 

 will be seen that twenty-four closely-packed transparent 

 bladders have been cut across ; whilst in the vertical 

 section, enlarged 100 diameters, at Fig. 59, five of the 



x-ioo 



FIG. 59. 



Vertical section through a Conceptacle of Erysiphe graminis, D.G 

 Enlarged 100 diameters 



central bladders have been cut through vertically. 

 With a little careful manipulation these contained 

 bladders or asci may be squeezed out of the perithecium, 

 and then, upon the application of a higher power of the 

 microscope, it will be seen that each bladder contains 

 eight spores, technically termed sporidia. A single 

 bladder or ascus, with its spores in situ, is shown both in 

 Figs. 58 and 59. Three of these asci, farther enlarged to 

 500 diameters, are illustrated at Fig. 60. One ascus con- 

 taining the normal eight sporidia in situ is shown at A, 

 whilst the figure at B shows the sporidia emerging from 

 the top of the ruptured ascus. The transparent asci are 

 so small that it would take 2j millions of them, contain- 

 ing 21 millions of sporidia, to cover a square inch. The 

 peculiar shape of the oval sporidia, slightly flattened in 

 their longest dimensions like the egg of a tortoise, is 

 worthy of note. One of these bodies is shown, enlarged 



