200 



Methods in Plant Histology 



the omnipresent Erysiphe commune on Polygonum aviculare is excep- 

 tionally favorable, because, after the material has been fixed and has 

 been brought into alcohol, the whole mycelium, with the develop- 

 ing perithecia, may be stripped from the leaf without the slightest 

 difficulty, thus avoiding the necessity of cutting the leaf in order to 

 get the fungus. The stage in which the perithecia are still white or 

 yellowish is the most favorable for sections. At this stage the 

 material, when abundant, can be stripped off from the leaves before 



FIG. 54. Uncinula necator on Ampelopsis quinquefolia: A, four asci containing asco- 

 spores have been forced out by pressing on the cover; stained in fuchsin and mounted in 

 balsam; B, a conidiospore; and C, an appendage of Microsphaera alni, drawn from 

 living material. X 192. 



fixing. Sections should not be thicker than 5 JJL. About 3 p is best 

 for free nuclear stages in the ascus and for the development of the 

 ascospores. The safranin, gentian-violet, orange combination seems 

 to give the best results, although cyanin and erythrosin are quite 

 satisfactory when the stains are properly balanced. 



The Xylariaceae. Most of these forms, in their mature con- 

 dition, are black. In younger stages the color is lighter, often show- 



