264 



METHODS IN PLANT HISTOLOGY 



development of ascospores, and dark brown or black perithecia con- 

 tain the mature asci with fully developed ascospores. In early stages 

 while the perithecia are still yellow or very slightly brownish, the 

 material can be stripped off from the leaves before fixing. An air-pump 

 will remove any air. Use iron-alum haematoxylin and orange, or the 

 safranin, gentian violet, orange combination. Sections thicker than 

 5 fjL will be hard to stain effectively. Erysiphe cojnmune also grows on 

 Polygonum eredum, from which it can be stripped even before fixing. 



The Xylariaceae. — Most 

 of these forms, in their ma- 

 ture condition, are black. In 

 younger stages the color is 

 lighter, often showing gray, 

 brick-red, or brownish tints. 

 Nummidaria is common on 

 dead branches of beech, elm, 

 oak, locust, and other trees. 

 It is generally flat, orbicular, 

 or elliptical in form. Ustilina 

 is a crustaceous form, rather 

 diffuse and irregular in 

 shape. It is most common on 

 the roots of rotten stumps. 

 Hypoxylon is more or less 

 globose in form, and the 

 color is brick-red, brown, or 

 black. It is found on dead twigs and bark of various trees, especially 

 beech, and is more abundant in moist situations. Xylaria (Fig. 70) is 

 found on decaying stumps and logs, and often apparently on the 

 ground, but really growing on twigs, wood, and bark just under the 

 surface. When mature it is black outside and white or light-colored 

 within. When young, it is easily cut in paraffin; in some forms the as- 

 cospores are fully formed before the stroma becomes hard enough to 

 occasion any difficulty in cutting. Even in the forms which become 

 the hardest in their mature stages, the young stages, up to the asco- 

 gonium or a little later, the stroma is not hard to fix or cut. With iron- 

 alum haematoxylin and orange, the ascogonium becomes sharply out- 

 lined against the vegetative hyphae. 



Fio. 70. — Xylaria: A, transverse section of a young 

 stroma, showing perithecia; X8. B, two asci with 

 ascospores; X245. 



