220 
METHODS IN PLANT HISTOLOGY 
sublimate, or alcohol be used for fixing, the appendages become brittle 
and very easily break off. However, the chromo-acetic mixtures are 
better if it is desired to make paraffin sections showing the develop¬ 
ing of the perithecium with its asci and spores. For this purpose 
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 
n in order to get the fungus. 
)l Material in which the peri- 
I thecia are still white or yel¬ 
lowish contain stages up to 
k the formation of the uninu¬ 
cleate ascus; brownish peri- 
I thecia show the development 
of ascospores, and dark- 
brown or black perithecia 
contain the mature asci with 
fully developed ascospores. 
In early stages while the 
perithecia are still yellow or 
a very slightly brownish, the 
C1 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 y will be hard to stain effectively. 
The Xylariaceae.—Most of these forms, in their mature con¬ 
dition, are black. In younger stages the color is lighter, often show¬ 
ing gray, brick-red, or brownish tints. Nummularia is common on 
dead branches of beech, elm, oak, locust, and other trees. It is gen¬ 
erally 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. 59) is found on decaying 
