1921] MILES—LEAF SPOTS OF ELM 173 
on account of the absence of any hyphae connecting them with 
the leaf tissue, they leave a smooth cavity or locule in the leaf. 
ASCI AND ASCOSPORES 
In mature perithecia the asci are very much confused in their 
arrangement, owing to the fact that the older ones are broken 
loose from their attachment and pushed toward the top of the 
perithecial cavity by the younger ones. There are no paraphyses. 
The asci are oblong-cylindrical or somewhat club-shaped in form, 
and have a short stalk at the base which may be either straight 
or bent toward one side. The wall is hyaline, thin below, but 
thickened in the upper half (fig. 19), and does not color with 
iodine. At the upper end of the ascus is a pore surrounded by a 
ring of thickened tissue which is strongly refractive toward light. 
In optical section as seen from the side this ring presents the 
appearance of two small spheres arranged side by side in the apex 
of the ascus. The asci measure 45-55 9-11 w. The spores are 
very characteristic also. They are hyaline, elongate-elliptical, or 
obovate-oblong, and have a septum near the lower end, thus 
becoming unequally two-celled. They are eight in number, sub- _ 
biseriate, and measure 8-10X3-3.5 4. The small cell at the 
lower end of the spore averages about 2, both in length and 
breadth. There is a slight constriction at the septum. Some 
epiplasm is present in the mature ascus along with the spores. 
EXPULSION AND GERMINATION OF ASCOSPORES 
As previously stated, the asci in a mature perithecium become 
loosened from their attachment at the base and crowded toward 
the apex of the perithecial cavity in a somewhat disordered mass. 
In the process of expulsion of ascospores an entire ascus enters the 
lower part of the ostiole and is held in place by the periphyses 
until the pressure produced by the absorption of water, which 
must be present to allow the ascospores to be discharged, becomes 
sufficient to bring about the discharge of the spores. These pass 
outward through the periphysis-lined ostiolar channel to the sur- 
face of the ostiole, where they are expelled with some force, and 
under natural conditions are evidently dispersed by currents of 
air. Early in March leaves were found which had passed the 
