6 SALMON: SUPPLEMENTARY 
marked difference of structure in the perithecial wall distinguishes 
the upper from the lower side. The wall of the upper side of the 
perithecium is composed of thick-walled cells, and is consequently 4 
rigid, while the Jower side, being composed of thinner-walled cells, 
is flexible. As the result of this structure the perithecium in 
drying becomes strongly concave on the lower surface—a 
fact, according to Neger’s interpretation, of great biological im- 
portance. 
Podosphaera resembles Microsphaera in the structure of the 
perithecia. 
The genus Uncinula is divided by Neger into two groups— 
Microsphaeroidea and Euuncinula. In the first named group, in 
which the species U. salicis, U. prunastri, U. macrospora, U. flexu- 
osa, U. necator, and U. clandestina were examined, the cellular 
structure of the perithecial wall resembles that found in the genus 
Microsphaera. Inthe species of this group Microsphacroidea, how- 
ever, the difference of structure between the tissues of the under 
and upper sides is even more marked. A rigid layer of cells with 
strongly thickened walls surrounds the perithecium on its upper 
surface and at the sides, while layers of delicate-walled cells having 
a wide lumen occur on the lower surface. 
The group Exuncinula comprises U. aceris, U. polychaeta, and 
U. circinata. Were a differentiation exists between the cell-struc- 
ture of the under and upper halves of the perithecium, but it is not 
identical with that found in the last group. In UV. aceris the cells 
toward the upper surface have only slightly thicker walls and a 
slightly smaller lumen than those towards the lower surface ; in 
U. polychaeta the same characteristics are more marked; in U 
ctrcinata, however, exactly the converse is found. The perithecia 
of U. aceris (see monograph, p. 92) and of U. polychaeta become, 
when dry, concave on the under surface; while in U. circivata, in 
consequence of the tissue towards the upper surface being com- 
posed of tender cells with a wide lumen and those towards. the 
under surface being relatively thick-walled, the perithecia never 
become concave on the under surface, but usually become slightly 
so on the upper. 
In Phyllactinia the cells of the tissue towards the base of the 
perithecium have a narrower Jumen than those towards the upper 
