80 BORTHWICK AND WILSON 
of the appearance of the fungus, Mr. Grant found that the larch 
was attacked early in the season, and that the fungus was fully 
developed on leaves when they were about three weeks old. 
It is interesting to note that, in his subsequent observations, 
he found the branches had shed their diseased leaves by 30th 
July, and, in consequence, no further material was available 
for examination. This would indicate that the stage of the 
fungus upon the larch runs a rapid course, and may thus account 
to some extent for the fact that it has previously escaped 
observation.* 
The fungus is almost always found on the under surface of 
the leaves, but occasionally on the upper side. The aecidia are 
arranged in rows on one or both sides of the midrib, and are 
separated by short, irregular intervals (Figs. 1, 2, and 3, Pl. xiii). 
The number of aecidia on each leaf is variable, but usually 6-15. 
Each group of spores is enclosed by a delicate white protective 
covering, the pseudoperidium, which, at maturity, has the 
form of a cylinder, slightly flattened laterally and open at the 
upper end. The pseudoperidium varies from ‘5-7 mm. in 
height, 5-7 mm. long, and ‘3-4 mm. wide. In the earlier 
stages it is closed, and then the cylindrical part is terminated 
by a bluntly conical upper portion (long pointer, Fig. 1). The 
ripe aecidiospores are set free by the irregular rupture of the 
apex of the pseudoperidium, and, in consequence, after dehiscence 
has taken place, this is terminated by a ragged or laceraté margin 
(see short pointer, Fig. 1 and Fig. 4). Occasionally two ad- 
jacent pseudoperidia become partially or completely fused 
together (Fig. 2). The wall of the pseudoperidium is one cell in 
thickness, the cells being rhomboidal or polygonal in shape 
and the walls finely verrucose (Fig. 6). The numerous spores 
are orange-yellow in colour and rather irregular in shape, ellip- 
soidal or polyhedral, about 16-22 » in length and 14-18 yp broad. 
The spore wall is evenly verrucose except a small area, which 
is smooth, and thinner than the remaining portions. Fig. 4 
represents a spore in optical median section and Fig. 5 in surface 
view. It will be seen that the outer part of the wall consists 
of a number of rods of material placed perpendicularly to the 
surface (Fig. 4). Before dehiscence takes place the spores are 
found arranged in chains at the base of the pseudoperidium ; 
when the latter ruptures they easily separate and escape from 
the opening at the upper end. 
A fungus known as Caeoma Laricis bears a considerable 
resemblance to the form just described on the larch. Caeoma 
* A re-examination of diesen larch leaves, sent by Mr. Murray, forester 
at Murthly, Perthshire, in June the shows that t the fungus present is an 
early condition of Peridermium Larici 
