170 LEAF AND SEEDLING DISEASES 
favourable to the disease, and long summers assist in its 
wide distribution. : 
The mycelium lives through the winter in the infected 
needles lying on the ground. By the spring it is found to 
be composed of brown thick-walled hyphae, which in April 
and May give rise to perithecia—spherical, ascus-bearing 
fructifications, with a small aperture at the apex—which 
break through the epidermis like the conidial pustules. By 
‘the beginning of June these perithecia are the same colour 
as the conidial pustules, but are somewhat smaller (0-1 to 
0-15 mm. in diameter). Some are half submerged in the 
leaf tissue, some are nearly superficial. The pore at the 
apex is obscure and difficult to find until ascospores, or 
entire asci, are seen emerging from it. The asci are club- 
shaped, 50 to 60» long, and each contains eight ascospores, 
15-17» long, which are at first. one-celled, but later two- 
celled and colourless. They are forced out of the asci in 
a single mass. 
The ascospores germinate in water in twenty-four hours 
from both ends, and sometimes also from the middle. Pure 
cultures have been grown from the spores, and such cultures 
bear four-celled conidia, exactly like those in the conidial 
pustules, after twenty days. Hartig observed a red colora- 
tion at the border of the mycelium in gelatine cultures, and 
presumed that this was due to the same cause as the red- 
brown discoloration of the needles. In the forest the 
ascospores are lifted by the wind, and are capable of infect- 
ing the young leaves of the larch. Thus the life-cycle of 
the fungus is completed. 
It has been noticed that the disease is particularly severe 
in larch woods mixed with spruce. The explanation of this 
is that the dead needles fall on to the spruce shoots and 
remain there all the winter, and next year, as they are near 
the level of the fresh larch needles, reinfection is facilitated. 
Comparative immunity of larch woods at high altitudes is 
attributed to the shortness of the season, which restricts 
the number of conidial generations which can be produced 
during the summer. 
When larch woods are heavily attacked by the leaf-cast 
