66 BoRTHWICK AND WILSON—Two Rust DISEASES OF SPRUCE. 
shoots of the previous year. The uredospores are oval in form 
and are produced in chains; they may further propagate the 
disease on the Rhododendron. Slightly later the development 
of the teleutospores commences, and in the following spring 
the sori appear as small dark red cushions on the lower surface 
of the leaf. 
The teleutospores are formed closely together in groups 
covered by the epidermis, and each spore consists of a series 
of superimposed cells. A section of a mature sorus is shown 
in fig. 1. Immediately before germination the epidermis is 
ruptured and the terminal cell of the teleutospore, the only 
one capable of germination, gives rise to a four-celled pro- 
mycelium, each cell of which produces a sporidium. The 
sporidia are set free in June, and if they alight on the young 
leaves of the spruce may cause infection. A well-developed 
mycelium is produced in the leaf, and the infected area becomes 
yellow in colour. In some cases almost the whole of the leaf is 
infected, but generally the fungus is confined to certain zones 
and the remaining portions retain their normal greencolour. As 
a result of infection small yellow spermagonia are first produced, 
and these are soon followed by the aecidia. Each aecidial 
sorus is surrounded by a long white pseudoperidium which, 
in the early stages, completely encloses the spores. The aecidio- 
spores are arranged in chains and are produced from the base 
of the aecidium ; they are orange-yellow in colour. At maturity 
the pseudoperidium breaks down at its apex and allows the 
aecidiospores to escape as a powdery orange-yellow mass. 
Before dehiscence the pseudoperidium has the form of a cylinder 
terminated by a rounded cone and possesses a yellow tint due to 
the colour of the enclosed spores. After the spores are shed 
the form is that of an open cylinder and the pseudoperidium is 
perfectly white. The aecidiospores are distributed by the wind, 
and if they alight on the leaves of the Rhododendron are capable 
of producing infection. 
The material of the aecidial stage of the fungus was obtained 
in October, some time after its maturity and the greater part of 
the spores had already been shed. An examination showed that 
in addition to the aecidia small yellow spermagonia are present. 
The number of aecidia present on a leaf varies. 
Fig. 2 is from a photograph of a leaf bearing two pseudo- 
peridia ; several other aecidia were present, but the pseudo- 
peridia surrounding them had fallen away. The pseudoperidium 
‘consists of a single layer of thick-walled pitted cells with strongly 
verrucose walls (see fig. 3, in which the upper cells are shown in 
surface view and the lower in optical section). The aecidio- 
spores are subglobose or ellipsoid, usually with a somewhat 
