176 LEAF AND SEEDLING DISEASES 
peridium is white, but the yellow or orange spores show 
through it and impart a dull yellow colour. When ruptured 
the peridium has a lacerated margin. The aecidia are very 
like those of Peridermium pini acicola (or, as it is now called, 
Coleosporium Senecionis, Fr.) on the needles of Scots pine, 
which being more common is far better known. The aecidio- 
spores borne on the larch are orange-yellow, 14-21 X 11-16 pn, 
and rather irregular in shape (fig. 72). The spore wall is 
finely verrucose, except on a small area which is smooth, 
and is rather thick (fig. 72, 5and 6). The spores are borne in 
chains from the base of the aecidium. The aecidiospores 
are incapable of infecting another larch needle, but infect 
the leaves of the birch (Betula verrucosa and B. pubescens). 
The mycelium to which they give rise bears uredosori and 
teleutosori on the under-surface of the leaf, and causes 
yellow discoloration on the upper side. As this is the 
only rust known in this country on the birch, its identifica- 
tion is not difficult. The uredosori differ from those of 
‘Melampsora by being covered when young by a layer of 
fungal cells, which expand into a kind of peridium when the 
sorus is ruptured. The uredospores are abstricted singly 
and, carried away by the wind, infect more birch leaves. 
The teleutospores are produced late in the season, and are 
borne in a single layer under the epidermis of the birch 
leaf. They pass the winter on the ground and in the spring 
give rise to promycelia, which bear sporidia. These are 
ready to attack the larch needles as soon as they open. 
The identity of the forms on the Jarch and the birch was 
first demonstrated in 1890 by Plowright, who obtained cross 
infections from an infected larch, which he found at King’s 
Lynn, to the birch and vice versa. It has been subsequently 
noted near Oxford by Jones (1911), and was identified by 
Borthwick and Wilson (1913) in specimens from Fersit in 
Inverness-shire. As the rust on birch is common nearly 
everywhere, it is remarkable that the form on the larch is 
not more frequent, but it is probably often overlooked. 
The damage caused by this fungus is very slight. It is 
fortunate for the larch-grower that his crop is attacked, not 
