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 11-1 6 fi, 

 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, 5 and 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 larch 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 



