INJURIES INDUCED BY PLANTS 



165 



stromata are distinguished by their smaller size from those which 

 occur on P. balsamifera (M. Balsamifera T/ium.\ and it would 

 also appear that the form M. populina Jacq. which is often 

 met with in luxuriant development on P. nigra is distinct from 

 the first two. Owing to the development and increase of the 

 uredospores in the course of the summer, the foliage may 

 appear quite golden yellow in August, and poplars sometimes 

 suffer so severely from this 

 rust that even in September 

 the trees may be entirely 

 leafless. 



The teleutospore- layers 

 are primarily concealed by 

 the epidermis of the leaf, but 

 ultimately appear above the 

 surface as smooth brownish- 

 yellow cushions, which after- 

 wards become dark brown 

 (Fig. 96) ; while the yellow 

 uredo-layers, after rupturing 

 the epidermis, may be recog- 

 nised as loose clusters of 



FIG. 96. Aspen-leaf showing the teleuto- 

 'spore-layers of M. Tremula 1 . 



spores. 



It would appear desirable 

 that these various forms of 

 poplar-rust should be made 



the subject of more exact investigation, seeing that their aecidium 

 forms have not yet been determined with certainty. 



I have, so far, investigated only the Melampsora that affects 

 Populus tremula. Even as early as iS/4 1 I drew attention to 

 the fact that, with scarcely an exception, aspens present 

 in young Scotch pine woods are infested by CcBoma pimtor- 

 quum, and that some connection might possibly exist between 

 Cczoma and some fungus that occurs on the aspen. 



I indicated that such a connection was doubtful in the 

 case of Melampsora Tremulce, because this fungus occurs in 

 districts where Cceoma pinitorquum is unknown. In the in- 

 terval, however, Rostrup proved experimentally the connection 

 1 Wtchtige Krankheitcn der Waldbaume, p. 91. 



