XIL] PINE-BLISTER. 265 



that the leaves of these plants, and of several allied 

 species, are attacked by a fungus, the mycelium of 

 which spreads in the leaf-passages, and gives rise to 

 powdery masses of orange-yellow spores, arranged in 

 vertical rows beneath the stomata : these powdery 

 masses of spores burst forth through the epidermis, 

 but are not clothed by any covering, such as the 

 cecidia of Peridermium Pint, for instance. These 

 groups of yellow spores burst forth in irregular 

 powdery patches, scattered over the under sides of 

 the leaves in July and August : towards the end of 

 the summer a slightly different form of spore, but 

 similarly arranged, springs from the same mycelium 

 on the same patches. From the differences in their 

 form, time of appearance, and (as we shall see) 

 functions, these two kinds of spores have received 

 different names. Those first produced have numerous 

 papillae on them, and were called Uredospores, from 

 their analogies with the uredospore of the rust of 

 wheat ; the second kind of spore is smooth, and is 

 called the Teleutospores, also from analogies with the 

 spores produced in the late summer by the wheat-rust. 

 The fungus which produces these uredospores and 

 teleutospores was named, and has been long dis- 

 tinguished as, Coleosporium Senecionis (Pers.). We 

 are not immediately interested in the damage done 



