178 LEAF AND SEEDLING DISEASES 



or aecidial stage. The following account is chiefly taken 

 from the work of Klebahn, who has made a very special 

 study of the genus Melampsora, and in nearly every case 

 he has been the first to determine the identity of the coeoma 

 with specifically distinct uredo-forms. The identification of 

 the different species is often difficult, even in the uredo- 

 stage, and a large number of biological races have arisen. 

 Thus species which have their uredo -stage on Salix Caprea 

 may have their aecidial stage on larch or spindle-tree, and 

 those with uredo -stage on aspen may have their aecidial 

 stage on larch, Scots pine, or dog's mercury. For the 

 features by which these species may be distinguished, the 

 reader is referred to Klebahn (1904) or Grove (1913), and 

 only points of biological interest are mentioned here. 



Melampsora Larici-Caprearum, Kleb. Uredo- and teleuto- 

 sori on Salix Caprea, less frequently on Salix aurita and 

 other species. 



Melampsora Larici-epitea , Kleb. Uredo- and teleuto-sori 

 on many species of Salix. There appear to be many bio- 

 logical races of this species, all with their coeomata on the 

 larch, but with uredo -stage on different willows. These 

 races are indistinguishable morphologically, and it is only 

 by infecting the larch with the rust obtained from a par- 

 ticular species of willow and then reinfecting various species 

 of willow that'their validity can be determined. For instance, 

 it is found that the rust of this species taken from Salix 

 Caprea will, after its generation on the larch, infect S. 

 fragilis or S. viminalis, but not S. purpurea, while the race 

 which grows on S. purpurea will not infect the other species. 



Melampsora Larici-populina, Kleb. Uredo- and teleuto- 

 sori on Populus balsamifera, P. canadensis, P. nigra, and 

 P. pyramidalis (italica). The association of this species 

 with the coeoma on the larch was first demonstrated by 

 Hartig in 1889. 



Melampsora Larici-tremulae, Kleb. Uredo- and teleuto- 

 sori on Populus alba, P. tremula, and rarely P. balsamifera. 



The aecidial stage of all these species grows on the larch, 

 and produces what is described as Coeoma lands. On the 



