BIOLOGIC FORMS 147 



the Powdery Mildews. This degree of specialisation is due 

 to the evolution just as in the Rusts (Uredineae) of 

 numerous specialised, or biologic forms, as they are called, 

 within a morphological species. Neger, Marchal, and 

 Salmon have demonstrated by numerous infection-experi- 

 ments that in the case of a species parasitic on several 

 genera of host-plants the individuals on any one host- 

 species have become specialised in such a way as to be 

 incapable of growing on the other hosts ; that is to say, 

 we find on each host-plant a biologic form which is con- 

 fined either to that one species or to a few closely related 

 species of the same genus. Thus the morphological species 

 Erysiphe gramim's, DC, is found on barley, oat, wheat, rye, 

 and a number of wild grasses belonging to various genera 

 (Bromus, Poa, Dactylis, Agropyrum, etc.). Experiments 

 have proved, however, that on each of these host-plants the 

 fungus has become specialised, with the result that the 

 form on barley cannot infect oat, wheat, rye, etc. that on 

 wheat cannot infect barley, oat, rye, etc. and so on. 

 These biologic forms are morphologically indistinguish- 

 able from one another, but differ physiologically or bio- 

 logically in possessing distinctive and sharply defined 

 infection-powers. 



Further, it has been found that this phenomenon, which 

 was first observed in the conidial (Oidum} stage of the 

 fungus, occurs also in the ascigerous stage. In experi- 

 ments carried out by Salmon, ascospores of individuals 

 of E. gramim's grown on barley were sown on barley, 

 Hordeum Zeoc* iton and H. trifurcatum (two closely allied 

 species), and on H. maritimum, H. secalinum, H.jubatum^ 

 H. bulbosum, oat, wheat, and rye. Infection resulted only 

 on barley, and on the two species closely allied to it, H. 



