150 TEXT-BOOK OF FUNGI 



The biologic form on B. interruptus is characterised by 

 producing full infection when sown on B. mollis^ B. inter- 

 ruptus, and B. tec forum ; it produces either slight infection 

 on B. velutinus, or passes over this species ; and causes no 

 infection on B. commutatus^ B. secalinus, B. racemosus^ 

 B. arvensis, and B, sterilis. The biologic form on B, 

 hordeaceus is distinct in nearly always infecting B. com- 

 mutatus > and in sometimes causing slight infection on 

 B. secalinus. The biologic form on B. commutatus differs 

 in being unable to touch B. mollis and B. interruptus. 

 The biologic form on B. racemosus differs from all the 

 others in infecting B. racemosus. 



The facts obtained show not only the high degree of 

 specialisation reached by the fungus in its adaptive para- 

 sitism to the various species of the genus 'Bromus^ but 

 demonstrate also that each species of Bromus possesses 

 distinctive physiological (or constitutional) characters ex- 

 isting concomitantly with the specific morphological cha- 

 racters. By using the index of the reaction to the attacks 

 of the biologic forms of the fungus, it can be demonstrated 

 that species of plants very closely allied systematically may 

 possess different physiological characters. Thus the two 

 plants B. commutatus and B. racemosus which are so close 

 morphologically that the majority of systematists hold that 

 B. racemosus is nothing more than a variety of B. com- 

 mutatus are proved to differ physiologically by the fact 

 that the fungus on B. commutatus cannot infect B. race- 

 mosus^ and vice versa. 



Now, although as a rule each species of Bromus shows 

 physiological characters which hold good for all examples 

 of the species derived from different localities, there are 

 exceptions to this rule, and we find that the inter-relations 



