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TEXT-BOOK OF FUNGI 



to be identical morphologically with B. mollis. These 

 results lead us to conclude that the morphological species 

 B. mollis includes two * races ' or sets of individuals pos- 

 sessing distinctive physiological (or constitutional) char- 

 acters, that is to say, an immune and a susceptible race with 

 regard to the form of the fungus in question. These may 

 be termed biologic forms of a host-plant. 



Another interesting fact is the discovery of certain host- 

 plants which function as ' bridging species.' It has been 

 found that a number of biologic forms may all be able to 

 infect a certain host-plant, although each one of these 

 biologic forms may be incapable of infecting the host- 

 plants on which the others occur. 



Thus fig. 45 represents diagrammatically the results of 

 certain infection-experiments, from which we see that the 

 biologic forms of the fungus on B. secalinus, B. interruptus^ 

 B. racemosus^ B. commutatus, and B. arduennensis are all 

 able to infect B. hordeaceus^ although they are incapable of 

 the reciprocal infection of their host-plants. With the 

 existence of such inter-relations as these the question at 

 once presented itself whether B. hordeaceus might not act 

 as a c bridge,' affording a passage for certain biologic forms 

 to species of host-plants which they are unable to attack 

 directly. Actual experiments have proved this to be the 

 case. It has been ascertained by repeated inoculation- 

 experiments that the biologic form on B. racemosus is 

 unable to cause any infection whatever on B. commutatus ; 

 infection by means of a ' bridging species ' (of host) has, . 

 however, been found to take place. In certain experiments 

 conidia were taken from B. racemosus and sown on B. 

 hordeaceus. Infection resulted at once, a crop of conidia 

 being produced on B. hordeaceus in a few days. These 



