72 IMMUNITY 



characters, the latter bring dominant (Biffen, L907, 1912; Pole- 

 Evans, 1911). But owing to the minute specialisation which is 

 characteristic of many Rusts, a variety may be immune to one 

 Rust while susceptible to another, or may even be immune in 

 one country but susceptible to the same Rust in a different 

 climate. The latter change would depend upon a slight 

 disturbance (by climatic factors) of the delicate balance which 

 existed between the attacking and resisting powers of the two 

 organisms. 



It may be pointed out here that this affords an opportunity 

 for dealing a final blow at the moribund "mycoplasm" hypo- 

 thesis. For when a susceptible and an immune variety of 

 wheat were crossed (Biffen, 1905) both the reciprocal crosses 

 were susceptible. Yet in that cross in which the pollen used 

 was taken from the susceptible parent, while the other was 

 immune, the only means by which the "mycoplasm" could be 

 conveyed would be in the generative nuclei of the pollen-tube, 

 which is inconceivable. An attempted criticism of this con- 

 clusion (Butler, 1905) misses altogether the point of the 

 argument: the maternal parent could not be classed as 

 immune, if it usually contained the "mycoplasm''' already in 

 its tissues. 



