72 IMMUNITY 
characters, the latter being dominant (Biffen, 1907, 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. 
