246 PLANT DISEASES 



a. The yellow rust appears in certain varieties of wheat 

 and barley that are especially susceptible, uniformly four 

 to five weeks after sowing. 



b. The intensity has sometimes been stronger in sunny 

 than in shady places in the same wheatfield. 



c. Plants of a variety of barley extremely susceptible to 

 yellow rust, grown in sterilised soil, and protected from 

 external infection in isolated glass-houses, have sometimes 

 become rusted. 



' These results prove beyond doubt that the disease 

 must come from internal germs inherited from the 

 parent plant. But in what form are these internal 

 germs of disease living? Is it easy to follow and identify 

 them with the microscope ? Not at all. They can only 

 be detected just before the breaking-out of the young 

 pustules. The microscope examination induces me to 

 suppose that 



' The fungus lives for a long time a symbiotic life as a myco- 

 plasma in the cells of the embryo and of the resulting plant, 

 and that only a short time before the eruption of the pustules, 

 when outer conditions are favourable, it develops into a visible 

 state, assuming the form of a mycelium? 



Eriksson, Bot. Gaz., vol. xxv. p. 26 (1898). 



It has been proved experimentally that rust-shrivelled 

 wheat used as seed produces as good a crop, and one as 

 free from rust, as when plump seed is used. If Eriksson's 

 theory is correct, it is not wise to sow rust-shrivelled grain 

 for seed, as probably the amount of rust produced may be 

 due to the internal mycoplasma, consequently more grain 

 is produced which presumably would contain mycoplasma, 

 and if this should be used as seed the same thing would 



