42 



PUCCINI A GRAM I MS 



Fi^'. 2~>. Puccinia graminis. a, 

 fficidia on Berberis ; b, uredo- 

 spore ; c, teleutospores. 



culms; for these differences the systematic part can be con- 

 sulted (Fig. 25). 



There is, however, one point 

 of difference connected with P. 

 graminis which possesses greal 

 biological interest — its virtual in- 

 dependence of the aicidial stage. 

 For a long time it had been 

 known that Barberry bushes in 

 the hedges caused " mildew : ' on 

 the corn in the neighbouring 

 fields, and when, in 1864-5, De 

 Bary proved the hetercecism by 

 experimental cultures, it was too 

 hast ilv assumed that the a-cidiuni 

 on the Berberis was just as es- 

 sential to the rust on the corn as 

 that on the Nettle is to the rust on the Sedge. Many facts 

 new tend to show that this is not the case. 



In Australia and the plains of India the Barberry is un- 

 known except as an introduced plant, yet the Puccinia occurs 

 everywhere and does enormous damage. McAlpine records, in 

 his Rusts of Australia that he made numerous attempts to 

 infect imported species of Berberis with the rust of Australian 

 wheat which is morphologically undistinguishable from the 

 P. graminis of Europe, but all his efforts were in vain. The 

 inevitable inference is that P. graminis, as it occurs in those 

 countries, is a " biological " race which maintains itself by other 

 than the primitive means. A similar thing is true, according 

 to Lagerheim, in Ecuador, where also rust flourishes and does 

 great harm. 



The facts now known concerning the specialisation of the 

 Black Rust are treated of in a separate chapter, but there is 

 one point which must be mentioned here. This concerns the 

 mode by Avhich fresh epidemics are produced each year. Even 

 if the Barberry is present, it is by no means certain that it 

 plays any important part in these annual attacks. Apart from 

 that, there are several possibilities: (1) the fungus may winter 



