VARIATION AND INHERITANCE 



403 



that the physiologic races may vary in Init a single gene and that they may 

 arise by hybridization or by mutation. 



There is abundant evidence that the haploid and diploid stages of some 

 fungi may differ in pathogenicity. The haploid phase of a number of 



Fig. 77. Panus stypticus grown on malt agar. A, diploid mycelium, 4 weeks old, from 

 a pairing between a haplont of the luminous American form and a haplont of the 

 nonluminous European form, photographed by reflected light; B, the same culture as A 

 photographed by its own light; C, a 2-weeks-old pairing between a nonluminous 

 haplont, on the left, and a luminous haplont, on the right, photographed by reflected 

 light; D, the same pairing as C photographed by its own light. (Courtesy of Macrae, 

 Can. Jour. Research, Sec. C, 20: 424, 1942.) 



smuts is apparently unable to cause infection, while the diploid mycelium 

 is pathogenic. Since the haploid and diploid mycelia of the heteroecious 

 rusts parasitize different hosts, we must conclude that they also differ in 

 pathogenicity. 



