MORPHOLOGICAL DIFFERENCES 261 



and 5". cruerita. The phenotypes obtained had characters common 

 to both parents. Other interspecific smut hybrids, such as those 

 between Ustilago avenae and U. levis, and between U. hordei and 

 U. medians, have been produced that are intermediate in the Fi 

 generation but segregate in the F 2 generation. The status of 

 present knowledge of genetic factors as applied to hybridization 

 in smuts, and especially to the origin of physiologic races by this 

 means, is summarized by Rodenheiser (1940). New specialized 

 races of smuts are known to arise in nature, presumably by hy- 

 bridization. Reed (1935) isolated from a collection of loose smut 

 of oats from Texas two distinct new races, one capable of infect- 

 ing Red Rustproof oat and the other Fulgum oat. 



It should be recalled that in some species of smuts infection and 

 production of chlamydospores occur only if there has been fusion 

 of lines of opposite sex. In such species hybridization between 

 biotypes undoubtedly is of common occurrence. Moreover, inter- 

 specific hybrids between Ustilago hordei and U. medians, U. levis 

 and U. avenae, Tilletia levis and T. tritici, and Sphacelotheca 

 omenta and S. sorghi have been produced. Certain intergeneric 

 crosses, as between Sorosporiwn reilianum X Sphacelotheca 

 sorghi, and Sorosporhtm reilianum X Sphacelotheca omenta, have 

 also been effected [Tyler and Shumway (1935), Christiansen and 

 Rodenheiser (1940)]. 



Morphological differences between physiologic species. It 

 has been pointed out that minor morphologic differences have 

 been noted between urediniospores of varieties of Pnccinia granii- 

 nis. This observation has led to a search for distinctive differences 

 by means of which to separate specialized races of this rust as it 

 occurs on wheat. The outstanding of these attempts is that of 

 Levine (1928), who by the aid of biometrical methods was able 

 to show minor differences in size and shape of spores between 

 the several physiologic forms. Newton and Johnson (1927) 

 were able to show that a bright orange form species and a greyish 

 brown one can be distinguished from the normal form 9 of P. 

 graminis tritici. Similar segregation of species followed from 

 monographic studies on Peronospora by Gaumann (1923). By 

 making numerous measurements of the lengths and widths of 

 sporangia of Peronospora parasitica and then plotting these data 

 as population curves, he was able to separate the species into a 

 number of distinct groups. 



