THE FUNGI WHICH CAUSE PLANT DISEASE 379 



P. graminis Pers. "• ""-""• ""• '"'• ^^^ 



I (=^. berberidis). Spots generally circular, thick, swollen, 

 reddish above, yellow below. Peridia cylindrical, with whitish 

 torn edges. Spores subglobose, smooth, orange-yellow, 15-25 /i. 



II. Uredinia orange-red, linear, but often confluent, forming 

 very long lines on the stems and sheaths, pulverulent. Spores 

 elliptical, ovate, or pyriform, with four very marked, nearly 

 equatorial germ pores, echinulate, orange-yellow, 25-38 x 15-20 /i. 



III. Telial persistent, naked, linear, generally forming lines on 

 the sheaths and stems, often confluent. Spores fusiform or clavate, 

 constricted in the middle, generally attenuated below, apex much 

 thickened (9-10 m), rounded or pointed, smooth, chestnut-brown, 

 35-65 X 15-20 /i. Pedicels long, persistent, yellowish-brown. 



and I on Berberis and Mahonia. 



II and III on Avena, Hordeum, Secale, Triticum and nearly 

 fifty other grasses. Of great importance on wheat in the Great 

 Plains and along the Ohio. 



This fungus was the subject of the classic researches of de 

 Bary ^^ begun in 1865 and has since repeatedly served as the basis 

 of fundamental investigations in parasitism, cytology and biologic 

 specialization. That the barberry seciospores can bring about 

 cereal infection seems to have been shown as early as 1816. "* 

 Inoculations in the reverse order were made in 1865. '^ Extensive 

 studies by Eriksson ^^^ are interpreted by him to show that what 

 was formerly regarded as one species must be separated on bio- 

 logic groimds into several races which he finally erects as species, 

 though others do not agree that their rank should be specific. 

 These are: P. graminis secalis. P. graminis avenae. P. graminis 

 tritici. P. graminis airse. P. graminis poae. P. phlei-pratensis. 



These words from Butler and Hayman ^^ show the complexity 

 of the status of these biologic forms. 



"Of late years it has become more and more established that 

 parasitic fungi, which are capable like these rusts of living on 

 several hosts, tend to develop 'races' on their different host- 

 species, marked off from each other by definite characters. Some- 

 times the characters are such as are capable of being detected 

 microscopically. Usually, however, the fungi are, to all appear- 

 ance, identical, and differences only appear when their manner of 



