SCOTTISH PERISPORIACE^E 181 



experimental researches into the life -histories of parasitic 

 fungi is to show that where a " species," in the structural 

 sense, attacks numerous host plants, especially of widely 

 different orders, the " species " is apt to be composite, and 

 to include groups incapable of passing from host to host 

 beyond a limited range of plants. Thus what have been 

 called " biological " or " physiological species " are dis- 

 tinguished, though these are not practically separable under 

 the microscope. For one of these (Erysiphe Polygon!) over 

 200 hosts are enumerated with confidence, and 400 have been 

 stated to nourish it. For several other fungi the numbers of 

 host plants are also very large, and the range of orders 

 represented by them is often very wide. Over half the 

 species of Erysiphacea, on the other hand, have been found 

 on only one species, or not beyond one genus of host plants. 



In 1902 F. W. Neger ("Flora," pp. 242-267) showed 

 the existence of such races under Erysiphe cichoracearum 

 and of E. Polygoni ; and E. Marchal (" Comptes Rendus," 

 cxxxv. pp. 210-212) asserted that he had/ound seven such 

 races under E. graminis. In the "Journal of Botany" 

 (May and June 1903), Mr. Salmon gives an account of 

 important experiments made by himself to test the infective 

 power of ascospores of E. graminis, and concludes that they 

 " seem to give conclusive evidence that E. graminis com- 

 prises in its ascigerous stage specialised biologic forms." 



On comparing our records for Scotland with this mono- 

 graph, our list would stand as below, the names under which 

 the forms appear in my Revision following the mark = in 

 brackets. 



ERYSIPHACE^, LEV. 



PODOSPH/ERA OXYACANTHLE (DC.), De Bary ( = P. Oxyacantha, 

 (DC.), De Bary + P. myrtillina, Kze. and Schm.). 



var. TRIDACTYLA (Wallr), De Bary ( = P. tridactyla, as 

 a species). 



P. leucotricha (Ell. and Everh.) has been determined by Mr. Salmon 

 to be common in parts of Europe, where, as in America, its 

 Oidium causes the formation of a white "powdery mildew," 

 very hurtful to apple trees. As it rarely develops perithecia 

 it is difficult to be certain of the species. A mildew much like 

 that described as due to P. leucotricha certainly occurs at 



