284 S a Im on , On Specialization of Parasitism in the Erysiphaceae. 



the latter cannot., These two Oidia we will call respectively 02 6 

 and 03r. Now, it is possible that the üidium produced on a, 

 whether derived from the spores of 26 or 03c may become at 

 once, through its residence on «, identical mth 1 a, i. e. capable 

 of infecting B. sterilis, which both 02 6 and 03c are unable to 

 touch. If so, we shall have an instance of two Oidia which, when 

 growing on two species, viz. B. hordeaceits and B. interruptus, are 

 incapable of infecting a species, B. sterilis, becoming capable of doing 

 so through their power of infecting another species, viz. B. tectorum. 

 On the other band, if the Oidium on B. tectorum derived from 026 

 and 03c do not at once acquire the characters of Ola, we shall 

 have on a more than one ,biologic form.' The possible infection- 

 powers of these transferred Oidia on a are considered in the 

 hypothetical cases discussed above. The fact must not be lost sight 



fSerrq/alciisf 



B.commutaUn d \ 



t Serra/aicus) y' 

 B.internt ptiis c 



03C---"" 



(Stenobromus) 

 vf B. slerilis e \/^i 



Oia 



i Stenobromus) 

 ß. tectorum a 



fis:. 6. 



[SerrafalcuS] 

 hordeaceus b 



-"~02Ö 



of that a third Oidium, 04 c?, on B. commutatus exists, which is 

 capable of causing at any rate ,subinfection' on B. tectorum (see 

 Table 4), so that the problem is really more complex than that 

 considered above. 



Another point of considerable interest suggests itself when we 

 consider the inter-relations of the ,biologic forms' represented in the 

 above diagram. The ,biologic forms* 02, 03, and 04 on -B. Jiordea- 

 ceus, B. interruptus, and B. commutatus — species belongüig to the 

 section Serrafalcus of Bromus — are all, as Table 8 shows, quite 

 incapable of infecting the species of the section Stenobromus, B. 

 madritensis, B. mazimus, B. crinitus. and B. sterilis. They are all, 

 however, able to infect one species of the section Stenobromus, viz. 

 B. tectorum, 02 and 3 causing füll infection, and 04 ,subinfection.' 

 Now, in nature, we find an Oidium on B. tectoruin which proves 

 itself (Table 5) able to cause füll infection on B. sterilis. We are 

 led to ask, therefore, whether it is not possible, or even probable, 

 that the ,biologic forms' on B. hordeaceus, B. interruptus., and B. 

 commutatus may not through the help of B. tectorum, pass on to 



