. Beihefte zum Botanischen Centralblatt. Botany. 284 S aIm on , On Specialization of Parasitism in the Erysiphaceae. the latter These two Oidia we will call respectively 02 6 and 03c Now, it is possible that the Oidium produced on a, whether derived from the spores of 0 26 or 03c may become at once, through its residence on a, identical with 01 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. hordeaceus and B. interruptus, are incapable of infecting a specie
. Beihefte zum Botanischen Centralblatt. Botany. 284 S aIm on , On Specialization of Parasitism in the Erysiphaceae. the latter These two Oidia we will call respectively 02 6 and 03c Now, it is possible that the Oidium produced on a, whether derived from the spores of 0 26 or 03c may become at once, through its residence on a, identical with 01 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. hordeaceus 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 hand, 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 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. 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. hordea- ceus, B. interruptus, and B. commutatus — species belonging 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. maximus, B. crinitus. and B. sterilis. They are all, however, able to infect one species of the section Stenobromus, viz. B. tectorum, 02 and 0 3 causing Ml infection, and 04 ,subinfection.' Now, in nature, we find an Oidium on B. tectorum which proves itself (Table 5) able to cause füll infection on B. sterilis. We are led to ask,
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