Abstract
Being heterothallic, Puccinia graminis produces haploid infections of two mating types the mycelial cells of which are uninucleate except for an occasional cell in which the nucleus has divided and in which the two daughter nuclei are still present. The pycniospores of both types of infection seem incapable of germination; but those of one type, when transferred to the upper surface of infections of the other type, fuse with the flexuous hyphae that protrude from the pycnia. By such fusions the pycniospore nuclei gain entry into the mycelia of the infections. When such transfers are made or when infections of opposite mating types coalesce to form compound infections, the introduced nuclei or their progeny migrate through the mycelia to the protoaecia of the infections, where they become conjugately associated with nuclei of the protoaecial cells. Meanwhile an occasional cell of the vegetative mycelium may become temporarily binucleate either by the division of its own nucleus, of by the passage through it of a migrating introduced nucleus or one of its progeny. Such a cell may become temporarily multinucleate by the division in it of one (or possibly both) of the daughter nuclei. In the protoaecium, conjugate associations are made possible by nuclear migration, in which activity both introduced and indigenous nuclei seem to be involved.
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