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1. Out of a total of 491 investigated plants in the progeny of Crepis capillaris X-rayed during the seedling stage 84, or 17.1 per cent, proved to deviate from the normal as regards their karyotype. Most of these aberrants (74) were characterized by structural chromosome changes. The data are summarized in Table 1 (p.2).2. In a number of cases several individuals with identically altered chromosomes arose from one mother plant, indicating their origin from a single aberrant cell of the latter, which had given rise to an entire aberrant sector or part of the mother plant.3. The most common types of visible cytological changes were translocations, shown by the shortening of one chromosome and the corresponding lengthening of another, and inversions, resulting in alterations in the morphology of the chromosomes concerned.4. All of these changes were first obtained in a heterozygous state, and afterwards produced in a hcmozygous state.5. In addition to balanced changes, cases of duplication were also obtained, both of whole choromosomes (all three types) and of separate parts of chromosome D, distal (of various kinds) and proximal. The latter change was also obtained in the form of a tetrasomic.6. Some of the investigated structural aberrants were characterized by lowered fertility. In most cases this decrease in fertility was more marked in the heterozygous state; in others-in the homozygous state. In the former this was due, presumably, to cytological reasons; in the latter-to genetical, perhaps to a lethal expression of the “position effect.”7. In the progeny of structural heterozygotes structurally altered homozygotes occur, as a rule, less frequently than normal plants.8. Among the karyotypes of pollen grains formed by structurally heterozygous aberrants normal karyotypes are, as a rule, considerably more numerous than balanced aberrant karyotypes. In two of the investigated heterozygous translocants there were formed a considerable number of pollen grains with karyotypes having an additional chromosome or part of a chromosome. The total diversity of chromosome sets of the male gametophyte reached as many as nine types.

A Cytological Study of the Progeny of X-rayed <i>Crepis capillaris</i> Wallr