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62 nils nybom
Ihe ]jöllen milosis and phvsiologically unhalanced comhinations will
never be observed.
2. Mechanically. A cerlain mechanism at meiosis causing Ihe St
chromosomes to be möre often inclosed in the largest anaphase group
at Ihe first division. Concerning the first alternative, it might be
supposed that the relation between the nucleolar organizing Sx
chro-mosome and Ihe other chromosomes is of importance, leading to an
»elimination» of pollen grains with comparativelv too few or too many
Sj chromosomes. In fact, such an elimination would lead to the found
deviation in Ihe distribution of the Sj chromosomes, i.e. an elimination
of pollen grains with none extra St in the high classes would increase
the frequency of St chromosomes in Ihese classes, and a corresponding
elimination of pollen grains with one extra S, in the löw classes would
decrease the frequency of St chromosomes in these classes.
The optimal percentage of Sx chromosomes of all chromosomes
must be assumed to be that of the mother plant, i.e. 12.5 °/o. lf we,
however, consider the found percentages of Sj chromosomes in Table 1
we shall find an excess of the probably optimal percentage of Sx in the
high classes and a corresponding deficit in the löw classes. The
signi-ficance of this may be tested by comparing the found and expected
distributions of Sx chromosomes in an analysis of covariance. v2 for
correlation, i.e. that the two distributions are moved in relation to each
other, was 6.37, which gives p = ca. 0.01.
An elimination of the kind mentioned is, thus, not very j)robable,
and I can hardly find any other kind of elimination that might be
responsible for these deviations from randomness. Consequently I am
not inclined to think that we are here concerned with physiological
unbalance of certain chromosome comhinations leading to delay or
failure of pollen milosis. 1 have very kiiidlv been given an opportunity
of studying the preparations used by Levan for these Si chromosome
counts. They were eight in number (8X6 anthers), and the pollen
grains seemed to be quite normal and without any visible disturbances.
An excess of pollen grains with 2SX in the high classes might be
caused by some seleclive orientation of Ihe Sj trivalent so that it tended
to arrange itself in the same direction as the majority of the olher
trivalents. Thus, 2Sj ought to come möre often lo the largest anaphase
group at the first division. Such a selective orientation would be the
reverse to that observed by Darlington (1929) in Tradescantia and by
Beadle (1935) in Drosophila, hut it may very well account for the
deviations from randomness observed by Satina and Blakeslee (1937)
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