936 Gates.—Pollen Formation in Oenothera gigas. 
cell-walls all showed outgrowths or proliferations, and the spindles were 
constantly very much wider. 
There is much variation in the shape of the daughter chromosomes of 
the heterotypic mitosis, and in the time when the fission of the chromosomes 
for the homotypic mitosis occurs. In the anaphase or telophase of the 
homotypic mitosis the chromosomes usually show a median constriction. 
A type of pollen sterility was studied in certain flowers, in which the 
reduction processes are normal or nearly so but the surrounding tissues fail 
to grow, remaining in contact with the mother-cells, which therefore retain 
their polyhedral shape. In some cases the four nuclei resulting from 
reduction move together to form a group in the centre of the mother-cell. 
In other cases a wall may be formed after the heterotypic mitosis, or the 
cytoplasm of the mother-cell may undergo cleavage to form a tetrad. The 
usual irregularities observed in hybrids also sometimes occur. 
It is considered most probable that the mutant O. gigas , with the 
tetraploid number of chromosomes, originated through a suspended mitosis, 
either in the fertilized egg or in the megaspore mother-cell. 
Literature cited. 
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