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307
be brought to germinale in sugar solution (in R. rubiginosa 16%
of the pollen grains). Though these pollen grains must have a
rather small chromosome number, less than half of the somatic
number, sometimes only one-fourth of it (counts were made in
the pollen grain nucleus during its first division), they must be sup-
posed to be capable of fertilizing the eggs. Otherwise the forma-
tion of hybrids between roses of the section Caninae could not be
explained. The large amount of abortive pollen, immediately cau-
sed by the irregular distribution of the unpaired chromosomes in the
reduction divisions and the formation of numerous dwarf pollen grains,
is connected with the hybrid origin of these roses (see point 24).
14. The tetraploid and hexaploid species with 7 gemini plus 14
singles and 7 gemini plus 28 singles respectively perform the di-
visions of the pollen mother cells in all details in the same way
as the pentaploid with 21 unpaired chromosomes. This is true
also in regard lo the divisions of the embryo-sac mother cells.
15. The diakinesis of the embryo-sac mother cell nucleus is of
quite the same appearance as in the pollen mother cell, but from
the metaphase all the development is another. In most of the
embryo-sac mother cells the 7 bivalents assemble below the singles
— the bivalents next to the chalaza, the singles next to the micro-
pyle (fig. 3 a). In the anaphase the partners of the gemini pass
in normal way to the poles, but usually all the singles pass
undivided to the micropylar pole. The result is that the
micropylar daughter nucleus receives 28 chromosomes, the chalazal
only 7 (3 b). The second division is a regular homotype one, all
chromosomes being divided. The tetrad then is composed by two
large megaspores containing 28 chromosomes each and two small
one with only 7 chromosomes in their nuclei (fig. 3 c). This type
of distribution of the chromosomes is the most common. Only
in about one-fifth of the studied cases a greater or smaller number
of the unpaired chromosomes passed to the chalazal pole.
16. Eggs developed from such megaspores containing 28 chro-
mosomes, i. e. */5 of the whole somatic number (35), are qualified
for fertilization. This can be proved by the chromosome sets in
hybrids derived from pentaploid species and normally sexual roses
with a known chromosome number (see point 18).
17. In the pentaploid species of the section Caninae the chro-
mosome number of the eggs will then oflen be about four times
as large as that of the pollen grains.
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