307 



be broughl lo germinale in sugar solulion (in R. ruhiginosa 16% 

 of Ihe pollen grains). Tlioiigh lliese pollen grains must have a 

 rather small chromosome number, less llian balt" of the somatic 

 number, somelimes only oiie fourlh of il (counls were made in 

 tlie pollen grain nucleus duiing ils lirsl division), Ibey must be sup- 

 posed lo be capable of feriilizing Ibe eggs. Olberwise Ibe forma- 

 tion of bybrids belween roses of Ibe seclion Caninae could iiol be 

 explained. Tbe large amounl of aborlive pollen, immediately cau- 

 sed by the irregular distribulion of Ibe unpaired chromosomes in the 

 reduclion divisions and Ibe formation of numerous dwarf pollen grains, 

 is connecled wilb tbe hybrid origin of tbese roses (see poinl 24). 



14. Tbe letraploid and bexaploid species with 7 gemini plus 14 

 singles and 7 gemini plus 2(S singles respeclively perform Ihe di- 

 visions of Ihe pollen mother cells in all dctails in tbe same vvay 

 as Ihe penlaploid wilb 21 unpaired chromosomes. This is true 

 also in regard lo Ihe divisions of the embryo-sac mother cells. 



15. The diakinesis of the embryo-sac mother cell nucleus is of 

 quile tbe same appearance as in the pollen mother cell, but from 

 the melaphase all the development is anolher. In mosl of the 

 embryo-sac molher cells Ihe 7 bivalents assemble below the singles 

 — the bivalents next to Ihe cbalaza, the singles next to the micro- 

 pyle (fig. 3 a). In the anaphase the partners of tbe gemini pass 

 in normal way to the poles, but usually all tbe singles pass 

 undivided to the micropylar pole. The result is that the 

 micropylar daughter nucleus receives 28 chromosomes, the chalazal 

 only 7 (3 /)). The second division is a regular homotype one, all 

 chromosomes being divided. Tbe lelrad then is composed by two 

 large megaspores conlaining 28 chromosomes each and two small 

 one wilb only 7 chromosomes in their nuclei (lig. 3 c). This type 

 of distribution of the chromosomes is the most common. Only 

 in about one lifth of the studied cases a grealer or smaller number 

 of the unpaired chromosomes passed to the chalazal pole, 



16. Kggs developed from sucb megaspores containing 28 chro- 

 mosomes, i. e. '/.T of the whole somalic number (35), are qualified 

 for fertilization. This can be proved by tbe chromosome sets in 

 bybrids derived from penlaploid species and normally sexual roses 

 wilb a known chromosome number (see point 18). 



17. In Ibe penlaploid species of the section Caninae the chro- 

 mosome number of the eggs will then oflen be aboul four times 

 as large as Ibal of tbe pollen grains. 



