307 



1)6 hroughl lo gerininale in sui^ar solulioii [\n Ji. riihit/inosa IG/w 

 of Ihe pollen grains). Though Ihese pollen grains must have a 

 rather small chromosome numher, less than half of Ihe somalic 

 number, somelimes only one-fourlli of it (counts were made iii 

 Ihe pollen grain nucleus (Inring ils lirst division), they must be sup- 

 poseti lo be capable of rerlili/.ing Ihe eggs. Otherwise Ihe forma- 

 lion ol hybrids belween roses of Ihe seclion Caninae could not be 

 explained. The large amonnl of aborlive j)ollen, immedialely cau- 

 sed by Ihe irregular dislribnlion of Ihe unpaired chromosomes in Ihe 

 reduclion divisions and Ihe fornialion of numerous dvvarfj)ollen grains, 

 is connecled ^vilh the hybrid origin of Ihese roses (see point 24). 

 11. The letraploid and hexaploid species wilh 7 gemini plus 14 

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

 visions of Ihe pollen molher cells in all delails in Ihe same way 

 as Ihe penlaploid wilh 21 uni)aired chromosomes. This is true 

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



15. The diakinesis of Ihe embryo-sac molher cell nucleus is of 

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

 the melaphase all the development is another. In mosl of Ihe 

 embryo-sac mother cells the 7 bivalents assemble below the singles 

 — the bivalents next to the chalaza, the singles next to the micro- 

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

 in normal way lo the poles, but usually all the singles pass 

 undivided to the micropylar pole. The resull is Ihat Ihe 

 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 lelrad then is composed by two 

 large megaspores conlaining 28 chromosomes eacli and two small 

 one wilh only 7 chromosomes in Iheir nuclei (lig. 3 c). This tyi)e 

 of distribution of Ihe chromosomes is the mosl common. Only 

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

 of the unpaired chromosomes passed to the chalazal pole. 



16. Eggs developed from such megaspores containing 28 chro- 

 mosomes, i. e. ^1', of Ihe whole somalic number (35), are qualilied 

 for fertilization. This can be proved by the chromosome sels in 

 hybrids derived from penlaploid species and normally sexual roses 

 with a known chromosome number (see point 18). 



17. In the penlaploid species of the seclion Caninae the chro- 

 mosome number of Ihe eggs will then ollen be aboul four times 

 as large as Ihat of Ihe pollen grains. 



