SEX-CHROMOSOMES 



221 



During the heterotypic division of meiosis M disjoins from ni^ and 

 nig, and at the homotypic division all three split longitudinally, 

 and thus two of the microspores receive seven chromosomes (six 

 autosomes and M), and the other two receive eight chromosomes 

 (six autosomes and m^ and mg). The somatic nuclei of the 

 female plant contain twelve autosomes and two M-chromosomes. 



/ 



V\ 



'A 



^ 



/, 



\ 



\ 





^^m 



A^^ 



i*«.'3r% 



^ 



\ 



G 



Fig. 80. — Sex chromosomes in heterophytic Angiosperms. (A) Vallisneria 

 sjjiralis, showing eight autosomes. (B) F. spiralis showing eight 

 autosomes and one long constricted X-chromosome. (C) Melandrium 

 album showing XY pair about to disjoin. (D) Humulus japonicus 

 showing XY pair on right. (E) Populus balsamifera with XY pair. 



(F) Valisneria dioica showing X and Y disjoining at first division. 



(G) Rumex acetosella showing Mm disjoining from m at first division. 

 (H) Polar view of the same. (A-D after Winge, E-H after Meurman. 

 From Sharp, Introduction to Cytology.) 



Thus at fertilisation with a male gamete carrying M, the result 

 will be a female plant, and fertilisation with a male gamete carry- 

 ing nil and nig will give rise to a male plant. Kihara and Ono 

 considered that the M-chromosome is equivalent to X and that 

 the nil ^1^^ ^^2 elements compose the Y-chromosome. Meurman 

 reported a similar state of affairs for R. thyrsiflorus, but according 

 to this author in R. acetosella, M and one of the m's disjoin from 

 the other m at the first meiotic division. 



