POLYPLOIDY 223 



Winge has reported the case of the XO-type of sex-chromo- 

 somal mechanism in Vallisneria spiralis. In the staminate plant 

 there are 17 chromosomes in the somatic nuclei. In tlie micro- 

 sporocyte there are eight paired gemini and one unpaired X-chro- 

 mosome. Since in the first division of meiosis the X-chromosome 

 passes to one pole of the spindle and divides equationally in the 

 second mitosis, half of the microsporocytes receive nine chromo- 

 somes (8 + X) and the other half receive but eight. The chromo- 

 somal complement of the carpellate plant has not been investigated 

 as yet. A similar state of affairs has been reported for Dioscorea 

 sinuata by Meurman. Jorgensen, however, in a re-investigation 

 of Vallisneria has failed to confirm the presence of the XO 

 mechanism described by Winge. 



Meurman has confirmed the statement of Blackburn and 

 Harrison that there is evidence for the presence of heterosomes in 

 the Salicacese and that probably they occur in both genera of this 

 family. In Populus balsamifera, P. Simoni, and P. trichocarpa, 

 Meurman has demonstrated an XY mechanism. 



POLYPLOIDY 



Generally the number of chromosomes in the somatic nuclei 

 of a plant is constant for a given species. When a comparison is 

 made of related species it is very frequently found that the chromo- 

 some complement is some multiple of a basic number. It has also 

 been found that different varieties of some species possess twice 

 the number of chromosomes normal for that particular species. 

 This relationship is practically exclusively confined to the vege- 

 table kingdom, only some three cases being known in the animal 

 world. 



If n represent the basic haploid number of chromosomes in the 

 somatic cells of some fertile type, then exact multiples of n can be 

 represented as follows : — 



Number of chromosomes in somatic nucleus. 



n ...... Haploid 



2n ...... Dij^loid 



3n ...... Triploid 



