222 BOTANY 



In the genus Melandrium the presence of sex-chromosomes 

 was independently discovered by Blackburn and Winge. These 

 workers reported that an unequal pair of XY chromosomes are 

 present in the staminate plant and that the members of this pair 

 disjoin at the heterotypic division in the microsporocytes. In the 

 carpellate plant Blackburn described the presence of an XX-pair. 

 She also found the same state of affairs in Lychnis dioica and in 

 the hybrid L. alba X L. dioica. In this case Blackburn is of the 

 opinion that the larger and not the smaller member of the pair 

 represents the Y-chromosome. 



The presence of sex-chromosomes has now been reported in the 

 genus Humulus. Winge described in both H. lupulus and H. 

 japonicus the somatic number of chromosomes in the male plant 

 as 20. This number is made up of 18 autosomes and an XY-pair 

 of chromosomes. The X-member of the pair can be readily 

 distinguished from the Y-member by its larger size and the 

 presence of a constriction in the middle. At meiosis this XY-pair 

 disjoin and at the second division they divide longitudinally. 

 The question of sex-chromosomes has been further investigated 

 by Kihara in H. japonicus. Here there are 17 chromosomes in 

 the male and 16 in the female plant. In the male plant the 17 

 chromosomes are made up of the following, 14 autosomes and 

 3 sex-chromosomes. One of these is large and V-shaped, and 

 is presumably the X-chromosome, while the other two are J-shaped 

 and comprise the Y-chromosome, made up of a pair (y^ and yg). 

 The sex-chromosomes here are very large and larger than the 

 largest autosomes. In the female plant the two X-heterosomes 

 are large and V-shaped structures. At meiosis of the pollen 

 mother-cell, the sex-chromosomes form a tripartite complex of 

 which the central X-chromosome passes to one pole of the spindle 

 and y^ and yg pass to the other. The behaviour of the sex- 

 chromosomes is therefore identical with that of the corresponding 

 stages of Rumex acetosella. The chromosomal formulae of these 

 plants are given as follows : 



Staminate (diploid) . . 14-|-X+yi+y2 



(haploid) . . 7 -f X, 7 + yi + y2 



Carpellate (diploid) . , 14 ~|- X -|- X. 



