212 THE THEORY OF THE GENE 



to the next generation, produced by back-crossing the F x 

 male to either parent stock, provided, however, the trip- 

 loid nature of the offspring be taken into account. (See 

 Chap. IX.) 



There are no grounds for supposing that the chromo- 

 somes involved in the XX-XY and in the WZ-ZZ types 

 are the same. On the contrary, it is difficult to imagine 

 how one type could change over directly into the other. 

 There is no theoretical difficulty, however, in supposing 

 that the change in balance that gives the two sexes may 

 have arisen independently in the two types, even although 

 the actual genes involved are the same or nearly the same 

 in both. 



Sex-Chromosomes in Dioecious Flowering Plants. 



One of the surprises of the year 1923 was the simul- 

 taneous announcement by four independent workers that 

 in some of the flowering plants with separate sexes a 

 mechanism is present that follows the XX-XY type. 



Santos found in the male of Elodea that 48 somatic 

 chromosomes are present (Fig. 123), consisting of 23 

 pairs of autosomes and an XY unequal pair. At matura- 

 tion the X and Y separate. Two kinds of pollen grains 

 result, one with X, the other with Y. 



Two other cytologists, Kihara and Ono, found in male 

 plants of Rumex 15 somatic chromosomes consisting of 

 6 pairs of autosomes and 3 heterochromosomes (m x , m 2 , 

 and M). These three come together at maturation of the 

 germ-cells to form a group (Fig. 123). The M goes to one 

 pole, the two small m's to the opposite. Two kinds of pol- 

 len grains result, 6a-f M and 6a+m+m. The latter are 

 male-determining. 



Winge found an XY pair of chromosomes in two spe- 

 cies of hops (Humulus lupulens and H. japonica). Nine 



