THE PROBLEM OF SEX IN PLANTS 



359 



in the gametophytes of the Horsetails (Chapter XXIV), 

 which are usually differentiated into larger ones (female) 

 and smaller ones (male); but under certain conditions 

 (apparently involving differences of nutrition), the female 

 gametophytes may bear antheridia, and the male gameto- 

 phytes archegonia. 



2. Efect of Constitution of Germ-cells. — If sex is not de- 

 termined by external conditions — by environment — then 



Zygote 

 Mal9 



(zo ox my 



Sperm 

 No-X,or y 



Sperm 

 Xr class 



(2Z) 

 Zygote 

 Female 

 Fig. 265, — Diagram to illustrate determination of sex by the 

 ic-chromosome. 



its explanation must lie in the internal constitution of the 

 germ-cells — in their chemical, physical, or morphological 

 differences. Remarkably careful and accurate observa- 

 tion has, in fact, revealed a constant and fundamental 

 morphological difference in the germ-cells of animals. 

 It has been found, for example in some insects, that the 

 nucleus of every egg possesses a certain clearly distinguished 

 chromosome, called the rr-chromosome,^ while in the 



' The X, as in algebra, indicating an unknown, or not understood, 

 factor. 



