THE PROBLEM OF SEX IN PLANTS 359 



in the gametophytes of the Horsetails (Chapter XXIY), 

 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. Effect of Constitution of Germ-cdls. — If sex is not de- 

 termined by external conditions — by environment — then 



Zygote 



Male " 



Sperm 



No-x 



Sperm 



x-class 



Zygote^ 

 Female 



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

 a;-chromosome. 



its explanation must lie in the internal constitution of the 

 germ-cells — in their chemical, ph3'sical, 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 x-chromosome,^ while in the 



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

 factor. 



