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. Effect of Constitution of Germ-cells. If sex is not de- 
termined by external conditions by environment then 
Zygote 
Male " 
sgg 
Sperm 
No-x 
Sperm 
x -class 
^Zygote 
Female ' 
FIG. 265. Diagram to illustrate determination of sex by the 
^-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 ^-chromosome, 1 while in the 
1 The x, as in algebra, indicating an unknown, or not understood, 
factor. 
