360 STRUCTURE AND LIFE HISTORIES 
sperms one-half possess it and one-half do not. This 
a>chromosome is always associated with femaleness. If 
an egg is fertilized by a sperm possessing the ^-chromosome 
a female zygote is determined, otherwise the zygote will 
be male, thus: 
Egg x + sperm no x = zygote x (male). 
Egg x -f- sperm x = zygote xx (female). 
This condition is illustrated diagrammatically in Fig. 
265. 
In sea-urchins, and some other animals, this condition 
may be reversed the sperms being all alike, and the eggs 
unlike. Careful investigations have, so far, failed to 
reveal anything corresponding to the ^-chromosome in 
plants, but undoubtedly there are fundamental dif- 
ferences between the eggs and sperms of plants. Possibly 
these differences are chemical only, and not accompanied 
by differences of cellular structure. 
326. The Meaning of Sex. Just what is accomplished 
for plants by the occurrence of two sexes is not entirely 
understood. Among the lower plants the primitive expres- 
sions of sex seem in some cases, to have met a need for better 
nutrition, or to have resulted in rejuvenating the protoplasts 
of the gametes; but these explanations are not satisfactory, 
especially for the higher plants. We do know that fertili- 
zation always results in increasing variation. When 
plants are propagated vegetatively, as by cuttings or by 
grafting, the characters remain constant in the new 
plants, 1 but when reproduction is by seeds (resulting from 
fertilization) we always observe great variation. This 
is of course, an advantage, for it is only by variation that 
new characters may appear, and without the appearance 
1 Except in the special case of bud-sporting (p. 532, and Fig. 400). 
