360 STRXJCTURE 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 x-chromosome 

 a female zygote is determined, otherwise the zygote will 

 be male, thus: 



Egg X -\- sperm no — x = zygote x (male)'. 

 Egg X -\- 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 a;-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 accompKshed 

 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,^ 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). 



