358 STRUCTURE AND LIFE HISTORIES 
male and female gametophytes are as unlike as could well 
be imagined. Illustrations of this condition will be 
found when we study the Gymnosperms (zamia, cycas, 
ginkgo), and the Angiosperms (willow, poplar, hop, etc.). 
325. Determination of Sex. i. Effect of Nutrition. 
Nobody knows the real cause of sex of maleness and 
femaleness. We may arrange plants (and animals) in 
a series so as to show the gradual transition from the 
simple non-sexual condition to complete differentiation of 
males and females, but in doing this we should clearly 
recognize the fact that we have really explained nothing. 
We have only described events and structures in the order 
in which it seems probable that they have occurred, in 
the gradual development of the earth's vegetation. 
But while we have never yet been able to determine in 
advance the sex of a plant or animal, we have been able 
to determine which sex shall gain expression. For ex- 
ample, we have seen above that male plants are frequently 
less vigorous and more poorly nourished than female 
plants. We would, therefore, expect that poor nutrition 
would cause a suppression of femaleness, and this is pre- 
cisely what has been found in certain experiments that 
have been made. When the prothallia of certain ferns 
that normally bear both antheridia and archegonia are 
grown under conditions that result in their being poorly 
nourished, the antheridia develop, but not always the 
archegonia. In such cases we know that we have not 
changed the sexual nature of the prothallus, but have 
only modified its expression. 1 This is further illustrated 
1 In exceptional cases perfect flowers may appear on staminate or pistil- 
ate plants, as in papaw (Carica Papaya) and willow, suggesting that 
dwecious plants may, in reality, be of double sex-value, but that only one 
of the sexes comes normally to expression. 
