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.^ This is further illustrated 



' In exceptional cases perfect flowers may appear on staminate or pistil- 

 ate plants, as in papaw {Carica Papaya) and willow, suggesting that 

 dioecious plants may, in reality, be of double se.x-value, but that only one 

 of the sexes comes normally to expression. 



