CHAP. VII. POLYGAMOUS PLANTS. 279 



be left existing, of which many instances occur; and if 

 the female organs of the hermaphrodite were afterwards 

 to abort, the result would be a dioecious plant. Con- 

 versely, if we imagine the female organs alone to abort 

 in some individuals, males and hermaphrodites would 

 be left; and the hermaphrodites might afterwards be 

 converted into females. 



In other cases, as in that of the common Ash-tree 

 mentioned in the Introduction, the stamens are rudimen- 

 tary in some individuals, the pistils in others, others 

 again remaining as hermaphrodites. Here the modifi- 

 cation of the two sets of organs appears to have occurred 

 simultaneously, as far as we can judge from their equal 

 state of abortion. If the hermaphrodites were sup- 

 planted by the individuals having separated sexes, and 

 if these latter were equalised in number, a strictly dioe- 

 cious species would be formed. 



There is much difficulty in understanding why her- 

 maphrodite plants should ever have been rendered 

 dioecious. There would be no such conversion, unless 

 pollen was already carried regularly by insects or by the 

 wind from one individual to the other; for otherwise 

 every step towards diceciousness would lead towards 

 sterility. As we must assume that cross-fertilisation 

 was assured before an hermaphrodite could be changed 

 into a dioecious plant, we may conclude that the con- 

 version has not been effected for the sake of gaining 

 the great benefits which follow from cross-fertilisa- 

 tion. We can, however, see that if a species were 

 subjected to unfavourable conditions from severe com- 

 petition with other plants, or from any other cause, the 

 production of the male and female elements and the 

 maturation of the ovules by the same individual might 

 prove too great a strain on its powers, and the separa- 

 tion of the sexes would then be highly beneficial. 



