Chap. VU. POLYGAMOUS PLANTS. 279 



be left existing, of which many instances occur ; and 

 if the female organs of the hermaphrodite were after- 

 wards to abort, the result would be a dioecious plant. 

 Conversely, if we imagine the female organs alone to 

 abort in some individuals, males and hermaphrodites 

 would be left ; and the hermaphrodites might after- 

 wards be converted into females. 



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

 mentioned in the Introduction, the stamens are rudi- 

 mentary in some individuals, the pistils in others, 

 others again remaining as hermaphrodites. Here the 

 modification 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 supplanted by the individuals having separated 

 sexes, and if these latter were equalised in number, a 

 strictly dioecious 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 dioeciousness 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. 

 13 



