106 THE STORY OF THE PLANTS. 



different blossoms or even on separate plants. 

 All the flowers we have so far considered have 

 contained both male and female portions have 

 been made up of stamens and carpels united to- 

 gether in the self-same blossom. But many of 

 them, as you will recollect, have not been actively 

 both male and female at the same moment. The 

 stamens ripened first, the sensitive surface of the 

 carpels afterwards ; and this, as we saw, tended 

 to promote cross-fertilisation. But if in any spe- 

 cies all the stamens in certain flowers were to be 

 suppressed or undeveloped, while in other flowers 

 the same thing happened to the carpels, self- 

 fertilisation would become an absolute impossi- 

 bility, and every blossom would necessarily be 

 impregnated from the pollen of a neighbour. 

 Natural selection has accordingly favoured such 

 an arrangement in a considerable number of thr 

 higher plants. In such cases some of the flowers 

 consist of stamens only, with no carpels ; while 

 others consist of carpels alone, with no stamens. 

 But as all are descended from ancestors which 

 had both organs combined in the same flower, 

 remnants of the stamens often exist in the female 

 flowers as naked filaments or barren threads, 

 while remnants of the carpels equally exist in the 

 male flowers as central knobs without seeds or 

 ovules. 



The beautiful begonias, so much cultivated in 

 conservatories, give us an excellent example of 

 such single-sex flowers. In these plants the males 

 and females are extremely different The male 

 flower has four coloured and petal-like sepals, 

 surrounding a number of central stamens. The 

 female flower has five coloured and petal-like 

 sepals, surrounding a group of daintily-twisted 



