CHAP. XII. GENERAL RESULTS. 445 



self-fertilisation and preventing to a certain extent 

 cross-fertilisation, are wonderfully diversified; and it 

 is remarkable that these differ widely in closely allied 

 plants,* in the species of the same genus, and some- 

 times in the individuals of the same species. It is 

 not rare to find hermaphrodite plants and others 

 with separated sexes within the same genus ; and it is 

 common to find some of the species dichogamous and 

 others maturing their sexual elements simultaneously. 

 The dichogamous genus Saxifraga contains proter- 

 androus and proterogynous species, t Several genera 

 include both heterostyled (dimorphic or trimorphic 

 forms) and homostyled species. Ophrys offers a 

 remarkable instance of one species having its structure 

 manifestly adapted for self-fertilisation, and other 

 species as manifestly adapted for cross-fertilisation. 

 Some con-generic species are quite sterile and others 

 quite fertile with their own pollen. From these several 

 causes we often find within the same genus species 

 which do not produce seeds, while others produce an 

 abundance, when insects are excluded. Some species 

 bear cleistogamic flowers which cannot be crossed, as 

 well as perfect flowers, whilst others in the same genus 

 never produce cleistogamic flowers. Some species exist 

 under two forms, the one bearing conspicuous flowers 

 adapted for cross-fertilisation, the other bearing in- 

 conspicuous flowers adapted for self-fertilisation, whilst 

 other species in the same genus present only a single 

 form. Even with the individuals of the same species, 

 the degree of self-sterility varies greatly, as in Keseda. 

 With polygamous plants, the distribution of the sexes 



* Hildebrand has insisted natsbcricht K. Akad. Berlin,' Oct 



strongly to this effect in his 1872, p. 763. 



valuable observations on the fer- t Dr. Engler, < Bot. Zeitung, 



tilisatioc of the Graminese : ' Mo- 1868, p. 833. 



