244 CONCLUDING EEMAEKS Chap. VI 



CHAPTER VI. 



CONCLUDINQ EeMAKKS ON HeTECOSTTLED PLANTS. 



The essential character of heterostyled plauts— Stimmary of the 

 differences in fertility between legitimately and illegitimately fer- 

 tilised plants — Diameter of the pollen-grains, size of anthers and 

 structure of stigma in the different forms — Affinities of the genera 

 which include heterostyled species — Nature of the advantages 

 derived from heterostylism — The means by which plants became 

 heterostyled — Transmission of form — Equal-styled varieties of 

 heterostyled plants — Final remarks. 



IN the foregoing chapters all the heterostyled plants 

 known to me have been more or less fully described. 

 Several other cases have been indicated, especially by 

 Professor Asa Gray and Kuhn,* in which the indi- 

 viduals of the same species differ in the length of 

 their stamens and pistils ; but as I have been often de- 

 ceived by this character taken alone, it seems to me 

 the more prudent course not to rank any species as 

 heterostyled, unless we have evidence of more impor- 

 tant differences between the forms, as in the diameter 

 of the pollen-grains, or in the structure of the stigma. 

 The individuals of many ordinary hermaphrodite plants 

 habitually fertilise one another, owing to their male 

 and female organs being mature at different periods, 

 or to the structure of the parts, or to self-sterility, &c. ; 

 and so it is with many hermaphrodite animals, for 

 instance, land-snails or earth-worms ; but in all these 

 cases any one individual can fully fertilise or be ferti- 



* Asa Gray, ' American Joum. elsewhere as already refeiTed to, 

 cf Science,' 18fi5, p. 101 ; and Kuhn, ' Bot. Zeitung,' 1867, p. 67. 



