CHAPTER XII. 



1 DIFFERENT FORMS OF FLOWERS ON PLANTS OF THE SAME 

 SPECIES.' 1877. 



[THE volume bearing the above title was published in 1877, 

 and was dedicated by the author to Professor Asa Gray, "as 

 a small tribute of respect and affection." It consists of cer- 

 tain earlier papers re-edited, with the addition of a quantity 

 of new matter. The subjects treated in the book are : 



(i.) Heterostyled Plants. 



(ii.) Polygamous, Dioecious, and Gynodioecious Plants. 



(iii.) Cleistogamic Flowers. 



The nature of heterostyled plants may be illustrated in the 

 primrose, one of the best known examples of the class. If a 

 number of primroses be gathered, it will be found that some 

 plants yield nothing but " pin-eyed " flowers, in which the 

 style (or organ for the transmission of the pollen to the ovule) 

 is long, while the others yield only " thrum-eyed " flowers with 

 short styles. Thus primroses are divided into two sets or 

 castes differing structurally from each other. My father 

 showed that they also differ sexually, and that in fact the bond 

 between the two castes more nearly resembles that between 

 separate sexes than any other known relationship. Thus for 

 example a long-styled primrose, though it can be fertilised by 

 its own pollen, is notfut/y fertile unless it is impregnated by 

 the pollen of a short-styled flower. Heterostyled plants are 

 comparable to hermaphrodite animals, such as snails, which 

 require the concourse of two individuals, although each pos- 

 sesses both the sexual elements. The difference is that in 

 the case of the primrose it is perfect fertility ', and not simply 



