ADAPTATIONS FOE INTERCROSSING. 235 



Cinchona, sometimes only a part of them, as in Primula and 

 Linum. In Hottonia, a Primulaceous genus of two species, the 

 European one has heterogonous dimorphism * for cross-fertiliza- 

 tion : the American one has homogenous showy flowers with 

 only the general chance for intercrossing, and earlier flowers 

 which are cleistogamous for self-fertilization. 



425. The nature of heterogone dimorphism may be well under- 

 stood from a single example. The most familiar one is that of 

 Houstonia ; but, in larger blossoms, Gelsemium is a fine illus- 

 tration in the Southern United States, and Mitchella (Fig. 467) 

 mostly in the Northern. Raised from the seed, the individuals 



are about equally divided between the two forms : namely, one 

 form with long style and short or low-inserted stamens ; the 

 other with short style and long or high-inserted stamens. The 

 stigmas in one rise to about the same height as the stamens in 

 the other, both in the tall or exserted organs and in their low 

 and included counterparts, as is shown in Fig. 468, answering 

 to the left hand and Fig. 469 to the right hand flowers of Fig. 467. 

 A bee or other insect with proboscis of about the length of the 

 corolla-tube, visiting the blossoms of Mitchella, will brush the 

 same part of its body against the high anthers of the long- 

 stamened and the high stigmas of the long-styled forms ; and 



1 C. C. Sprengel, as Darwin mentions, had noticed this, before 1793. He, 

 " with his usual sagacity, adds that he does not believe the existence of 

 the two forms to be accidental, though he cannot explain their purpose." 

 Darwin, Forms of Flowers, 61. 



Some heterogonous Primulas are said to produce homogonous varieties hi 

 cultivation. In Primula, and in other genera, there are species which seem 

 as if of one sort only, no reciprocal sort being known, as if one form had 

 become self-fertile and the other had disappeared. 



FIG. 467. Partridge Berry, Mitchella repens, In the two forma, viz. long-stamened 

 and short-styled, and short-stamened and long-atyled. 



