CHAP. VI. ON HETEROSTYLED PLANTS. 267 



of some dimorphic species, namely, Primula vulgaris 

 and Pulmonaria angustifolia; but it entirely fails in 

 other cases, as with Hottonia palustris and Linum 

 grandiflorum. We shall, however, best perceive the diffi- 

 culty of understanding the nature and origin of the 

 co-adaptation between the reproductive organs of the 

 two forms of heterostyled plants, by considering the case 

 of Linum grandiflorum: the two forms of this plant 

 differ exclusively, as far as we can see, in the length 

 of their pistils ; in the long-styled form, the ^stamens 

 equal the pistil in length, but their pollen has no more 

 effect on it than so much inorganic dust; whilst this 

 pollen fully fertilises the short pistil of the other form. 

 Now, it is scarcely credible that a mere difference 

 in the length of the pistil can make a wide difference 

 in its capacity for being fertilised. We can believe this 

 the less because with some plants, for instance, Am- 

 sinckia spectabilis, the pistil varies greatly in length 

 without affecting the fertility of the individuals which 

 are intercrossed. So again I observed that the same 

 plants of Primula veris and vulgaris differed to an ex- 

 traordinary degree in the length of their pistils during 

 successive seasons; nevertheless they yielded during 

 these seasons exactly the same average number of seeds 

 when left to fertilise themselves spontaneously under a 

 net. 



We must therefore look to the appearance of inner 

 or hidden constitutional differences between the indi- 

 viduals of a varying species, of such a nature that the 

 male element of one set is enabled to act efficiently 

 only on the female element of another set. We need 

 not doubt about the possibility of variations in the 

 constitution of the reproductive system of a plant, for 

 we know that some species vary so as to be completely 

 self -sterile or completely self -fertile, either in an appar- 



