266 CONCLUDING KBMABKS Chap. VI. 



the anthers in size, and the pollen-grains in diameter. 

 It appears, therefore, at first sight probable that 

 organs which differ in such important respects could 

 act on one another only in some manner for which 

 they had been specially adapted. The probability of 

 this Tiew is supported by the curious rule that the 

 greater the difference in length between the pistils 

 and stamens of the trimorphio species of Lythrum and 

 Oxalis, the products of which are united for reproduc- 

 tion, by so much the greater is the infertility of the 

 union. The same rule applies to the two illegitimate 

 unions of some dimorphic species, namely. Primula 

 vulgaris and Pulmonaria angustifolia ; but it entirely 

 fails in other cases, as with Mottonia palustris and 

 Lirmm grcmdiflorvm. We shall, however, best perceive 

 the difficulty of understanding the nature and origin 

 of the co-adaptation between the reproductive organs 

 of the two forms of heterostyled plants, by consider- 

 ing the case of lAwum grandiflorum : the two forms of 

 this plant differ exclusively, as far as we can see, in 

 the length of their pistUs ; in the long-styled form, 

 the stamens equal the pistil in length, but their 

 pollen has no more effect on it than so much in- 

 organic dust ; whilst this poUen fully fertilises the 

 short pistU 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, Amsinckia speataMUs, 

 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 extraordinary degree 

 in the length of their pistils during successive seasons ; 

 nevertheless they yielded during these seasons exactly 



