40 PRIMROSE, LY THRUM. [CHAP. 



diameter against ytnnr> or nearly in the proportion of 

 three to two ; a difference, the importance of which 

 is probably due to the fact that each grain has to 

 give rise to a tube which penetrates the whole length 

 of the style, from the stigma to the base of the flower; 

 and the tube which penetrates the long-styled pistil 

 must therefore be nearly twice as long as in the 

 other. Mr. Darwin has shown that much more seed 

 is set, if pollen from the one form be placed on the 

 pistil of the other, than if the flower be fertilised by 

 pollen of the same form, even taken from a dif- 

 ferent plant. Nay, what is most remarkable, such 

 unions in Primula are more sterile than crosses 

 between some nearly allied, though distinct species 

 of plants. 



The majority of species of the genus Primula 

 appear to be dimorphous, but this is not the case. 

 (Scott, " Proc. Linn. Soc." vol. viii. 1 864, p. 80.) 



Mr. Darwin has since pointed out (" Jour. Linn. 

 Soc." 1863) that several species of Linum are di- 

 morphous, in the same manner as those of Primula; 

 and has shown that the existence of three forms in 

 Lythrum (Figs. 7780) already observed by Vaucher, 

 is to be explained in the same manner. I shall refer 

 to this case more in detail when we come to that 

 family. Nor are these by any means the only cases 

 of Heteromorphism now known. I have already 

 mentioned that of Oxalis, and Hildebrand gives the 

 following list of genera as containing Heteromorphous 

 species, viz., Hottonia, Primula, Linum, Lythrum, 

 Pulmonaria, Cinchona, Mitchella, Plantago, Rhamnus, 

 Amsinckia, Mertensia, Leucosmia, Drymospermum, 



