II.] PRIMROSE. LYTHRUM. 35 



part further from the tip ; which, when the iasect sub- 

 sequently visited a long-styled flower, would again 

 come just opposite to the head of the pistil. Hence 

 we see that by this beautiful arrangement, insects 

 must carry the pollen of the long-styled form to the 

 short-styled, and vice versd. 



There are other points in which the two forms differ 

 from one another ; for instance, the stigma of the long- 

 styled form is globular and rough, while that of the 

 short-styled is smoother, and somewhat depressed. 

 The pollen of the two forms (Figs. 38 and 39) is 

 also dissimilar ; that of the long-styled being con- 

 siderably smaller than the other — 7-5^5- of an inch in 

 diameter against \^^, 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. v. viii. 1864, p. 80.) 



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



D 2 



