32 COWSLIP AND PRIMROSE. [CHAP 



The two forms differ also in some other respects. In 

 the long-styled form, the stigma (st) is globular and 

 rough, while that of the short-styled is smoother, and 

 somewhat depressed. These differences, however, are 

 not sufficiently conspicuous to be shown in the figure. 

 Again, the pollen of the long-styled form is consider- 

 ably smaller than the other, a difference, the import- 

 ance of which is obvious, for each pollen 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 one has therefore to produce a tube 

 nearly twice as long as that of the other. The 

 careful experiments made by Mr. Darwin have shown 

 that, to obtain the largest quantity of seed, the flowers 

 must be fertilised by pollen from the other form. 

 Nay, in some cases, the flowers produce more seed, if 

 fertilised by pollen from another species, than by 

 that from the other form of their own. 



This curious difference in the Primrose and Cowslip, 

 between flowers of the same species, which Mr. Dar- 

 win has proposed to call Dimorphism, is found in most 

 species of the genus Primula, but not in all. 



The Cowslip and Primrose resemble one another in 

 many respects, but the honey they secrete must be 

 very different, for while the Cowslip is habitually visited 

 during the day by humble-bees, this is not the case 

 with the Primrose, which, in Mr. Darwin's opinion, is 

 fertilised almost exclusively by moths. 



The genus Lythrum affords a still more complex 

 case, for here we have three sets of flowers. The 

 stamens are in two groups ; in some plants, the pistil 

 projects beyond them ; in the second form it is shorter 



