BEES AS FERTILISERS. 299 



putting pollen from form A upon the same or another 

 flower of form A, that the best seed, and the largest 

 number per capsule, could only be obtained by cross- 

 ing, not only two flowers, but also the two forms, 

 which are naturally bound together by mutual depend- 

 ence. This leads to the inquiry, Does any ascer- 

 tainable difference exist between these pollens ? Micro- 

 scopic measurements show that the pollen grains of 

 form A (pg) only contain about one-third the material 

 of those of B, seeming to indicate, although Darwin 

 hazards no opinion here, that the larger grain is best 

 suited to forming the longer pollen tube required in 

 the long-styled form. By further experiment, the 

 existence of that which is commonly called prepotency 

 was proved — i.e., pollen placed on the stigma of the 

 flower form whence it had been derived, would be 

 rendered powerless by subsequently adding pollen from 

 the complementary blossom. 



Dimorphism (or double form) is more common than 

 even botanists, not long since, suspected, and amongst 

 dimorphic plants we find those of the highest utility to 

 the bee, because to many such the bee, or some other 

 insect, is a sine qua non. In the Linaceae, or flax 

 family, e.g., certain species are only capable of pro- 

 ducing seed at all when intercrossed. In Linum grandi- 

 florum (A, Fig. 62), the stigmas (s, s) and anthers (a, a) 

 are so placed, that intercrossing must be generally 

 brought about by bees reaching after the nectar 

 secreted, at five points, at the outside of the anther 

 bases; and experiment has fully shown, that if the pollen 

 of B be placed on its stigmas, or those of any other 

 similarly formed flower, not only is fertilisation not 



