148 BRITISH FLOWERING PLANTS chap. 



wings, the keel goes with them ; and when the weight 

 is removed, all three rise together again. 



As a good illustration of the general arrangement 

 of the flowers we may take the Sweet Pea, because, 

 though it is not an English species, it is so common a 

 garden flower, and is of a convenient size. When visit- 

 ing it for the sake of the honey, insects do not generally 

 alight directly on the keel, but rather on the wings, 

 which are more conveniently situated ; the two, however, 

 are relatively so arranged, that when a bee alights on 

 the wings it presses down, the keel, which is locked 

 with the two wings by a projection and corresponding 

 depression ; thus a portion of the pollen and also the 

 tip of the pistil are forced out at the point of the 

 keel, and against the breast of the bee, until on the 

 removal of the pressure the elasticity of the flower 

 causes the various organs to resume their former 

 position. 



The Leguminosse are, as a rule, adapted for fer- 

 tilisation by bees, though in some cases by Lepi- 

 doptera. 



As regards the mechanism of the flower, Delpino has 

 divided the Papilionacese into four divisions, which, 

 however, are connected by numerous intermediate 

 stages : — 



1. Those in which not only the pollen but also 

 some of the stamens are pressed out. The stamens and 

 the pistil remain outside the keel as long as the pressure 

 of the visiting bee lasts, but afterwards return imme- 

 diately to their former position. Such blossoms permit 

 several successful visits. 



(a) Honey is excreted : Melilotus, Trifolium, Galega, 

 Onobrychis, Astragalus, Oxytropis, Phaca, Ornithopus, 

 Hedysarum. 



(6) The sweet juice remains in the cellular tissue, 

 and must therefore be bored for : Cytisus (some 

 species of this genus show gradations towards 3 a). 



2. Explosive flowers : those in which the flower 

 bursts on pressure and ejects the pollen, the stamens 



