IV.] BROOM, MEDICAGO. 87 



keel. They also lock at the base with the keel, and 

 when they are pressed downwards the flower bursts 

 open. 



In the Laburnum, the tip of the pistil is protected 

 from its own pollen by a ring of close hairs ; when, 

 however, the pistil has grown to the very point of 

 the keel, these hairs shrivel and turn outwards, so as 

 to expose the tip of the pistil, which thus comes in 

 direct contact with the breast of any bee which may 

 alight on the flower. 



In the Broom {Sarothammis scoparius) the flowers 

 also explode. If, however^ the- bee alights on a newly- 

 opened blossom, the shorteE stamens only emerge 

 and dust the abdomen of the insect. If, on the con- 

 trary, the flower is a day or two> older, the pistil and 

 longer stamens spring out, and the hairs on the pistil 

 deposit pollen on the bee's back. The pistil gradually 

 curls, and the stigmatic surface turns up, so as to 

 stand close to the anthers of the shorter stamens. 

 In this position it is so placed that it would come in. 

 contact with the abdomen of the bee. " Thus," says 

 Mr. Darwin, " both the upper and lower surface of 

 the bee get dusted with pollen, which will be trans- 

 ferred to the stigma at two different periods." {Linn. 

 Jour. V. ix. p. 358.) 



In Medicago sativa, as in Genista and the. Broom, 

 the flowers open once for all ; but the elastic power 

 is confined to the upper stamens. In the Broom and 

 in Genista, the resistance is obtained by the union of 

 the upper edges of the keel. These are also united 

 in Medicago, but even if they are separated no ex- 

 plosion takes place; the flower being still locked 



