THE DICOTYLEDONES 



1 67 1 



minor differences in the mechanism in various genera but we may describe 

 that of Genista as typical (Fig. 1528). 



Fig. 1528. — Genista anglico. Longitudinal sections of flowers illustrating 



pollination. 



The yellow flowers are arranged in racemes and are devoid of nectar 

 and of nectar guides. The two pentamerous whorls of stamens and the 

 projecting style are closely surrounded by the keel. While still in the bud 

 the four upper stamens of the outer whorl dehisce and their pollen is pushed 

 towards the lips of the keel by the elongation of the five inner stamens. The 

 filaments then wither. As soon as the standard expands the remaining six 

 stamens discharge their pollen so that now the upper part of the keel 

 encloses all the pollen and the lower part of the style. The style together 

 with the staminal tube makes a compressed spring exerting upward pressure, 

 while the claws of the keel petals and the interlocking alae form a second 

 spring which presses downwards. These two springs operate against one 

 another and the parts remain in equilibrium until the upper edges of the 

 keel petals are separated. The wing petals interlock with angular pro- 

 jections from the two keel petals and it follows that when a bee settles on 

 the flower with the legs on the wing petals and thrusts its head under the 

 standard, the wing petals are pressed down and slip off the keel while the 

 two keel petals open out and release the style and stamens. This sudden 

 movement presses the pollen on to the lower surface of the insect, which is 

 also touched by the style simultaneously. If the insect has previously 

 visited another flower, cross-pollination occurs. If cross-pollination is not 

 effected, self-pollination is ensured as the insect backs out of the flower, 

 since the stigma will be dusted with the pollen that the insect has received 

 on its ventral surface. This explosive mechanism can apparently be 

 brought into action only by the application of external pressure. 



In the Trifolioideae, a small sub-family of only six genera, five occur 

 in Britain. One of these, the genus Ononis, is essentially a Mediterranean 

 one. It contains about seventy species. Trigonella, with the same number 



