228 A BOOK OF INSECTS 



the visiting insect, as in the well-known birdVfoot trefoil 

 {Lotus corniculatm). The corolla consists of five petals. 

 The largest, which stands up at the back of the flower, 

 is called the standard. Below this are two smaller petals, 

 called wings, which overlap two more petals that are 

 united to form a flattened, tubular structure tapering to 

 a point, where the tube is open. This structure, known 

 as the keel, completely encloses the essential organs of 

 the flower. One stamen is free ; but the filaments of nine 

 are joined together and form a kind of trough within 

 which lies the ovary. The slender style lies between the 

 free ends of the stamens and extends beyond them, the 

 stigma being actually surrounded by the pollen which is 

 discharged by the anthers into the tip of the keel. It is 

 not receptive, however, until it is rubbed. Nectar is 

 secreted round the base of the ovary, and is accessible 

 only through openings at the base of the free stamen. 

 The flower is visited especially by bees, which alight upon 

 the wings. As these interlock with the keel, the weight 

 of the insect serves to depress the latter, and the rigid 

 stamens expel pollen from the orifice at its tip. The 

 stigma is also forced out, and rubs against the insect's 

 body at the spot where pollen is likely to have been de- 

 posited by another flower. Experiment has shown that 

 the pumping mechanism of the bird's-foot trefoil may be 

 worked by an insect eight times in succession before the 

 store of pollen is exhausted. 



In the foxglove tribe (Scropliulariacece) we find very 

 diverse adaptations. The foxglove itself (Digitalis pur- 

 purea) is exclusively cross - fertilised by humble-bees, 

 which alone are large enough to fill the tunnel-like tube 

 of the flower. Insects which are too small to touch the 

 anthers are kept at bay by a palisade of hairs across the 

 mouth of the corolla. Self-pollination is possible, how- 



