174 THE BOOK OF NATURE STUDY 



To complete the description of the flower it must be pointed 

 out that the receptacle is widened out to form a shallow cup 

 (cf. p. 106). Nectar is secreted around the base of the ovary, and 

 is concealed within the tube formed by the stamens. Entrance 

 to the nectary can only be obtained to either side of the base 

 of the free stamen. 



It might at first appear as if self-pollination were a necessary 

 result of the stigma being surrounded by pollen in the tip of the 

 keel. The stigma is not receptive, however, until it is rubbed, so 

 that it remains unfertilised while the flower is opening and 

 becoming ready for the visits of insects. The flower is visited 

 especially by bees. These come in search of the nectar, to which 

 they are guided by the converging red lines on the standard. 

 The bee alights on the wings, and the weight of its body is trans- 

 mitted to the keel owing to the interlocking of the petals. The 

 keel will thus be depressed, its open tip resting against the under 

 side of the body of the visiting insect. This depression does not 

 extend to the stamens, so that the keel is forced down over the 

 group of filaments, which fill its cavity as the piston of a syringe 

 fills the cylinder. The effect of this is to force some of the 

 pollen from the tip of the keel, and this pollen will be deposited 

 on the region of the under surface of the insect. When 

 the weight of the insect is removed the keel rises up and 

 the mechanism is ready to work in the same way on another 

 insect coming to the flower. When the keel is sufficiently 

 depressed the stigma emerges and will rub against the same 

 spot on the insect where pollen may have been deposited 

 from a flower previously visited. The whole arrangement, in 

 the study of which Fig. 82 will be found of great assistance, 

 is beautifully adapted to render cross-pollination likely if the 

 flowers are visited by insects. There is some reason to think 

 that in the absence of the latter the flower's own pollen is 

 effective. 



The fruit of the Bird's-Foot Trefoil is, when ripe, a straight 

 brown pod. This has the calyx at the base, but the other parts 

 of the flower have fallen away. The pod contains a number of 

 smooth round seeds like small peas, and opens by the two valves 

 splitting completely apart. 



