46 THE FERTILISATION OF FLOWERS. [CHAP. 



general rule, where we find within the limits of one 

 genus some species which are much more conspicu- 

 ous than others, we may suspect that they are also 

 more dependent on the visits of insects." 



Among the plants with papilionaceous flowers there 

 is a remarkable arrangement of the petals and stamens 

 to ensure cross-fertilisation. In these flowers of 

 which the Pea, Bean, Lupin, Broom, and Laburnum 

 are examples the corolla consists of five petals, as 

 in fig. 52. The upper one (S) is called the " Standard,'' 



FIG. 52. FIG. 53. FIG. 54. FIG. 55. 



the two lateral ones (W) are called " Wings," and the 

 two lower ones (K) are united at their lower edges, 

 and form what is known as the " Keel." Within this 

 keel the stamens and pistil lie hidden, and the honey- 

 glands are also contained. To get at the honey the 

 insect has to alight on the wings which form a con- 

 venient platform for the purpose. Fig. 53 shows a 

 flower of the Lupin with one of these wings removed. 

 The wings are so locked to the keel that the weight 

 of the insect, whilst pressing down the wings, presses 

 down the keel also. From the curvature of the 

 stamens (fig. 55, An) this has the effect of forcing out 

 some of the pollen which has previously been shed 

 within the tip of the keel. Fig. 54 shows the wings 

 depressed and the pollen being forced out from the 

 tip of the keel against the bee's body. When the 



