46 THE FERTILISATION OF FLOWERS. [cuHapP. 
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, Bear, Lupin, Broom, and Laburnum 
are examples—the corolla consists of five petals, as 
in fig. 52. . The upper one (S) is called the “‘ Standard,” 
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 
