252 EVOLUTION OF PLANTS 



stigma the pollen brought from it. In both Pelargo- 

 nium and Tropoeolum self-fertilization is impossible. 



The pea family offers many striking examples of 

 flowers which are entirely dependent upon insects for 

 pollination. The peculiar butterfly-shaped flowers of 

 most of these have the stamens and pistil enclosed in 

 the " keel " formed by the union of the two lower petals 

 (Fig. 56, G, &). The pollen is discharged and forms 

 a loose, powdery mass within the keel, but cannot reach 

 the stigma owing to the presence of a brush of hairs 

 between it and the stigma. If a bee alights upon the 

 flower, in searching for the honey the sides of the keel 

 are forced downward, and the apex of the pistil is ex- 

 posed, usually springing out with some force and brush- 

 ing out the pollen, which is thus dusted upon the visitor, 

 which carries it to the next flower, where it is deposited 

 upon the stigma. In some Leguminosae, like the species 

 of broom (Sarothamnus) (Fig. 56, G, H), the stamens 

 and pistil are set free with a good deal of violence, and 

 there is a small explosion when the keel is depressed, 

 and the pollen is ejected with considerable force. 



Certain parts of the flower may be sensitive to touch, 

 and this is almost always connected with pollination. 

 Thus in the common barberry the stamens are extremely 

 sensitive and on being touched near the base, as happens 

 when an insect is seeking for nectar, they spring in- 

 ward with a quick motion and deposit the pollen upon 

 the visitor. The trumpet-creeper (Tecoma), and other 

 related plants have the stigma sensitive, the two lobes 

 of which it is composed closing slowly after they are 

 touched. This is possibly a provision for holding the 

 pollen-grains deposited upon it, and perhaps hasten- 



