252 EVOLUTION OF PLANTS 
stigma the pollen brought from it. In both Pelargo- 
nium and Tropeolum 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 Leguminosz, 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- 
