SEED-BEARING PLANTS 
469 
forming the standard, the two lateral petals are the 
"wings;" while the two lower petals adhere along their 
adjacent edges to form the keel. In these flowers there 
are usually ten stamens (rarely five), which commonly 
occur in two groups or "brotherhoods" (diadelphous) , 
nine in one group with their filaments united into a tube, 
cleft on the upper side, the other standing alone above the 
cleft. 
FIG. 353. Alsike, or Alsatian clover (Trifolium hybridum). Inflores- 
cences, showing carpotropic movements of the flowers after pollination by 
an insect. At the extreme left, flowers in bud, the outermost ones just 
beginning to open; next to the last, at the right, only one flower remaining 
erect on account of not having yet been pollinated; at the extreme right, 
every flower pollinated, turned down, and withering. 
Pollination is usually accomplished in this family by 
means of insects which visit the flowers for the nectar 
secreted by glands. In the case of white clover and 
alsike, each flower of the head, when pollinated, turns 
down, and the corolla becomes brown (Fig. 353). This 
change has been interpreted by some as a sign to the insects 
that the nectar has been taken, and therefore that another 
