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 



