THE FLO WEB 



121 



another by the water, or water and wind together ; the 

 staminate flowers of the fresh-water Eel-grass, for instance, 

 after being detached from the submerged heads, are driven 

 like minute rafts before the wind, and collect about the 

 much larger pistillate flowers on the surface.^ 



237. A few species of plants are regularly cross-polli- 

 nated by snails, and 

 others by birds. 



238. Pollination by 

 insects. — Cross-fertili- 

 zation in flowering 

 plants is brought about 

 by aid of insects far more 

 frequently than by all 

 other agencies combined; 

 A few cases will be de- 

 scribed in some detail. 



239. Lady^s Slipper 

 (^Cypripediuni) and the 

 South American Seleni- 

 pedium^ Fig. 168, show 

 a very perfect mode of 

 compelling the insects 

 that visit them to serve 

 as pollen bearers. One 

 of the petals is shaped 

 into a sac, or labellum, 



open above and on either 168. Flower of South American Seleni- 



jjedium Schlunii. The dotted lines 

 with arrow tips show the course fol- 

 lowed by a visiting bee. In b, the 

 flower is seen from the side, the 

 labellum, or saccate petal, being cut 

 open ; p, a pollen mass ; s, the stigma ; 

 e, exits. 



side near the base (e). 

 The bee alighting on 



this labellum in search 



of the honey secreted by 



glandular hairs within, 



and entering through the main opening, is prevented by 



the incurved edges of the latter, as well as by the depth 



of the labellum, from escaping except by one of the two 



1 See Kerner and Oliver, "Natural History of Plants," Vol. II., p. 182. 



