TBE FLOWER 



121 



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

 staniinate 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 combine(h 

 A few cases will be de- 

 scribed in some detail. 



239. Lady's Slipper 

 (^Cypripediurii) and the 

 South American Seleni- 

 pediimi. Fig. 168, show 

 a very perfect mode of 

 compelling the insects 

 that visit them to serve 

 as pollen bearers. One 

 of the petals is sliaped 

 into a sac, or labellum, 

 open above and on either 

 side near the base (e). 

 Tlie bee alighting on 

 this labellum in search 

 of the honey secreted by 

 glandular hairs within, 

 and entering through the main oiDening, 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 



168. 



Flower ot South American Seleni- 

 pedium Schliinii. The dotted lines 

 ■n'ith arrow tips sliow the course fol- 

 lowed hy a visiting bee. In h, the 

 flower is seen from the side, the 

 labellum, or saccate petal, being cut 

 open ; p, a i^ollen mass ; s, the stigma ; 

 e, exits. 



1 See Kerner and Oliver, "Natural History of Plants," Vol. II., p. 1,32. 



