232 



Plant Life and Evolution 



cross-pollination, their flowers being often abso- 

 lutely sterile unless the proper insect visitors are 

 available. One of the simplest cases is seen in 

 the genus Orchis (Fig. 22). In Orchis, and the 

 same is true of very many other orchids, the 



FIG. 22 



A Flower of Orchis spectabilis; L, the lip ; gy, the column 

 or gynostemium. 



B A flower with the upper segments bent back so as to ex- 

 pose the column ; an, the anther ; d, adhesive disc of the pol- 

 linium; st, one of the stigmatic surfaces; sp, the spur; o, 

 ovary. 



C The two pollinia, adhering to a slender straw thrust into 

 the flower. 



D The same pair of pollinia, a few minutes later, showing 

 the change of position; if the straw is now thrust into the 

 flower, the pollinia will come in contact with the stigmatic 

 surfaces. 



pollen is aggregated in small masses, or pollinia. 

 These pollinia are held in little pockets, or recepta- 



