ADJUSTMENT TO TEMPERATURE 



125 



autogamy. This fact explains the large number of allo-autogamous 

 plants. 



147. Cross-pollination. The benefits arising from the transfer 

 of pollen from one flower to another, as well as the necessity for 

 su-^h a transfer in diclinic species, have led to the production of 

 numerous devices for bringing it about. These have been developed 



Fig. 32. Geitonogamy in an aster, Machwranthera aspera, in which pollina- 

 tion regularly occurs Ijetween neighboring flowers or heads. 



in relation to one of three factors, water, wind, and animals, 

 especially insects. With reference to the agent concerned, cross- 

 pollinated species are accordingly termed hydrophilous, anemophi- 

 lous, or zoophilous. Hydrophilous species may be pollinated 

 under water, as in Zostera or Ceratophyllum, or more often the 

 pollen is borne on the surface of the water, as in Ruppia, Calli- 



