REPRODUCTION AND DISPERSAL 



829 



ever, flowers possess conspicuous advantages, and these will now be 

 considered. 



The role of flowers and the essential organs involved. Pollination. 

 Pollination, that is, the transfer of pollen grains to the stigma (or to the 

 ovule in gymnosperms) is the chief activity 

 associated with flowers. When pollen is trans- 

 ferred from a flower of one plant to a flower of 

 another, the phenomenon is termed crass polli- 

 nation or xenogamy, and when pollen is trans- 

 ferred from tKe~anTHers to the stigma of the same 

 flower, it is termed close pollination or autogamy. 

 Geitonogamy, in which pollen is transferred from 

 one flower to another on the same plant, is 

 intermediate between xenogamy and autogamy, 

 and often is classed with the former, but in reality 

 it is much closer to the latter. In many species 

 autogamy is the only kind of pollination possible, 

 and in other species (probably a greater number) 

 only xenogamy is possible, but it is probable that 

 in the great majority of plants both autogamy (or 

 geitonogamy) and xenogamy are possible, though 

 usually it is believed that the latter is the more 

 advantageous. In all cases xenogamy is possible 

 only through the action of external agents, of 

 which wind and insects are the most important. 

 In geitonogamy and autogamy (especially the 

 latter) pollination may occur through the direct 

 contact of anther and stigma, but gravity, wind, 

 and insects often effect autogamy and geitonog- 

 amy as well as xenogamy; in some cases in- 

 sects are as necessary for autogamy as for 

 xenogamy (as in Yucca). 1 



The dehiscence of the anthers. When the 

 pollen grains are mature, the anther dehisces, 

 usually by longitudinal slits (fig. 1142), but sometimes by transverse 

 slits, by valves (fig. 1176), or by terminal pores (as in Solanum and 



FIG. 1141. A com- 

 pound raceme of Coleus, 

 the individual flowers 

 being arranged in paired 

 cymes; c, calyx; c' , the 

 sympetalous bilabiate co- 

 rolla, composed of an as- 

 cending upper lip (i>) and 

 a boat-shaped lower lip 

 (/) ; note the partially ex- 

 serted stamens (a) and 

 style (*); the developing 

 cymes are subtended by 

 caducous bracts (6). 



1 On this account the term self-pollination, if used, should be restricted to contact polli- 

 nation, rather than be made synonymous with autogamy in general. 



