THE FL O WER PER TILIZA TION AND POLLINA TION 1 47 



some of the anthers mature simultaneously with the pistils, 

 so that there is chance for self-pollination in case for- 

 eign pollen does 

 not arrive. Even 

 when the stigma 

 receives pollen 

 from its own 

 flower, cross-fer- 

 tilization may 

 result. The hol- 

 lyhock is proter- 

 androus. Fig. 

 198 shows a 

 flower recently F IG. 198. FLOWER OF HOLLYHOCK; proterandrous. 



expanded. The center is occupied by the column of sta- 

 mens. In Fig. 199, showing an older flower, the long 

 styles are conspicuous. 



Some flowers are so constructed as to prohibit self-polli- 

 nation. Very irregular flowers are usually of this kind. 



With some of them, 

 the petals form a 

 sac to inclose the 

 anthers and the pol- 

 len cannot be shed 

 on the stigma but is 

 retained until a bee 

 forces the sac open ; 

 the pollen is rubbed 

 on the hairs of the 



bee and transported. 

 FIG. 199. OLDER FLOWER OF HOLLYHOCK. -r, , a 



Regular flowers usu- 

 ally depend mostly on dichogamy and the selective power 

 of the pistil to insure crossing. Flowers that are very 



