362 



GENERAL BOTANY 



be protandrom, while flowers in which the stigmas ripen before 



the anthers are said to be protogynous. It is evident that protan- 



drous and protogynous flowers are 

 necessarily either close-pollinating 

 or cross-pollinating. Homogamous 

 flowers are flowers in which the sta- 

 mens and pistils ripen together, thus 

 making self-pollination possible. 



It is evident that where flowers 

 are imperfect either close-pollination 

 or cross-pollination is insured, since 

 self-pollination would be impossi- 

 ble in such flowers. 



Heterostylous flowers. Flowers 

 in which the stamens and styles 

 are of different lengths are said 

 to be heterostylous. Heterostylous 

 flowers may be either dimorphic 

 (Fig. 219) (with two lengths of 

 stamens and pistils) or trimorpJiic 



(with three lengths of stamens and pistils). In either 'case 



each set of stamens matches one length of pistils, so that 



insects carry pollen from the anthers of one flower to stigmas 



of the same height 



in other flowers on 



the same or on a 



different plant, thus 



effecting either a 



close-pollination or a 



cross-pollination. 

 Odor, nectar, color, 



and movements. In 



flowers adapted to 



insect pollination, or 



entomophily, the insects are undoubtedly attracted to many 



flowers by their odor, nectar, or color, a fact which insures 



close-pollination and cross-pollination. The odor is due to the 



FIG. 218. Dichogamy in flowers 

 of Clerodendron 



a, the pistil is hent to one side away 

 from the ripe stamens in the young 

 flower; 6, in older flowers the sta- 

 mens wither and the stigmas are 

 exposed for the reception of pollen 



a 



A B 



FIG. 219. Dimorphic stamens and pistils in bluets 



A, form with long style ; 13, form with short style ; 

 a, anthers; s, stamens 



