MORE ABOUT POLLINATION 



319 



F. Prevention of Close-pollination. — Some plants with 

 monoclinous flowers possess devices which prevent close- 

 pollination; for diclinous flowers, of course, close-polH- 

 nation is impossible anyhow. You have already noted 

 (see page 306) that in some monocUnous flowers the pollen 

 will not germinate upon the stigma of the same flower; 

 this is one such device. 



Another thing which prevents close-pollination is what 

 is called dichogamy. This is the ripening of the stamens 

 and the pistils of the same 

 flower at different times. 

 Many flowers have this habit. 

 Among wind-pollinated plants, 

 the common plantain furnishes 

 an example of it. (See Figure 

 iig.) Among those which are 

 insect-polHnated, the figwort 

 {Scrophularia) is one of a 

 number of common dichoga- 

 mous forms. (See Figure ijg.) 

 Sometimes the stigmas ripen 

 before the anthers, and some- 

 times the reverse is true. 



Some plants have two kinds 

 of monocKnous flowers ; in one 

 the anthers are above the 

 stigma, in the other the stigma is above the anthers. This 

 is called dimorphism. (See Figure 140.) The primrose, 

 the forget-me-not, and the bluets have such flowers. That 

 part of the insect which brushes the higher anthers will 

 later come in contact with a high stigma in another flower, 

 while that part which is dusted by low anthers will prob- 



A 



Fig. 139. — Flowers of the figwort 

 {Scrophularia) , illustrating dichog- 

 amy. , In A the imripe anthers are 

 not yet visible, while the ripe 

 stigma is prominently exposed. In 

 B, which is older, the anthers can 

 be seen, and the top of the old 

 style has drooped down, out of the 

 way of visiting insects. 



