432 



POLLINATION OF FLOWERS 



407. Flowers with stamens and pistils each of two lengths. 



The flowers of bluets, partridge berry, the primroses, and a few 

 other common plants secure cross pollination by having stamens 

 and pistils of two forms (Fig. 333). Such flowers are said to be 

 dimorphous (of two forms). In the short-styled flowers, B, the 

 anthers are borne at the top of the corolla tube and the stigma 

 stands about halfway up the tube. In the long-styled flowers, 

 A, the stigma is at the top of the tube and the anthers are borne 



about halfway up. An insect 

 pressing its head into the throat 

 of the corolla of B would be- 

 come dusted with pollen, which 

 would be brushed off on the 

 stigma of a flower like A. On 

 leaving a long-styled flower the 

 bee's tongue would be dusted 

 over with pollen, some of which 

 might readily be rubbed off on 

 the stigma of the next short- 

 styled flower that was visited. 

 Cross pollination is insured, 

 since all the flowers on a plant 

 are of one kind, either long- 

 styled or short-styled, and since 

 the pollen is of two sorts, each 

 kind sterile on the stigma of any flower of similar form to 

 that from which it came. 



Trimorphous flowers, with long, medium, and short styles, 

 are found in a species of loosestrife and in the pickerel weed 

 (Pontederia). 



408. Cleistogamous flowers. In marked contrast with such 

 flowers as those discussed in the preceding sections, which bid 

 for insect visitors or expose their pollen to be blown about 

 by the wind, are certain flowers which remain closed even 

 during the pollination of the stigma. These flowers are called 





FIG. 333. Dimorphous flowers of 

 the primrose 



A, a long-styled flower; B, a short- 

 styled one. After Frank 



