THE DICOTYLEDOXES 1923 



only insect which normally visits the flower (Fig. 1849). The style and 

 stamens lie against the upper side of the corolla tube. The two long stamens 

 dehisce first, followed by the shorter ones, and it is only after the latter have 



Fig. 1849. — Digitalis purpurea. Flowers in section, to 

 illustrate pollination. See in text. 



completely liberated their pollen that the stigma becomes receptive. Should 

 the flower not be visited by an insect, self-pollination is possible, for as 

 the stigmatic lobes diverge they come into contact with the stamens. 



Most of the other genera in this sub-family are visited by humble bees 

 and their flowers are adapted to receive them. There are however excep- 

 tions. The alpine Rhinanthus angustifoUus is specially modified for butterfly- 

 pollination. The nectar is secreted at the base of the ovary and stored in the 

 corolla tube. To reach this nectar the insect must thrust its proboscis into 

 the flower and in so doing the head is dusted from above by the anthers 

 which lie protected from rain beneath the upper lip of the corolla. As 

 compared with other species of Rhinanthus, the entrance is too narrow for 

 the proboscis of any but butterflies to penetrate. In other species a wider 

 opening is formed by the upper lip which thereby admits both humble 

 bees and butterflies. The genus Euphrasia (Fig. 1850) is mostly visited 

 by hive bees, the flower tube being considerably shorter than in most 

 other genera. 



