POLLINATION AND FERTILIZATION 



145 



TIG. 128. Flower of 



potato, with widely 



expanded corolla 



138. The periwinkle ; a flower with concealed nectar. The 

 common periwinkle, 1 a familiar old-fashioned flower, is an ex- 

 cellent illustration of one way in which nectar is concealed 

 and protected from undesirable insect visi- 

 tors. The tube of the corolla is moderately 

 long and is partly closed by a sort of disk- 

 shaped enlargement of the style (fig. 129). 

 Part of the under surface of the disk does 

 the work of a stigma. 



The disk is surrounded by gummy mate- 

 rial and bears a crown of hairs at the top. 

 The anthers open inward and so fill the crown of hairs with 

 pollen. The long, slender tongue of an insect visitor (fig. 130), 

 in being thrust through the fringing hairs and down the tube 

 in search of nectar at its base, becomes covered with pollen. 

 In this way some of it will be 

 left on the stigma of the next 

 periwinkle flower visited, which 

 will secure cross-fertilization. 



Many other instances of con- 

 cealment of the nectar supply 

 can be discovered by the observ- 

 ing student. One of the most 

 obvious is in such flowers as 

 snapdragon and butter-and-eggs, 

 in which the two-lipped corolla 

 is rather firmly closed, so that 



it can only be pried open by a with the nectar not accessible to 



most small insects 

 moderately strong insect. 



There is a large class of 

 flowers in which the nectar is 

 not so much concealed as out 



of the reach of ordinary insects, since it is at the bottom of 

 a long and narrow corolla tube or in a slender spur of the 

 corolla. Excellent instances of this are found in the flowers 



i Vinca minor. 



FIG. 129. Lengthwise section of 

 flower of periwinkle (Vinca minor), 

 the corolla with a closed throat and 



s, disk-shaped expansion of the style, 

 stigmatic on its lower surface ; n, nec- 

 tar glands at the base of the ovary 



