REPRODUCTION BY SEED 



171 



so much honey in the tube that it is sometimes half-full r 

 when it can just be reached, though with difficulty, by 

 the humble-bee. The only effective visitor to the large- 

 flowered climbing convolvulus Convolvulus sepium is 

 the hawk-moth. In many highly specialized flowers 

 heavy structures have to be pushed aside before the 

 honey can be obtained e.g., snapdragon, calceolaria, 

 gorse, broom. Mock glands occur in some flowers e.g., 

 grass of Parnassus. Sham nectaries, in the form of little 

 green knobs, are found at the base of the corolla-lobes 

 in the woody nightshade. 



Some flowers have very 

 few visitors. This is either 

 because they are so 

 specialized that very few 

 insects can get at the 

 honey, or because they 

 are disagreeable to all but 

 one or two insects. In 

 extreme cases, flowers, 

 though seemingly well 

 adapted to insects, seldom 

 get a visitor, and have to 

 rely upon their own pollen. 

 The violet is a case in 

 point. By scent, form, 

 and honey it seems a 

 plant flaunting everything 

 that could attract insects ; 

 but few come, and seeds 

 are usually formed by self- 

 fertilization in closed buds. 

 Other flowers seem to be 

 adapted to one insect, and no other. They are probably 

 disagreeable to other insects. For example, Lysimachia 

 vulgaris, the yellow loosestrife, is only visited by one 

 insect-' a bee (Macropis labiata) ; Trifolium repens, the 

 white clover, relies on the humble-bee ; and the white 

 bryony (Bryonia dioica) is pollinated exclusively by the 

 bee Andrena florea. 



Since the position of the nectaries determines so 

 largely the kind of insects visiting the flowers for 

 food, we may roughly divide entomophilous flowers 



FIG. 71. MONK'S - HOOD : LONGI- 

 TUDINAL SECTION OF FLOWER. 



a, honey-leaf ; b, carpels ; c, stamens ; 

 d, sepals ; e, receptacle. 



