26o ANGIOSPERMAE—DICOTYLEDONES 



to attract insects. Their power of attraction is often enhanced by more or less 

 marked fragrance. The gamosepalous calyx holds the petals erect, in a position 

 adapted to the insect visitors. The vexillum serves in the bud to cover the inner 

 parts of the flower. In the open blossom it is erect and acts as a signboard ; 

 it often possesses nectar-guides in the form of colour-streaks. The vexillum is also 

 used as a support by bees, which press their heads against it as they suck. The 

 alae have a threefold function. — i. They serve as a resting-place for bees; 2. They 

 act as levers, depressing the carina, so that the stigma and pollen protrude during 

 insect-visits, and are brought into contact with the under-sides of the visitors 

 (bees) ; 3. They hold the carina in place as regards stamens and pistils, and 

 after the departure of a visitor help these parts to regain their original position. 

 The carina is a protective structure, sheltering the stamens and pistil from rain, 

 and keeping away unbidden guests (Lepidoptera and flies). When all ten filaments 

 cohere, the flowers afford only pollen, but when the upper one is free, there is a 

 slit on either side of it, leading to the nectar secreted inside the bases of the 

 stamens. The closed or split cyhnder formed by the filaments envelops the pistil, 

 of which the style is usually upwardly curved at the tip, and projects somewhat 

 beyond the anthers, so that the stigma first protrudes from the carina when an 

 insect visits the flower, and first touches its under-side. Cross-pollination therefore 

 results if the insect has previously visited another flower of the same species. In 

 some species the stigma is completely covered by the pollen of the same flower, 

 but self-fertilization does not usually follow, for it only becomes receptive after 

 visitors have rubbed against its papillae. 



In our native Papilionaceae Delpino distinguishes four types of flower 

 mechanism connected by transitional forms. — 



1. Simple valvular arrangement. Stamens and pistil project from the carina 

 as long as the pressure of the bee continues, and then return to their former position. 

 Such flowers permit of a number of effective visits. 



(a) Nectar : Melilotus, Trifolium, Galega, Onobrychis, Astragalus, Oxytropis, 



Phaca, Ornithopus, Hedysarum. 



(b) Enclosed sap, reached by boring : Cytisus (some of the species are 



transitional to 3 (a) ). 



2. Explosive arrangement. Stamens and pistil suddenly spring out of the 

 carina. Such flowers permit of only one effective visit. 



(a) Nectar present : Medicago. 



(b) No nectar. 



(a) The ventral surface of the bee comes into contact with the pollen and 



stigma : Genista, Ulex. 

 (0) The bee is struck on the back by the pollen and stigma : Sarothamnus. 



3. Pump arrangement. The thickened ends of the filaments press out the 

 pollen in successive portions from the tip of the carina. Several insect-visits are 

 necessary for pollination. 



(a) Nectar present : Lotus, Anthyllis, Tetragonolobus, Hippocrepis. 



(b) No nectar : Ononis, Lupinus, Coronilla. 



