TAKING UP OF POLLEN BY INSECTS. 



249 



The same mechanism exists in so-called "revolver-flowers," i.e. flowers which 

 exhibit within their outer portals the open ends of a number of small tubes resem- 

 bling the barrels of a revolver. These tubes are arranged in a great variety of ways. 

 In Bindweeds and Gentians (Convolvulus, Gentiana) the filaments are adnate to 

 the corolla-tube and project in the form of ridges towards the central ovary, and 

 so divide the main tube into four or five separate pipes. In some Geraniums and 

 several species of Flax (e.g. Geranium Rohertianum, Linum viscosum), a ridge 

 arises from the middle of each petal and projects towards the centre of the corolla, 



Fig. 267.— Contrivances for ensuring the deposition of pollen on insect-visitors. 



1 Flower of the European species of Lady's Slipper (Cypripedium Calceolus). A bee (Andrena) is forcing its way out through 

 one of the holes at the side of the stigma niid smearing its shoulder with pollen. 2 Longitudinal section through the 

 labellum and column of the Lady's Slipper. 3 An Andrena on the wing. < Flower of Grass of Parnassus {Pamassia 

 palustris) with the front sepals, petals, nectaries, and stamens cut away; of the stamens which are visible three are 

 despoiled of their anthers, the fourth has assumed a position placing the anther in the middle of the flower. * Single 

 nectariferous scale from the flower of Pamassia. « Flower of Malcolmia maritirna with the front sepal, the two front 

 petals, and two stamens cut away; the visible portion of the ovary bearing a longitudinal row of stiff bristles (white). 

 ' Longitudinal section through the flower of Leonurus heterophyllus. « Flower of Eernera saxatilis in the first stage of 

 developinent, seen from above, s I'he same at a later stage of development, seen from above, lo The same flower, seen 

 from the side, with the front sepal and the two front petals removed. " Vertical section through a flower of Trollius 

 Europceus (Globe-flower), i and 2 nat. size ; the rest x 2-8. 



dividing it into five tubes, which surround the styles and stamens in a circle. In 

 Flaxes of the group Catltartolinum each petal is swollen in the middle and 

 attenuated, and concave near the edges; the juncture of the concave margins of 

 adjacent petals gives rise to a flat canal which leads to the floral receptacle. The 

 corolla-tube of the Winter Cherry (Physalis) is fluted by five grooves, which, 

 together with the villous filaments facing them, form as many tubes. In all these 



