RANUNCULA CEAE 



49 



a stalk about 15 mm. long, which follows the curve of the upper sepal, and then 

 gradually expands into a receptacle that is open below and provided with tapering 

 mouth-lobes, while its upper end is closed and dilated. The dilatation is bluish- 

 black externally, greenish internally, and its inner surface secretes so abundantly that 

 a large drop of nectar hangs in the narrowed neck of the receptacle. 



The numerous stamens lie at first in the entrance to the flower with their 

 immature anthers directed downwards. They then become erect as the anthers 

 dehisce, and are so placed that the pollen must dust the ventral surfaces of humble- 

 bees sucking nectar. During this first stage the 3-5 carpels are immature, and 

 are so closely surrounded by the stamens as to be completely covered. As the 



Geographical Distribution 



Bonibus 



Aconitum 



Fig. 18. Map showing the distribution of the genera Aconitum and Bonibus (after Kronfeld). 



stamens wither they bend downwards again, so that at the end of the first stage 

 the anthers of the anterior stamens are directed backwards, those of the lateral 

 stamens inwards, and those of the posterior stamens chiefly sideways. When all 

 the stamens have withered the stigmas become mature, and being now free from 

 their staminal investment occupy the entrance to the flower, so that humble-bees 

 dusted with pollen from a blossom in the first stage, necessarily touch the stigmas 

 and thus effect cross-pollination. Automatic self-pollination is thus normally excluded, 

 but it sometimes happens that one or two pollen-covered stamens have not bent 

 down again towards the base of the flower when the stigmas have matured, and 

 in such exceptional cases self-pollination may take place. 



Visitors. Those which act as pollinators are exclusively humble-bees. 

 Wherever I have observed this plant in gardens (Kiel, North Frisian Islands, 

 Mecklenburg, Rugen, Thuringia, and elsewhere), I have seen the garden humble-bee 



