THE ANGIOSPERMAE 1315 



2. Flowers with Partly Concealed Nectar 



It is obviously impossible to draw a hard and fast line between nectar 

 which is exposed and concealed nectar. The two states must be connected 

 by many intermediate gradations. The criterion upon which the present 

 group is distinguished is whether, on a bright sunny day, the nectar is 

 directly visible. Most of the flowers included here are actinomorphic, 

 opening completely on sunny days but closing in dull weather, there- 

 by hiding the nectar from view. White and yellow flowers predominate 

 also in this group, which includes members of such genera as Ranunculus, 

 Caltha, Berberis, Anagallis, Potentilla, Sanguisorba and many members of 

 the Cruciferae. 



The insect visitors belong to quite different groups to those mentioned 

 in the preceding section. The flowers are often visited by the Honey Bee 

 and other species of medium-tongued insects, but the type of visitor is to 

 some extent dependent upon the local insect flora. For example, in the 

 Alps, where members of the Lepidoptera are common, they often pollinate 

 flowers with partly concealed nectar. On the other hand in areas where 

 flowers more suited to these insects abound, the butterflies tend to leave 

 these flowers alone, since they are not ideally shaped for their use. 



We may regard flowers of this type as being at a higher stage in develop- 

 ment than the last, for they represent a step towards specialization to a 

 restricted type of insect visitor. A few purple flowers belong to this group, 

 e.g., Sanguisorba officinalis and Comarum pahistre; they are almost entirely 

 visited by flies. 



As an example of the group we may consider the flowers of Berberis 

 vulgaris (Fig. 1234), not only because they have partly concealed nectar, but 

 also because of the movable stamens which are characteristic of the genus. 



Berberis vulgaris is an uncommon British wild shrub, though it and 

 many other species and varieties of the genus are frequently grown in 

 gardens. Individually the yellow flowers are small and not very showy, but 

 they are aggregated together into racemes which are very conspicuous. Each 

 flower has a perianth made up of a number of whorls, each consisting of 

 three members. The two outer whorls are usually regarded as representing 

 the perianth proper, while the inner segments are referred to as the " honey 

 leaves " and bear the nectaries at their bases. These nectaries take the form 

 of a pair of thick, orange-coloured bodies lying at the base of each honey 

 leaf. Standing immediately against these nectaries are the filaments of the 

 stamens, so placed that nectar collects around the foot of each filament. The 

 filament bases are sensitive to touch on the inner side and are readily 

 stimulated by an insect in search of nectar. The central stigma is a 

 rather massive discoid body with a receptive margin. An insect probing 

 for nectar stimulates a sensitive filament, which contracts causing the 

 anther to strike inwards and discharge its pollen against the side of the 

 insect's head. This movement of the stamen usually disturbs the insect, 

 which immediately leaves the flower. If it visits a new flower and presents 



