THE ANGIOSPERMAE 1313 



much longer, greenish in colour and are neglected by the visiting insects. 

 These stamens, however, have anthers which also open by longitudinal 

 slits and discharge pollen on to the body of the visiting insect. These long 

 stamens vary in position, they may project either to the right or to the left 

 and since both types occur on the same plant they afford a peculiar case of 

 dimorphism. In either case, when the insect visits another flower the pollen 

 discharged on its body will be dusted on to the stigma if the flower is in a 

 later stage of development, for the flowers exhibit a marked protandry. 



(b) Flowers sought after for Nectar 



It is possible to trace a series of stages in the evolution of nectar-secreting 

 flowers from simple types in which the nectar is fully exposed to those in 

 which the nectar is so concealed that it can be reached only by insects of a 

 particular shape. In this way the variety of insect visitors can be limited 

 and the chances of cross-pollination correspondingly increased. Though 

 the development of nectar is probably a stage in the change from anemo- 

 phily to entomophily, it must not be assumed that nectar-less flowers 

 necessarily represent a primitive condition. Flowers such as we have just 

 been considering, in which pollen is offered to the insect, may be specialized 

 and derived from nectar-secreting types. There is some evidence in 

 Verbasciim, for example, that those species in which no nectar is present and 

 pollen provides the attraction, may have been derived from nectar-secreting 

 forms. Even false, i.e., non-excreting nectaries, may not always be primitive, 

 but may in some cases have been reduced from active nectaries, the pre- 

 sumption being that nectar production as a physiological process antedated 

 the development of special nectary structures, 



I. Flowers zcitli Exposed Nectar 



Flowers in which nectar is freely exposed are usually actinomorphic; 

 they are simple, open flowers, generally white, greenish white or yellow in 

 colour. Attraction may be achieved by the individual flowers or by their 

 aggregation into conspicuous inflorescence, as for example in the Umbelli- 

 ferae, which are all in this class. Other examples of simple flowers with 

 exposed nectar are Ilex aquifoliiim, Galium verum, Acer campestre, Euoriymus 

 europaeus, Alchemilla vulgaris, Chrysosplenium oppositifolium and various 

 species of Cotoneaster, Saxifraga and Euphorbia. 



According to the position of the nectar, the flowers may be visited by 

 any insect with a proboscis of suitable type. The quantity of nectar is 

 usually small and such flowers are often neglected by long-tongued insects 

 such as the Hive Bee, who can make use of more specialized flowers. Fur- 

 thermore the shape of the flowers is unsuited to large insects such as butter- 

 flies, which rarely visit them, even in the regions where these insects are 

 common. 



Some flowers with dull yellow petals but fully exposed nectar are visited 

 only by flies and beetles who can suck nectar from a thin layer, but the 

 principal visitors appear to vary in different parts of the world. For example, 



