THE ANGIOSPERMAE 1309 



may alight before entering the flower. Examples of this are found in many 

 of the Labiatae, Aristolochia, the Orchids and some of the Scrophulariaceae. 

 The arrangement of the wing and keel petals in Papilionaceae also affords a 

 seat, but since the keel is often movable it also serves as a pollination 

 mechanism, for the depression of the keel may at the same time cause the 

 release of pollen which strikes the insect. Some flowers on the other hand 

 provide no alighting platform and the visiting insect is forced to hang on to 

 the most exposed structures, possibly the stamens, thereby receiving pollen 

 during the act of ahghting on the flower. An example of this is seen in 

 species of Veronica. The capitula of the Compositae and the flat, actino- 

 morphic flowers of various species provide an open surface on which the 

 insect may settle. Where this surface consists of a single flower the insect 

 may receive pollen or may pollinate the stigma according to the stage of 

 floral development. In the capitulum or in the umbellate type of inflores- 

 cence, an insect crawling over it in search of nectar will visit one flower after 

 another and may in this way pollinate a number of separate flow^ers. 



Pendent, bell-shaped flowers like those of Fritillaria and Soldanella are 

 unattractive to insects which hover, but they are accessible to Humble Bees 

 which can climb up the stamens to reach the nectar. 



Floral shape may not only assist the desirable insect to perform pollina- 

 tion, it may in many instances protect the nectaries from thieving by useless 

 or unwanted visitors. Constrictions or the development of stiff hairs may 

 prelude the entry of crawling insects. Tiny flies may fail to reach to the 

 bottom of the floral tube, for viscid substances may prevent their movement. 

 Hairs may be developed either on the inside of the petals or on the filaments 

 of the stamens. They may be permanent or may wither at some period in 

 the floral development, thus opening a passage to the inside of the flower 

 which they had previously closed. 



The robbing of nectar by insects, especially bees who eat through the 

 corolla tube in the neighbourhood of the nectaries and sup the nectar w ith- 

 out entering the flower, is sometimes prevented by the development of a 

 large baggy calyx. Similarly the collecting of water by funnel-shaped 

 calyces will preclude crawling insects from reaching the flower. A curious 

 example of such protection against robbery is offered by the Orchid, 

 Coeloglossiim viride, in which the entrance to the spur is closed by a membrane 

 which must be pierced by the proboscis of the insect visitor before the 

 nectar can be reached. 



Kerner, in his book on " Flowers and their Unbidden Guests ", records 

 many other structures which can be interpreted as preventing or restricting 

 this type of insect visitor. P'or example he suggests that spines and prickles 

 may prevent soft-skinned creeping animals from visiting flowers; waxy coat- 

 tings may interfere with the movements of ants and similar walking insects; 

 while sticky stems such as occur in Silene may catch small unwanted insects. 

 It would be unwise to interpret all such structural modifications as explicable 

 in terms of protection against unwanted visitors. Some at least of the 

 structures mentioned serve other and equally important ends. 



