THE ANGIOSPERMAE 



1347 



flower opens. Xo nectar is secreted, but an unpleasant smell is emitted 

 and this, together with the purple colour of the ovary, attracts carrion flies, 

 because presumably it suggests decomposing flesh to these insects. If they 



Fig. 1256. — Paris qtiadrifoUa. A, Flowering shoot. B, Vertical section 

 of flower in female state. C, The same, later state with anthers dis- 

 charging pollen. 



have previously visited another flower they will readily cover the stigmas 

 with pollen. Later the anthers open and the pollen is very easily shaken oflF. 

 Hence a fly visiting a flower in this state will probably touch the filament 

 or the elongated connective thus scattering pollen on its body. The stigmas 

 remain receptive until all the pollen has been shed so that, if insect visits 

 should fail, and indeed insects rarely visit the flowers, self-pollination is 

 possible either by wind or because of the movements of the stamens, which 

 may converge tow'ards the styles. Unisexual female flowers occur in which 

 the stamens are devoid of anthers. 



(/) Hover Fly Flowers 



Most of the flowers which are pollinated by these insects are highly 

 decorative and the colours are such that the centre of the flower is sharply 

 distinguished from the surrounding petals. The mechanism is usually 

 very delicate and suitable for a small, light insect w'hich could not operate 

 a more robust structure. Several genera have been observed to make use of 

 Hover Flies as pollinators, but the best-known are Circaea and Veronica. 

 Not all the species of Veronica appear to be pollinated by these flies, indeed, 

 the type of visitors probably depends mainly upon the size of the flowers 



