1266 A TEXTBOOK OF THEORETICAL BOTANY 



mense spikes in Eremurus, a liliaceous plant frequently cultivated, fade 

 quickly, the petals shrivelling soon after the anthers open. In doing so they 

 expose the swollen green veins on their under surfaces, which resemble 

 Aphides so much that they attract Hover-flies to pierce and suck the veins 

 as they normally do to green-flies. At the same time they become covered 

 with pollen, which they carry away. 



Variations of colour may sometimes play an important part in the attrac- 

 tion of insect visitors. A change of colour with age is not uncommon and if 

 the flowers are closely massed the resulting contrast is more striking than if 

 all the flowers were of one colour. A familiar example is the Apple, in which 

 the petals in the bud may be bright red, but the expanded petals show only a 

 faint flush of pink. Mixed together in the umbel the two states of the flower 

 contrast strongly. An analogous change may take place in withering. Many 

 yellow flowers turn white with age or blue [Myosotis versicolor). Blue 

 flowers, such as those of Myosotis, may turn pink or white. Flowers, after 

 pollination, may bend downwards and so display a differently coloured re- 

 verse. Lastly, the colour change may correspond to the sexual condition 

 of the flower, as in the varieties of the Sweet William [Dianthiis barbatus) 

 whose flowers are white in the stage when pollen is being shed from the 

 anthers, but turn red in the female stage, when the stigmas are receptive. 



There may be a seasonal progression of colours, if the season is long, 

 corresponding to the prevalence of different kinds of insects with diverse 

 colour preferences. There is even a geography of flower colour, scarlet 

 flow'ers, for example, being common in the Tropics, where they attract 

 humming-birds as pollinators, though scarlet is a rare colour in temperate 

 floras. These ideas open up interesting fields for consideration, but the 

 facts are almost unknown. 



Tubular or bell-shaped flowers, especially if they are pendent, need offer 

 no other inducement to ensure nocturnal visitors, for they are welcome 

 shelters for many small beetles, flies and Hymenoptera, which have no 

 permanent homes. Composites like Calendula which close their capitula at 

 night, are also favourite refuges. No special modifications have been 

 observed bearing on these visits, but there can be no doubt that the refu- 

 gees do carry pollen about from flower to flower. 



Flowers pollinated by flies often have strong odours, suggestive of decay, 

 which are repellent to us but must attract some types of fly. One would 

 imagine that such smells would be enough in themselves to ensure attrac- 

 tion, but many of the tropical flowers which attract flies in this way (Aris- 

 tolochiaceae, Rafflesiaceae, Araceae, etc.) are coloured to correspond with 

 decomposing flesh, excreta, etc. The odour may act as a distant signal to 

 the insect, but finding the source of the smell apparently depends, at least 

 partly, on sight. 



These odours are called indoloid, from their relationship to indol, 

 though it is probable that some of them are really diamines. Amine odours are 

 fairly plentiful in flowers and not all are obnoxious, at least when well 

 diluted. The Hawthorn (Crataegus) owes its attractive scent to trimethyl- 



