TI54 A TEXTBOOK OF THEORETICAL BOTANY 



especially lines and streaks, which have always been regarded as " honey 

 guides " and are not generally required in actinomorphic flowers. On the 

 other hand patterns of scattered dots and blotches are often found in fly- 

 pollinated flowers and they may perhaps be more readily noticed by flies 

 than are pure colours. Flies are short-sighted creatures and the flowers 

 which depend on their visits are usually small and relatively dull in colour. 

 No matter how brilliant the colour of a small flower might be, it would not 

 be visible to a fly at a distance. To attract long-sighted bees or butterflies, 

 flowers must either be individually conspicuous or else grouped into large 

 clusters. Colour patterns may be useful to such flowers by increasing 

 conspicuousness through contrast. Many flowers show very striking 

 colour contrasts in their perianths or between the perianth and the other 

 parts, while coloured bracts, leaves or stems may also be called into use 

 to increase the contrast eflfect, of which there are numerous examples {e.g., 

 Bougainvillea, Poinsettia, Davidia, etc.). 



Troll has pointed out the interesting fact that in the Compositae, which 

 are, of course, highly specialized Sympetalae, the colours of the ray florets, 

 which stand around the inflorescence like a ring of separate petals, tend to 

 resemble those characteristic of the simpler polypetalous flowers, i.e., 

 white, yellows and reds predominate, while blues are relatively scarce. It is 

 as if the inflorescence were behaving biologically like a simple polypetalous 

 flower. 



Flower colours are subject to alteration in tone and intensity by some 

 external factors, especially the brightness, quality and duration of light. 

 Bright sunshine and long days increase the depth and brilliancy of colour. 

 The same varieties, grown for comparison in Uppsala, Sweden, and in 

 Paris, showed a consistent advantage in favour of the northern plants, 

 grown under a longer summer day. A greater intensity of short wave- 

 lengths in the sunlight, such as prevails at high altitudes, also increases the 

 brilliancy of colour and enhances the attraction of alpine flowers. 



The second general function, that of protection, may include the whole 

 flower or only the pollen. The green perianths of small flowers, such as 

 those of Chenopodiaceae, are limited in their function to protection of the 

 young parts of the flower and sometimes of the young fruit, during growth. 

 More highly developed, dichlamydeous perianths usually show a division 

 of function, the calyx acting as a general protection in the bud stage and the 

 petals, apart from their attractive function, acting to protect the pollen. 

 In a great many flowers this is ensured very simply by the pendent or 

 inclined position of the flower, a feature especially notable in zygomorphic 

 flowers, when the corolla acts as an umbrella over the stamens. Actino- 

 morphic flowers are usually vertical, however, and in them pollen protection 

 involves, in many flowers, movements of the petals, either in moist air or 

 darkness, which cause them to bend inwards, closing the flower and covering 

 the anthers. Alternatively, the petals may be folded, hooded or pouched 

 in such a way as to afford protection. (See also p. 1305.) 



Pollen protection, it is true, is not confined to such means. Bracts, for 



