6 INTRODUCTORY [CH. 



upon the supply of materials from which the pigment is synthesised? 

 That low temperature favours pigment formation would seem to be 

 demonstrated by autumnal coloration, and the winter reddening of 

 leaves of Hedera, Ligustnim, Mahonia and other evergreens. Conversely, 

 Overton (333) found in Hydrocharis, the higher the temperature, the 

 less anthocyanin. Klebs (360) also notes that flowers of Campanula 

 and Primula may be almost white in a hot-house, but the same indivi- 

 duals kept in the cold will bear coloured flowers. The consideration 

 of temperature is perhaps more difficult than that of light; for low 

 temperature, on the one hand, retards photosynthesis by which sugars 

 are formed, but, on the other hand, it also retards growth, starch forma- 

 tion and probably translocation, thereby tending to raise the sugar 

 contents of the tissues. High temperature, on the contrary, accelerates 

 growth and respiration, and consequently prevents the accumulation 

 of any excess of synthetic products. 



An interesting application of these views upon light and tempera- 

 ture effects can be made in the case of Alpine flower coloration. The 

 subject has been extensively studied by Gaston Bonnier (288, 289, 

 294, 307, 328), Flahault (288, 289, 290, 292) and others, and has had 

 a great vogue with the writers on flower coloration in connection with 

 insect pollination. The special features of the case are intensity of 

 flower-colour and the formation of anthocyanin in the vegetative parts. 

 Gaston Bonnier & Flahault have compared individuals grown at heights 

 of 2300 metres with individuals grown in the plains, and have found 

 that the latter produce paler flowers and less anthocyanin in the leaves 

 and stems. It seems most reasonable to suppose that these phenomena 

 form a natural demonstration of some of the relationships which we 

 have just been considering between colour and factors. High Alpine 

 plants are stunted in growth, i.e. little material is expended vegetatively ; 

 they are exposed by day to intense insolation while the night tempera- 

 ture is low. One may therefore suppose photosynthesis to be very 

 active, whereas translocation and growth are retarded, these being 

 conditions which favour high sugar and chromogen concentrations in 

 the tissues and resultant abundance of pigment. 



The so-called functions of anthocyanin have provided material for 

 another main line of research. Two essentially different types of 

 function are readily distinguished, the biological and the physiological. 

 The biological function is a subject which, in itself, needs extensive 

 treatment, and does not lie within the scope of this book. It is solely 

 connected with the attractive value of the coloured floral organs for 



