i] INTRODUCTORY 7 



pollination by insects, and the subsidiary question of the attractive 

 value of ripe pigniented fruits for dispersal by birds. The relationship 

 between flower-colour and entomophily has received great attention 

 from botanists, and the whole matter is dealt with most thoroughly 

 in Knuth's Handbook of Flower Pollination (426), which includes an 

 excellent bibliography. 



The physiological function is a very difficult and far less satisfactory 

 matter. Several different functions of a physiological nature have 

 been attributed to anthocyanin. One of the most famous is the screen 

 theory, the idea of which was first based on work published in 1880 by 

 Pringsheim, who showed that chlorophyll was bleached by intense light, 

 but not if protected artificially by a red screen. Thus the view arose 

 that anthocyanin might be protective in function, but experimental 

 evidence does not altogether favour this hypothesis. For, in 1885, 

 Reinke pointed out that it is those rays absorbed by chlorophyll which 

 have the greatest destructive effect on chlorophyll, and Engelmann 

 (394), in 1887, demonstrated that the absorption spectrum of antho- 

 cyanin is on the whole complementary to that of chlorophyll. Hence 

 anthocyanin absorbs those rays which are least harmful to chlorophyll, 

 and cannot therefore be said to provide an effective screen. A second 

 suggestion, brought forward by Stahl (405) in 1896, and largely supported 

 by him, is that anthocyanin absorbs certain of the sun's rays, and by 

 converting them into heat, raises the temperature of the leaf, and this 

 may serve to accelerate transpiration in difficult circumstances, as in 

 damp regions of the tropics, or may protect leaves from low temperature 

 as in Alpine regions. The chief points in favour of this hypothesis are 

 the distribution of anthocyanin in leaves of shade-loving plants, and the 

 fact, observed also by Stahl, and confirmed in 1909 by Smith (420), that 

 the internal temperature of red leaves is greater than that of green. 



The next line of investigation, the chemical composition of the 

 pigments, is also difficult, and though spasmodically attacked from 

 time to time, met with no very serious consideration till 1906. So 

 intimately connected with its chemical composition that it can scarcely 

 be considered separately, is the question of the mode of formation of 

 anthocyanin, that is, the chemical reactions involved in the process. 

 Closely connected also, though in a lesser degree, is the part played by 

 anthocyanin pigments in heredity. It is proposed therefore to deal 

 with these three lines of research more or less together. 



As already pointed out, the reactions with acids and alkalies are 

 the most obvious and striking chemical properties of anthocyanins, 



