i] INTRODUCTORY 5 



affect those tissues which conduct away the synthetic products of 

 the leaf. 



There is little doubt that the chromogen of anthocyanin, in the 

 form of a glucoside, is also manufactured in the leaves, and the whole 

 trend of results goes to show that an enforced accumulation of this 

 chromogen, together with sugars, by stoppage of the translocation 

 current, leads to formation of anthocyanin in the leaf; and a similar 

 result may arise from artificial feeding with sugars. To the more 

 precise relationships between chromogen, anthocyanin and sugars 

 attention will be given in a later chapter. 



Thus it will be seen that in all problems connected with effects 

 of temperature, light, etc., on anthocyanin formation, we are confronted 

 with two distinct questions, i.e. the direct effect of light and temperature 

 on the actual production of pigment, and the indirect effect of these 

 factors on the supply of organic compounds from which the chromogen 

 of anthocyanin is synthesised. 



The relationship between pigment formation and light constitutes 

 a problem to which there is no very satisfactory solution. Sachs 

 (269, 271), Askenasy (282) and others have tried the obvious methods 

 of growing plants in the dark with controls in the light, of darkening 

 leaves while leaving inflorescences uncovered and so forth. The 

 outcome of these researches, as well as of several others, has been to 

 show that in many cases, for example, in flowers of Tulij)a, Hyacinthus, 

 Iris and Crocus, anthocyanin develops equally well in the dark ; in other 

 cases, such as Pulmonaria, Antirrhinum, and Prunella, the development 

 is feeble or absent. A general survey of anthocyanin distribution leaves 

 us in no doubt that, as far as organs where anthocyanin may be expected 

 to develop are concerned, the greatest production takes place in the 

 most illuminated parts. But we have on the other hand not a few 

 examples, of which the root of Beta is a good illustration, of development 

 of pigment in total darkness. In the absence of fuller evidence, the 

 most reasonable point of view is that the actual process of pigment 

 formation may in itself be entirely independent of light, should the 

 tissues contain sufficient reserve materials to supply the chromogen. 

 But if there is a shortage of reserve materials, such as would arise from 

 diminished photosynthesis, the anthocyanin may fail to appear from 

 lack of chromogen. 



The problem of the effect of temperature offers similar difficulties. 

 Does the temperature influence the actual formation of pigment, or 

 is it again an indirect cause, making itself felt only through its effects 



