4 INTRODUCTORY [CH. 



The matter of fundamental significance in all questions of factor 

 control, and one which it will be well to understand before turning 

 to the factors in detail, is the dependence of anthocyanin formation 

 upon the supply of an initial product or chromogen, as it is called, 

 from which the pigment arises. This substance, like all others in plant 

 metabolism, ultimately depends for its existence on a supply of carbo- 

 hydrates (sugars), the first products of synthesis in the plant. The 

 relationship between anthocyanin formation and the presence of sugars 

 in the tissues has provided an important subject for research. Overtoil 

 (333, 334), in 1899, first drew attention to its significance 1 ; this 

 botanist had noticed, while carrying out some experiments on osmosis, 

 that culture of Hydrocharis Morsus-ranae in sugar solutions leads to 

 greater production of anthocyanin in the leaves. Further experiments 

 showed the phenomenon to be constant for quite a number of species 

 when isolated leaves and twigs were fed on solutions of cane sugar, 

 dextrose, laevulose and maltose. Repetition of experiments on these 

 lines at later dates by Katie (354), Gertz (386) and others gave full 

 confirmation to Overtoil's results. This discovery led Overton to the 

 fairly obvious inference that possibly, in the normal plant, reddening 

 of leaves, etc., is correlated with excess of sugar in the tissues, and he 

 states that by tests upon red autumnal leaves he could detect more 

 sugar in red than in green leaves. 



More elaborate and conclusive work in this direction was commenced 

 by Combes in 1909 and carried on in the following years. Combes 

 (374, 385) had noted that decortication in some plants brings about 

 a considerable development of anthocyanin in the leaves above the 

 point of decorticatiou. Analyses were made by him, not only of leaves 

 reddened through this cause, but also of autumnal and other red 

 leaves. In all cases he (207) claims to have shown that the red leaves 

 contain greater quantities of sugars and glucosides than green ones 

 from the same plant. From other, more general, phenomena, in 

 addition to Combes's results, it may be safely inferred that an accumu- 

 lation of such synthetic products as are manufactured in the leaves 

 leads to production of anthocyanin. For example, we frequently find 

 abnormal reddening of a single leaf on a plant otherwise in full vigour, 

 and investigation almost invariably shows the reddening to be accom- 

 panied by injury, and the injury, whether it be due to mechanical 

 cutting or breaking, or to the attacks of insects, will be found to 



1 The connection between pigmentation and the presence of sugars was also pointed 

 out in 1899 by Mirande (332) as a result of his investigations on the genus Cuscuta. 



