86 PHYSIOLOGICAL CONDITIONS AND FACTORS [CH. 



The relationship between anthocyanin development and sugar- 

 feeding is reserved for a later paragraph (see p. 93). 



Effect of temperature. 



Most general observations bear out the conclusion that increase of 

 anthocyanin is correlated with lowering of temperature. The most 

 obvious demonstrations are the autumnal coloration of leaves and, to 

 a lesser extent, the reddening of evergreen leaves in winter (Ligustrum, 

 Hedera, Mahonia). The question of the relationship between low 

 temperature and anthocyanin formation has been specially considered 

 by Overtoil (333). This author had observed that Hydrocharis plants 

 grown in cane sugar solution became very strongly reddened, and the 

 idea occurred to him that an excess of sugar in the cell-contents might 

 similarly be the cause of autumnal and winter coloration of leaves. In 

 view of this suggestion, it is difficult to estimate any direct effect of low 

 temperature on anthocyanin formation, because of the indirect effect 

 produced by the same conditions on (1) photosynthesis, (2) formation 

 of starch from sugar, (3) growth in general and probably (4) transloca- 

 tion. A decrease of activity of (1) leads to a decrease of sugar contents 

 in the cell ; but a decrease of (2) and (4) has the opposite effect. The 

 process of removal of synthetic products from the leaves is, according 

 to Sachs, greatly retarded by low temperature. Hence similar condi- 

 tions of clogging to those brought about by injury, which were mentioned 

 in the previous section, might arise and there would be a resultant 

 production of pigment. The synthesis of starch from sugar is also a 

 process which is retarded by low temperature. Thus Miiller-Thurgau 1 

 has shown that at temperatures below 5 C. quite a considerable portion 

 of the starch contents of the potato is changed to sugar, and w r ith a 

 rise of temperature the greater portion of starch is again regenerated. 

 According to Lidforss 2 , evergreen leaves in winter are also completely 

 starch-free but contain very considerable quantities of glucose, which 

 is again to a large extent changed back to starch if the leaves are 

 artificially warmed. Overton himself examined the sugar content of 

 autumnal leaves and found considerable quantities present, considerably 

 more, at any rate, than in the same species at midsummer. 



From the above statements it will be seen that low temperature 



1 Miiller-Thurgau, H., 'Ueber Zuckeranhaufung in Pflanzentheilen in Folge niederer 

 Tempera tur,' Landw. Jahrb., Berlin, 1882, xi, pp. 751-828. 



2 Lidforss, B., 'Zur Physiologic und Biologic der wintergriinen Flora,' Bot. Centralbl., 

 Cassel, 1896, Lxvin, pp. 33-44. 





