144 On the Formation of Anthocyanin 



Hence we may state that in injured leaves the formation of pigment 

 commences in tissues which in the healthy plant are usually unpig- 

 mented. 



The same internal distribution of pigment is found in leaves red- 

 dened by low temperature, i.e. autumnal leaves and evergreen leaves in 

 winter, and also in the older dying leaves of plants at the end of their 

 vegetative season or after exposure to drought. 



It is an interesting coincidence that the phenomenon of increased 

 pigmentation accompanying age is also characteristic of young develop- 

 ing leaves. In these again the pigment is formed in the assimilating 

 tissue, chiefly palisade parenchyma, though it may also appear in the 

 epidermis. 



Anthocyanin is very frequent in variegated leaves and it is then 

 often limited to the stripes or patches free from chlorophyll (variegated 

 Zea Mais). In other cases (Codiaeum sp., Acalypha sp., Tradescantia 

 sp.), the whole leaf may be pigmented. 



Stems and Petioles. The distribution of pigment in petioles, herba- 

 ceous stems and the young stems of trees and shrubs is very much the 

 same as in the midribs of leaves. Anthocyanin is usually confined 

 either to the epidermis alone or to one or more sub-epidermal layers in 

 addition, of which the cells are frequently collenchymatous in structure. 



Flowers and Fruit. In the corolla, anthocyanin is located in the 

 epidermis, usually both upper and under, sometimes only upper. The 

 upper pigmented epidermal cells are almost always more or less pro- 

 longed into papillae but this prolongation is less characteristic of the 

 under epidermal cells. 



In fruits the colouring matter may be limited to the epidermis and 

 sub-epidermal layers or may extend into the inner tissues. 



Concentration of Sugars and Glucosides in various Tissues. 



To ascertain the relative concentrations of sugars and glucosides in 

 the different tissues of a leaf is a difficult problem. 



The presence or absence of glucose can be detected micro-chemically 

 by means of Fehling's solution (22), and to some extent glucose, fructose, 

 maltose and cane-sugar can be differentiated micro-chemically by a 

 modification, employed by Grafe(4), of Senft's(17) phenyl hydrazine 

 method. For detection of differences in amount I have not found these 

 methods reliable. 



