258 SUMMARY OF CUERENT RESEARCHES RELATING TO 



in large quantities, and in whicli they undergo transport from place 

 to place, are very commonly coloured red. This is especially the 

 case with the young branches of trees and other perennial plants, such 

 as the oak and rose, and with the earliest spring-leaves, the leaf-stalks, 

 and the principal veins of the upper surface of the leaf. But this 

 colouring is, as a rule, confined to those parts which are exposed to 

 the direct action of the sun, and is always directly connected with 

 the presence of tannin. Transverse sections through young leaf-buds 

 of the rose show that the entire epidermis of every leaflet up to the 

 cone of growth is impregnated by a hyaline and strongly refractive 

 mass, as also are those cells which are afterwards distinguished as 

 the conducting cells of the carbo-hydrates, such as the vascular 

 sheaths. This opalescent substance is readily proved to be tannin. 

 As the red tinge developes in these parts, the refringency gradually 

 diminishes, the tannin becoming transformed into the red pigment. 

 With regard to the localization of the tannin which undergoes this 

 transformation, it may occur either almost entirely in the epidermis, 

 as in the hazel, beech, vine, and many other plants, or both in and 

 below the epidermis, as in the horse-chestnut, privet, elder, &c. ; less 

 often it is not found in the epidermis, or only in slight traces, as in 

 the different species of poplar and willow. 



The conditions under which this red pigment is formed are the 

 direct action of sunlight and a low temperature, but more especially 

 the former, difibring in this respect from the red pigment of autumn 

 leaves, the formation of which is due chiefly to a low temperature. 

 If seeds of maize germinate in the dark, the young plant developes 

 without a trace of red colour, which, however, makes its appearance 

 as soon as they are exposed to the sun, especially in the tigellum. 

 The same is usually the case with the veins on the under side of the 

 leaf ; and the colour is always most intense on the side of the stem 

 which is most exposed to the sun. The leaves of Begonias, and some 

 other plants, form an exception to this rule, the colouring being most 

 intense in the veins on the under side. Plants in which only a very 

 small quantity of tannin is formed, as the Solanaceae, 01eace», the 

 laburnum, mulberry, &c., display scarcely any red coloration. 



The vertical position of the majority of stems removes them to a 

 large extent from the direct light of the sun, and they show, as a rule, 

 but little colour ; this is strongly contrasted with the prevalent red 

 colour of the upper side of creeping stems, such as the stolons of the 

 strawberry, species of Potentilla, &c. The petioles of leaves, and the 

 separate pedicels of flowers in an inflorescence are, on the other hand, 

 very commonly more or less deeply coloured. In tropical countries 

 the colouring is much more universal and intense than with us. 



Spectroscopic analysis of the red pigment shows that it completely 

 absorbs the yellow and green rays from D to 6, partially those 

 fi'om 6 to a little beyond F, and the ultra-violet. The rest of the 

 spectrum is bright, the brightest portion lying between B and C, and 

 on both sides of Gr. These are, on the other hand, the most strongly 

 absorbent portions of the spectrum of chlorophyll. 



The pigment is readily soluble in cold water, and the effect was 



