30 PHYSIOLOGY OF NUTRITION. 



hederacea. The leaves named exhibit the red coloration mainl y 

 on their upper surface, and microscopic investigation of delicate 

 transverse sections teaches us in fact that it is particularly in the 

 cells of the palisade parenchyma that the red pigment is contained. 

 The pigment is dissolved in the cell-sap. The epidermal cells 

 contain no pigment. In the autumnal yellowing of leaves the 

 disorganising chlorophyll grains assume a yellowish colour, as can 

 be determined for example by investigating maple leaves in 

 autumn. As the change in colour of the leaves advances, the 

 protoplasm and chlorophyll grains gradually dissolve; their sub- 

 stance passes over into the persistent structures of the plant, and 

 finally there only remains in the cells xanthophyll in the form of 

 small shining granules. Some leaves, e.g., those of oaks, turn 

 brown in autumn, a phenomenon which is to be referred to a 

 browning of the cell-membranes as well as of the cell-contents. 



The colour changes taking place in structures which last over 

 the winter are also interesting. When the first frosts have set in, 

 in autumn or winter, we observe that the surface of twigs of 

 Thuja orientalis which is exposed to the light has assumed a 

 brown colour. 1 This is due to a partial decomposition of the 

 chlorophyll and the appearance of red pigments in the chlorophyll 

 grains. If such brown Thuja twigs are brought into a warm 

 room the red pigment disappears, and the twigs grow green again. 

 Access of light is not necessary. Browned Thuja twigs which I 

 kept in the dark at a temperature of 15-20 C. had already become 

 green again at the end of eight days. 



The winter reddening of persistent plant structures is to be 

 referred to the formation of a red pigment soluble in the cell-sap, 

 the chlorophyll grains remaining intact, and at most undergoing 1 

 changes of position in the cells. If we examine in winter trans- 

 verse sections of the leaves of Mahonia aquifolium, it is found 

 that in particular the cells of the beautifully developed palisade 

 parenchyma contain red pigment. 2 



1 A detailed account of the anatomical structure of twigs of Thuja (Th. 

 occidentalis, however, not Th. orientalis) by Frank will be found in Pringsheim's 

 Jahrbilcher, Bd. 9, p. 159. 



2 On the winter colouring of persistent plant structures consult H. v. Mohl, 

 Vermischte Schriften, p. 375, and G. Haberlandt, Sitzungsberichte d. Akad. d. 

 Wiss. zu Wien, Bd. 72, Abth. 1, Aprilheft. 



