118 DERMAL SYSTEM 



season. A different explanation must of course be sought for the fact 

 that the lower sides of the leaves are coloured red or violet in so many 

 shade-plants. Here Kerner suggests that radiations which would 

 otherwise be reflected from the lower surfaces of the leaves are 

 absorbed by the coloured cell-sap and transformed into heat, which can 

 then be utilised by the plant for various purposes. 



Kny was the first to investigate this subject experimentally. He 

 found that an alcoholic chlorophyll-solution became discoloured more 

 slowly when shaded by a double bell-jar containing a solution of antho- 

 cyanin, than it did when the screen consisted of a colourless decoction 

 of sugar-beet. Kny was also able to establish the fact that insolation 

 causes an unequal rise of temperature in equal quantities of red and 

 green leaves of the same species, the heating effect being almost 

 always greater in the case of the red leaves ; the largest difference 

 observed amounted to 4 C. Kny's observations were subsequently 

 confirmed and extended by Stahl with the aid of thermo-electric 

 methods. The source of heat in Stahl's experiments was an ordinary 

 bat's- wing gas flame placed at a distance of 30 cm. from the leaf and 

 parallel to its surface. Under these conditions red leaves of Sarcanthus 

 rostratus became 1 , 5-1 , 82 C. warmer than green leaves of the same 

 plant ; in the case of Sempervivum tectorum the corresponding difference 

 was 1'67 C, in that of Begonia heracleifolia, var. nigricans 1*3 5 C, in 

 that of Pelargonium pdtatam *22 C. A method based upon the 

 melting of a coating of cacao-butter gave concordant results. In 

 Stahl's opinion the increased heat-absorption which results from the 

 red coloration entails a two-fold advantage. In the first place, it 

 leads to an acceleration of metabolism in general and of translocation in 

 particular, and, secondly, it causes an increase in the rate of transpira- 

 tion. With reference to the latter point, Stahl draws attention to the 

 fact that leaves which are ornamented with red spots or blotches, or 

 which possess a uniformly red under-surface are particularly prevalent 

 among plants that grow in very moist surroundings, and especially in 

 tropical rain-forests. In view of the fact, however, that the plants in 

 question for the most part grow in the deepest shade, it seems very 

 doubtful whether, in such cases, the presence of anthocyanin actually 

 produces heating effects sufficient to cause any appreciable increase of 

 transpiratory activity. In order to settle this question it will be 

 necessary to carry out transpiration experiments upon suitable plants 

 in their natural habitats. A further criticism applies to the experi- 

 ments of Kny and Stahl ; in many of the leaves examined by these 

 observers, the red cell-sap is not confined to the epidermal cells, but 

 occurs also in the photosynthetic tissues. It must in short be 

 admitted that, in spite of numerous interesting detailed observations, 



