260 SMITH : 



is, at any rate, true that the presence of red colouring matter 

 does raise the temperature of a leaf. His proof of the supposi- 

 tion that this higher temperature does really aid transpiration 

 is of a different kind. It consists in showing that the red colour 

 is usually found in places where transpiration is difficult, and 

 that other contrivances, such as silver-flecked and velvety 

 leaves, are also found in such situations, and can be explained 

 as being adaptations for the same purpose. The importance 

 of increasing transpiration is that then the salts necessary for 

 translocation and anabolic changes are not withheld from the 

 plant ^ven at night. He considers Kerner's results, and tries 

 to show that they can be explained on the supposition that the 

 red colouring in the Alps may by raising the temperature of 

 the leaves protect them from the very low temperature, which 

 would otherwise be the result of the excessive radiation going 

 on at night at high elevations. From the experiments already 

 brought forward in the earher part of the present paper, 

 showing how quickly leaves lose the high temperatures caused 

 by the absorption of sunlight, we can at once dismiss this 

 explanation of Stahl's, for it depends upon the supposition that 

 the red leaf for an appreciable part of the night retains a 

 temperature higher than that of the surrounding air. We 

 have seen that even in bright diffuse light a thin leaf only takes 

 three minutes and a fleshy leaf not more than fifteen minutes 

 to sink to the temperature of the air when it is screened from 

 direct sunlight. That a leaf should in the dark letain a tem- 

 perature above that of the air for any length of time seems 

 therefore impossible. 



Stahl's most important work was that he showed, by well- 

 devised experiments both with a thermo- junction and with 

 cacao-butter smeared on the leaf, that red leaves did attain 

 a temperature in sunhght 1-2° C. higher than the temperature 

 reached by green leaves, and that the same is true for a red 

 part of a leaf compared wdth the green part of the same leaf. 



Ewarts (7), 1896-7, favours the protective theory of antho- 

 cyan, but interprets it not so much as a protection against the 

 destruction of chlorophyll as against assimilatory inhibition 

 which he had shown to follow on too strong illumination. He 

 states, however, that this protection is not necessary in plants 



