INTERNAL TEMPERATURE OF LEAVES. 263 



from anthocyaa, especially at the more refrangible end. Even 

 if the destructiveness of strong light to chlorophyll under normal 

 conditions has not been over-estimated by them, yet it is not 

 enough to show that chlorophyll needs a screen, it must also be 

 shown that anthocyan actually affords the protection needed. 

 The chief supports of Stahl's position are: (1) that he has 

 shown by experiments that the internal temperature of red 

 leaves or parts is higher than that of green leaves or parts ; (2) 

 that he has shown that red colour does often exist in shade 

 plants; and (3) that he has suggested an interpretation of 

 Kerner's results which is compatible' with his own theory. 

 Overton's work bids us be chary of arguing on the assumption 

 that if a particular advantage might accrue from the presence 

 of anthocyan, therefore that advantage is probably the one 

 which does accrue, for it is quite possible that anthocyan is a 

 product necessarily formed in the leaf under certain chemical 

 and physical conditions and that its presence is of no advantage 

 to the plant at all. In this paper therefore no such conjec- 

 tural evidence will be brought forward, but experimental 

 results on the actual temperatures attained by red leaves 

 compared with yellow, white, and green leaves. Further 

 evidence will be brought forward in a later part of the paper 

 on the actual time and conditions of appearance of red colour- 

 ing matter in young leaves of certain tropical trees. It may 

 meanwhile be remarked that Stahl's explanation of Kerner's 

 results does not hold good in the light of experimental results 

 in the earlier part of this paper, and that there is a considerable 

 body of evidence by observation to the effect that anthocyan 

 is by no means confined to, even if it is prevalent in, shady 

 moist situations, where transpiration is difficult ; but that, on 

 the other hand, it is very prevalent in open situations exposed 

 to the strongest tropical sunlight. 



Effect of Colour (experimental). 



The leaves used in this part of the investigation were of 

 various kinds, and in some cases had, besides their colour, 

 other differences, e.g., of texture, from ordinary gi'een leaves. 

 These will, however, be described in each case. In the 

 following experiments the temperatures of young leaves of 



