292 SMITH : 



which do leaf out in the wet months find tliis habit a disadvan- 

 tage, but one wliich other compensating advantages are suffi- 

 cient to counterbalance. Now, if once this general result be 

 granted, we come upon a point of view which enables us to 

 connect the two investigations of this paper under one general 

 heading. For just as the necessarj^ copiousness of transpira- 

 tion stream for the rapid growth is only to be obtained in the 

 dry season, so the increase of internal leaf temperature caused 

 by the presence of a red colouring matter in the young leaves 

 would tend to increase the transpiration. The higher the 

 internal temperature of the leaf the more rapid the loss of 

 water from the leaf tends to be, and, if a sufficient supply of 

 water is forthcoming from the soil, the more copious the tran- 

 spiration stream becomes. Thus though the yomig red leaf 

 may not be any higher in temperature than the mature green 

 leaf, yet it is distinctly higher, as has been shown, than such 

 a young leaf would be without the anthocyan. The work of 

 the present paper has shown that a young leaf with anthocyan , 

 in bright sunlight on a fairly stiU day, is at a temperature 

 3° or 4° C. higher than such a leaf mthout anthocyan. Al- 

 though this may not seem a very large difference, yet when the 

 whole periphery of a large tree is covered with leaves, aU of 

 which are at this increased temperature, the iiicrease in the 

 transpiration stream must be considerable. This explanation 

 is in complete accord with that of Stahl, who pomts out how 

 in the tropics the raising of the internal temperature of leaves 

 promotes the transpiration stream. In the earlier part of the 

 paper reasons have been given for casting doubt upon the 

 function of anthocyan as a light screen for the chlorophyll, on 

 the ground that it does not, on the whole, screen from those 

 rays which the chlorophyll absorbs the only ones presumably 

 which can destroy the chlorophyll. One of the cliief supports 

 of the light screen theory has been the type of general distri- 

 bution of anthocyan. 



It is contended by Stahl that in the tropics leaves containing 

 anthocyan are often found in moist situations, e.g., in damp 

 forests, where it is an advantage that the transpiration stream 

 should be increased. No one who has visited the tropics 

 can doubt that they are also abundant in sunny and exposed 



