262 SMITH : 



found more sugar in Alps in autumn than in the same species 

 in summer. He then performed a large number of experi- 

 ments and came to the following conclusions from them : — (1) 

 In a large number of species the presence of red cell sap 

 stands in an exact relation with the richness of the sap in 

 sugar; (2) a lower temperature favours the red colour; (3) 

 greater light intensity shares in the effect. He considers the 

 chemical nature of the pigment and from the probability that 

 the grape sugar molecule is not much changed in forming 

 the pigment and from other considerations he supposes the 

 pigment to be a glucoside, of which a constituent is a tannic 

 acid. Red colours are, however, probably not all alike in 

 chemical composition. In reviewing the history of the 

 question, he is inclined to support Stahl in his view of the 

 function of the red pigment. 



The chief supports therefore of the screen tlieory are 

 Kerner's Alpine experiments, Pringsheim's experiments on the 

 destruction of chlorophyll by strong light, and the general 

 fact of distribution, namely, tliat the red colour is favoured by 

 exposed situations and is developed in specimens in strong 

 light when others of the same species in the shade are without 

 it, or even, as is often the case in the Copper Beech, that one 

 individual has its exposed leaves coloured and its shaded 

 leaves green. Against this screen theory are Reinke's experi- 

 ments tending to minimise the destructive effect of light on 

 chlorophyll, and Engelmann's careful analysis of the absorp- 

 tion spectrum of anthocyan, showing that it is complementary 

 to that of chlorophyll and only absorbs those rays wliich would 

 otherwise pass thro\igh tlie chlorophyll, not those rays which 

 would be absorbed by the chlorophyll. It is likely that a 

 screen which had the effect of protecting chlorophyll would 

 absorb some of those rays which would be absorbed by the 

 chlorophyll without such a screen. An effective screen should 

 have a similar, not a complementary, absorption spectrum to 

 that of chlorophyll. It is significant that later supporters of 

 the screen theory have never sliown experimentally that 

 anthocyan does protect the chlorophyll. Pringsheim's red 

 screen wliich protected the chlorophyll was iodine dissolved in 

 carbon bisulphide and had quite a different absoi^ption spwctrum 



