INTERNAL TEMPERATURE OF LEAVES, 261 



which are accustomed to exposure, and that it is seldom, except 

 in shade plants, that healthy and mature green leaves suffer 

 more than a sUght weakening of assimilation even after twelve 

 hours' exposure to sunlight. He mentions the distribution of 

 red colour, and quotes Johow (13) as having shown that the 

 presence of anthocyan is called forth directly by exposure to 

 light. In 1897 Ewart (8) in a further paper supports the 

 protective theory of the function of anthocyan and adversely 

 criticises Stahl. His view is still founded on Pringsheim (24), 

 who, he says, showed that the rays most destructive to 

 chlorophyll were the blue and green , and these are those most 

 absorbed by anthocyan. Against Stahl's theory of anthocyan, 

 as an aid to transpiration, he brings forward a new statement 

 that red leaves or parts of leaves have generally fewer stomata, 

 showing that the extra transpiration due to the higher tem- 

 perature caused by the anthocyan is a disadvantage which has 

 to be provided against. As opposed to the idea that young 

 leaves (often red in the tropics) require greater transpiration 

 to provide mineral salts to the growing organs, he looks upon 

 the young leaf not as an organ requiring copious mineral food, 

 but as one supplied with food already formed and presented 

 to it in a concentrated and soluble form. 



Overton (22). 1899, in a paper of striking value offered 

 evidence of a different character on the problem of anthocyan. 

 He accidentally observed that the leaves of Hydrocharis. a 

 plant on which he was experimenting with quite a different 

 object in view, turned red when cultivated in 3-5 per cent, 

 solution of cane sugar. New leaves formed in this solution 

 were all red. Travelling in the Alps, where such colour is 

 abundant, brought back the point to his mind and suggested 

 that perhaps the low temperature of those elevations favoured 

 the presence of sugar at the expense of starch. He quotes 

 as supporting this (1) Sachs, who showed that low tem- 

 perature hinders translocation and therefore increases the 

 amount of sugar; (2) observations of Miiller-Thurgau (21) on 

 potatoes to the effect that low temperatures tended to change 

 their starch into sugar ; (3) Fischer (9) on the reserves in 

 trees in winter; and (4) Lidforss (17) on sugar in evergreen 

 leaves in wmter. Overton himself hiade observations and 



