TEMPEEATUEE AND ITS CONDITIONS. 323 



X 



relationship with it. quite independently of the temperature 

 of the air through which the rays pass, though the latter 

 gradually rises also, particularly during bright sunshine. 

 As we have seen already, three-quarters of the radiant 

 energy during such conditions is not absorbed by the 

 plant. The temperature of the air round tjae leaves under 

 a diffuse illumination rises but slightly, as only about 

 four per cent, of the radiant energy falling upon them 

 remains unabsorbed by them. 



In certain cases, particularly where the temperature of 

 the air remains low for considerable periods, as in high 

 latitudes and on mountains, many of the light rays appear 

 to be transformed into heat. These are the rays which 

 are most vigorously absorbed by chlorophyll and by 

 anthocyan, and which cause the fluorescence of those 

 pigments. The importance of anthocyan in this respect 

 especially may be noticed. It is of very frequent occur- 

 rence among plants which grow in deep shade, and which 

 receive accordingly but little radiant energy. It is usually 

 found on the underside only of foliage leaves, and in special 

 leaves produced in summer upon the stems of deciduous 

 shrubs which occur upon the outskirts of forests or in 

 shady spots in their interior. It is found also at certain 

 times on the upper sides of foliage leaves, particularly when 

 the ordinary sources of heat are deficient, as in the cold 

 weather of early spring. Many grasses which when in 

 the lowlands are only green in colour develop a great 

 amount of the red or purple anthocyan when they grow 

 near the snow-line. 



In both shady and alpine habitats the function of the 

 anthocyan appears to be the same, to secure to the 

 plant a certain amount of heat by the transformation of 

 the light rays. 



The absorption of heat from the environment by the 

 processes of conduction is particularly noticeable in the 

 case of aquatic plants. Indeed any alteration of the 

 temperature of either the plant or the surrounding water 



