322 VEGETABLE PHYSIOLOGY 
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 the 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 under side 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 
