86 PLANT LIFE 
CHAPTER VIII 
CORRELATION OF FUNCTION AND FORM 
In the higher and more specialised green 
plants the organ principally charged with 
carrying on the important function of photo- 
synthesis is commonly, though not invariably, 
the leaf. Now a green leaf only “ pays its 
way ”’ for so long as it is adequately exposed 
to light. But the intensity of light which 
produces the best results varies greatly with 
different plants. Moreover, a plant may lose 
so much water when exposed fully to the 
light, and hence to the air, that any advantage 
of illumination may be more than balanced 
by the chance of wilting. We find, as a 
matter of fact, that all these various con- 
siderations are of practical importance in the 
practice of forestry. Some kinds of trees 
will tolerate shade, others speedily succumb 
if they are not fully exposed to the light. 
Beech, for example, when young will thrive 
under the shade of the birch, and in many 
places it is best raised under this latter tree, 
which shields it in various ways during its 
infancy. But birch will not grow under 
beech. One is a strenuously light-demanding 
