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 



