FUNCTIONS OF THE STEM 35 



planted in our hill stations, and sometimes even a 

 tree which has fallen or been forced slantwise away 

 from the perpendicular grows in consequence more 

 quickly in the vertical direction, becoming very much 

 thicker across the vertical diameter. 



An equally important function is the conveyance 

 of water and the contained mineral salts from the 

 root to the leaves, and of made-up food materials 

 sugar and proteids from the leaves down to all parts 

 of the shoot and the roots. 



To prevent loss of water by evaporation, and also 

 for protection against damage by accident or by animals, 

 the stem and branches are covered with a thick water- 

 proof bark. The outer part of the bark consists of 

 dead tissue, and its only use appears to be to protect 

 the inner parts against injuries and loss of water ; for, 

 not being strong and tough like wood, it does not con- 

 tribute in the smallest degree to the strength of the 

 part. 



We can now understand why leaves are flat and 

 branches and stems cylindrical in shape. The work 

 of a leaf is a surface-work, the greater the surface 

 exposed to the light and air, the greater (other things 

 being equal) the amount of carbon assimilated and 

 of water evaporated. The function of the stem and 

 branches on the other hand is to support the leaves, 

 and to conduct water to them and food from them ; 

 so the less the surface exposed to risk of injury and 

 loss of water the better. This is the reason why leaves 

 are thin thereby having the greatest surface, and 

 branches cylindrical. A stem or a branch may be 

 angular when young, but after a year or so, it always 



