36 THE HUMAN SIDE OF TREES 



off every eight or ten years without injury to the 

 tree. 



A tree is many-mouthed. Its immense system 

 of roots, often as extensive as the portion above 

 ground, is engaged in the sole business of sucking 

 and drinking in nutriment from the soil. Botan- 

 ically, a root is that portion of the plant axis which 

 does not bear leaves, normally grows downward 

 and is fixed in the soil. It is simpler in form than 

 the aerial branches, and is irregular in its method 

 of division. The growing point is just back of 

 the tip of each rootlet. At the tip is a sort of pro- 

 tective shield or cap which the expansive power of 

 cell division forces through the earth much as tun- 

 nel-diggers bore their way along. Still farther back 

 from the tip are the root hairs, long hollow single 

 cells which absorb moisture from the surrounding 

 soil by capillary attraction. In this moisture is dis- 

 solved valuable mineral salts essential to the tree's 

 economy. 



In many ways the leaves are the most wonder- 

 ful parts of the tree. They are stomachs and lungs 

 combined. Out of crude sap, with the assistance of 

 air and sunlight, they elaborate plant food capable 

 of assimilation. At the same time, they breathe 

 out through their stomates vast quantities of water 

 and other waste products. They are really thin 



