The Growth of the Tree 31 



elements obtained from the air into sugar and starch from 

 which the wood tissues are made. This passage of the 

 water up the stem in the early spring is known as the 

 "rising of the sap." Since this is the only way the tree 

 has of securing food, it is absolutely essential that these 

 necessary materials be in soluble form. Should they be 

 present ever so abundantly in insoluble combinations, or 

 poisoned by salts or acids, the tree would nevertheless 

 starve. This often happens with all kinds of plants, and 

 is well known by most farmers. 



The roots also fulfill the further purpose of holding 

 •the tree upright. Naturally the trees with the long tap- 

 root running deep into the ground are the firmest, because 

 they have the best grip on the soil. Such trees are prac- 

 tically never uprooted, — they break off first. Those 

 having the mixed root systems are also fairly firm and not 

 easily blown over unless they have been grown in a very 

 protected location and then suddenly deprived of their 

 protection, — for, as explained above, they easily adapt 

 themselves to conditions and would not take a deep hold 

 on the ground unless forced to it by constant strain. The 

 shallow-rooted species have a still weaker hold, and are 

 easily upset even when most carefully trained to with- 

 stand the wind. The roots are much quicker to respond 

 to the necessity of giving stronger mechanical support 

 than to the exigencies of a new moisture or soil condition. 



The stem, trunk or bole. ■ — ■ The trunk of the tree an- 

 swers the double purpose of transporting the water supplied 

 by the roots, and supporting the crown; its branches 

 spread the leaves to the light. 



If a cross-section of the stem is studied, its structure 



