104 THE NATURE AND WORK OF PLANTS 



the roots. These organs are constantly receiving 

 material from the leaves, but have a surplus on hand 

 almost all of the time, and some is also to be found 

 in the lower part of the stem. During the first year 

 this is sufficient for their nourishment, but it is gen- 

 erally exhausted before the beginning of the next 

 season. This material passes down very slowly 

 through the layer of living tissue immediately under- 

 neath the bark, and its flow is interrupted in any 

 girdling operation. If the tree lives several years 

 after girdling, it may be supposed that its roots have 

 formed partnership with fungi in such manner that 

 it receives its food from them (see § 52). As a mat- 

 ter of fact the stumps of many of the coniferous 

 trees are known to live for long periods, perhaps a 

 decade, by means of food obtained in this manner. 



Girdling of branches is used as a method of 

 increasing the size and quality of fruit borne on 

 them. 



139. Forces which carry the sap upward through 

 the stem. — Information upon the forces which pump 

 water up to the top of tall trees is very incomplete. 

 It is quite certain that it needs as much power to 

 carry water to the top of a tree a hundred feet high 



