24 INTRODUCTION TO BOTANY 



20. Amount of water pressure in woody plants. When very 

 little water is being lost by evaporation from the leaves, the 

 sap pressure in trees and large shrubs is often great. As a 

 result of this pressure, in early spring sap escapes freely from 

 cuts or borings made into the roots, trunks, or branches of 

 many kinds of trees and shrubs. The " bleeding " of grapevines 

 pruned too late in the season is familiar to many people, as is 

 also the flow of sap from sugar maples. Woody plants cut 

 off near the root may have a pressure gauge of any convenient 

 sort attached to the cut surface, and the pressure can thus be 

 measured with accuracy. A severed grapevine has been found 

 to exert a pressure sufficient to sustain a column of water 

 more than 43 feet high, and the root of a black birch tree 

 a pressure equivalent to about 86 feet of water. 



In the tallest trees, the giant redwoods and the Australian 

 gum trees (Sequoia, JHucafyptus), water is sometimes raised 

 to a height of from 300 to 400 feet. It is not yet known how 

 large a part of the force required for this is due to the pres- 

 sure with which the sap from the roots is impelled up into 

 the stem, nor what other causes are mainly responsible for 

 the rise of water into the highest portions of the tree. 



21. What roots do for the plant. All plants must have 

 water, at any rate during the part of their lives when they 

 are actively manufacturing plant food, and it is by means 

 of their roots that most familiar plants absorb water and the 

 substances that are dissolved in it. Ypt^absnrpti^n of water 

 is not the only function of roots. They often absorb oxygen ; 

 they jsommonly serve to anchor the plant ; they may aid it to 

 climb; they frequently store food, water, or both; andinjjiL 

 on them are sometimes carried on important chemical opera- 

 tions, which result in gaining material for the production of 

 plant food. Many kinds of roots reproduce the plant ; that 

 is, a root or part of one may grow into a new individual plant 

 like the one to which the root belonged. 



The great importance of roots to life and growth is well 

 shown by the results which follow from any severe injury to 



