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 :\.ustralian 
eum trees GSequota, Eucalyptus), 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. Yet absorption of water 
is not the only function of roots. They often absorb oxygen; 
they commonly serve to anchor the plant; they may aid it to 
climb; they frequently store food, water, or both; and in or 
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 
