ROOTS. 



37 



covers the inner surface of the cell-wall of the root-hair. 

 When the student has learned how active a substance proto- 

 plasm often shows itself to be, he will not be astonished to find 

 it behaving almost as though it were possessed of intelligence 



from cell to cell, a 

 root-hairs is forced 

 stem, just as the 

 the tube shown in 



with which the up- 

 may be estimated 



and will. Traveling by osmotic action 

 current of water derived from the 

 up through the roots and into the 

 contents of the egg was forced up into 

 Mg. 21. 



56. Root Pressure. — The force 

 ward flowing current of water presses 

 by attaching a mercury gauge to 

 the root of a tree, or the stem of 

 a small sapling. This is best done 

 in early spring after the thawing of 

 the ground, but before the leaves 

 have appeared. In Fig. 22 the ap- 

 paratus is shown attached to the 

 stem of a dahlia. The large glass 

 tube W, filled with mercury up to 

 the level g and with water from g 

 to near s, is fastened tightly to the 

 cut stem at s. As water absorbed 

 by the roots is forced over into W, 

 the mercury level in Q will rise 

 higher and the difference of level 

 in the two mercury-columns will 

 measure the root pressure. For 

 every foot of difference in level 

 there must be a pressure of nearly 

 six pounds per square inch on the 

 stump at the base of the tube g. 



A black-birch root tested in this way at the end of April 

 has given a root pressure of 37 pounds to the square inch. 

 This would sustain a column of water about 86 feet high. 



Fig. 22, — Apparatus for Measure- 

 ment of Root Pressure. 



if, cut-off stem of dalilia; c, a piece 

 of rubber tubing slipped over the 

 stump s and the glass tube g and 

 tied fast ; g^ bent glass tube ; W^ 

 water (sap forced up hy the 

 roots) ; Q, mercury-column sus- 

 tained by the root pressure. 



