FUNCTIONS OF ROOT 



47 



with a thin layer of protoplasm (Fig. 22). This soil-water 

 will pass rapidly into the plant, while very little of the 

 sap will come out. The selective action, which causes the 

 flow of liquid through the root-hairs to be almost wholly 

 inward, is due to the living layer of protoplasm, which 

 covers the inner surface of 

 the cell-wall of the root-hair. 

 When the studen t 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 and will. Trav- 

 eling by osmotic action from 

 cell to cell, a current of water 

 derived from the root-hairs is 

 forced up through the roots 

 and into the stem, just as 

 the contents of the egg was 

 forced up into the tube shown 

 in Fig. 21. 



61. Sap-Pressure. — The 

 force with which the upward- 

 flowing current of water 

 presses may be estimated 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. The experiment may also be performed 

 indoors upon almost any plant with a moderately firm 



Fig. 23. Apparatus to measure 

 Sap-Pressure. 



T, large tube fastened to the stump of 

 the dahlia stem by a rubber tube ; 

 rr, rubber stoppers; t, bent tube 

 containing mercury ; 1 1', upper and 

 lower level of mercury in T. 



