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 student has learned 
how active a substance proto- 
plasm often shows itself to l 
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 Fic. 23. Apparatus to measure 
force with which the upward- Bap-Fresaute: 
flowing current of water 7 arse be fstenel tthe sy 
presses may be estimated by rr, rubber stoppers; t, bent tube 
attaching a mercury gauge —_fontaming merous : oo ae 
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 
