ROOTS ‘20 
A black-birch root tested in this way at the end of 
April has given a root-pressure of thirty-seven pounds to 
the square inch. This would sustain a column of water 
about eighty-six feet high. 
67. Root-Absorption and 
Temperature of Soil. —It 
would not be remarkable if 
the temperature of roots and 
the earth about them had 
something to do with the 
rate at which they absorb 
water, since this absorption 
depends on the living proto- 
plasm of the root-hairs (see 
Sects. 64, 65). An _ experi- 
ment will serve to throw 
some light on this question. 
EXPERIMENT XVII 
Effect of Temperature on Absorp- 
tion of Water by Roots. — Trans- 
plant a tobacco seedling about four 
inches high into rich earth con- 
tained in a narrow, tall beaker or 
very large test-tube (not less than 
— mmm 
goon 
¥: 
teh 
Fic. 25. — Apparatus to Measure 
Root-Pressure. 
T, large tube fastened to the stump of 
the dahlia stem by a rubber tube; 
rr, rubber stoppers; ¢, bent tube 
containing mercury ; / /', upper and 
lower level of mercury in 7. 
1 inch in diameter and six inches high). When the plant has begun 
to grow again freely, in a warm, sunny room, insert a chemical ther- 
mometer into the earth, best by making a hole with a sharp round 
stick, pushed nearly to the bottom of the tube, and then putting the 
thermometer in the place of the stick. Water the plant well, then 
set the tube in a jar of pounded ice which reaches nearly to the 
top of the tube. Note the temperature of the earth just before 
placing it in the ice. Observe whether the leaves of the seedling wilt, 
