ROOTS 



55 



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

 April has given a root-pressure of thiity-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- 



. ... i , /? T 7 , large tube fastened to the stump of 



plant a tobacco seedling about four th d h]i t b bb t b 



FIG. 25. Apparatus to Measure 

 Root-Pressure. 



rr, rubber stoppers; t, bent tube 

 containing mercury ; 1 1', upper and 

 lower level of mercury in T. 



inches high into rich earth con- 

 tained in a narrow, tall beaker or 

 very large test-tube (not less than 

 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, 



