76 ROOT PRESSURE. [CH. Ill 



and serves as an index of the rate of flow as it travels 

 up the tube, which should be graduated. 



By watering the earth with warm water a greatly 

 accelerated rate of flow is obtained, but whether it is due 

 to increased root pressure or to the expansion of air in the 

 tissues is not easy to say. 



(89) Root pressure. 



To demonstrate the force of root pressure a striking 

 method is that used by Mr Gardiner in his lectures. He 

 uses a plant of Sparmannia africana growing in a large 

 pot. The stump is attached by rubber tubing to a poto- 

 meter tube 1 filled with a solution of nigrosin in water; to 

 one arm of the potometer a vertical glass tube, a few mm. 

 in diameter and several feet in length, is attached; the 

 other arm of the potometer is closed with a cork. The 

 nigrosin seems to have no bad effect on the plant and 

 makes the rising column of fluid easily visible. If the 

 tube is supported against a wall it can be elongated by 

 fresh lengths of glass tubing and thus a column of 8 or 10 

 feet can easily be shown. 



(89 A) Root pressure. 



The classical method of observing root pressure is that 

 described and figured by Sachs in his Physiologie (Fr. 

 Trans.), p. 223, of which the following (Fig. 14) is a 

 modification. A T tube (T) having one arm B bent so as 

 to be parallel to the two others, is tied into a piece of 



1 The arrangement is similar to that figured in Sachs' Vorlesungen, 

 p. 328, fig. 211. 



