TRANSPIRATION. 217 



In this apparatus the external conditions may be varied one at 

 a time (1) light, by shading in various ways ; (2) humidity, by 

 drawing air, by means of an aspirator, through calcium chloride 

 U -tubes, or through wet sponge, into the chamber ; (3) temperature, 

 by drawing the air through a glass tube heated by a spirit or Bunsen 

 flame, or cooled by ice or cold water. 



(c) Many of the transpiration experiments already given 

 should be repeated with the potometer, and various others 

 may be made. For instance, leaves of a shoot fitted into 

 a potometer should be smeared with wax and vaseline on 

 their upper or lower surfaces or both, the effect of this 

 treatment on the rate of the current being noted. To 

 imitate the effect of a hairy covering, tie cotton- wool over 

 the leaves with thread vary the experiment by covering 

 the upper surface, lower surface, or both surfaces and 

 note the slowing down of the current. The diminution of 

 transpiration by the rolling-up of leaves either temporary 

 as in Psamma, or permanent as in Erica or Empetrum 

 may be demonstrated by rolling up each of the flat thin 

 leaves of a shoot fixed in the potometer, to form a tube 

 with the lower epidermis on the inside, and tying it 

 with thread ; note the reduced rate of the transpiration 

 current. 



278. " Root Pressure." Evaporation from the leaves 

 will tend to suck up fresh supplies of water to replace that 

 removed by the air in the form of vapour this upward 

 suction is easily proved. Is water also forced upwards by 

 the root ? It is not easy to test the individual absorbing 

 root-hairs and rootlets, but we can ascertain the collective 

 action of these organs as shown by the forcing of water 

 upwards from root to stem. 



(a) The exudation of sap forced up by the root may be 

 demonstrated as follows. Cut across the stem of a vigorous 

 pot plant, at about 5 cm. above the soil. Over the stump 

 slip a tight piece of strong rubber tubing, with about 2 cm. 

 projecting, and into this insert a tight cork with a slender 

 bent glass tube leading into a graduated vessel containing 

 some oil to prevent evaporation. Keep the soil watered, 

 and measure the amount of water exuded. Suitable 



