214 



TRANSPIRATION. 



a Bean or other seedling in a pot with 0'5 per cent, solution of 

 common salt for a few days. Test with cobalt paper leaves from 

 this seedling (A) and leaves from a seedling (B) watered in the 

 ordinary way. The leaves of A redden the paper much more slowly 

 than those of B. A iveak salt-solution actually keeps cut or dug-up 

 plants fresh ; plants sent by post will wilt much less if sprinkled 

 with a weak solution of salt. 



275. Relation between Transpiration and Absorp- 

 tion. This may be determined roughly by means of 

 apparatus like that of Fig. 47. As the 

 plant in the jar absorbs water, the water 

 column in the narrow graduated tube 

 (covered with a layer of oil to prevent 

 evaporation) sinks, measuring the vol- 

 ume absorbed ; the amount of water 

 transpired is simply measured by weigh- 

 ing the whole apparatus. It is best to 

 use a rooted plant (e.g. Bean or Maize 

 seedling grown in culture solution), 

 though a cut shoot (e.g. a Willow twig) 

 will serve to show that the amounts 

 absorbed and transpired are roughly 

 equal. By increasing the temperature 

 of the air, transpiration may be in- 

 creased until the plant loses more water 

 than it absorbs ; if transpiration is then 

 diminished by covering the apparatus 

 with a bell- jar, more water will be ab- 

 sorbed than is transpired. 



An apparatus of this kind merges 

 into the forms called Potometers. 



Fig. 47. Apparatus for 

 estimating the Absorp- 

 tion of Water by the 

 Roots, and the Loss 

 by Transpiration from 

 the Leaves. 



276. Potometers. By making the 



;? 



graduated tube in Fig. 4 very narrow, 

 and sealing into the larger vessel a large 

 and actively transpiring plant, we could 

 measure with greater accuracy and convenience the passage 

 of the water through the plant. This is the principle of the 

 potometer, of which almost endless forms have 

 devised. 



