Exchange and Movements of Gases and Liquids. 69 



thus necessary as a medium for the transport of salts, and about 

 ninety-eight per cent, of the energy of the sunlight which falls 

 upon the plant is used in the work of lifting it. It may be 

 seen that a more or less continuous loss or transpiration of 

 water is an urgent necessity with the plant, and it may also be 

 said that the leaves of every species are adapted to carry on 

 this function in a manner 

 more or less exactly suited 

 to its environment. Not 

 only is the structure of the 

 leaf fitted to throw off an 

 amount of water defined 

 by the supply furnished the 

 plant, and under the condi- 

 tions existing in the atmos- 

 phere, but the stomata or 

 organs of transpiration are 

 under the direct control 

 of the organism. 



63. Evaporation 

 from an artificial mem- 

 brane. Soak a piece of 

 bladder or strong parch- 

 ment in water for an hour, 

 then stretch over the mouth 

 of a thistle tube, and wrap 

 tightly with a small, firm 

 cord. Seal the edges of the 



membrane to the glass with soft gelatine. Invert the tube and fill 

 with boiled water. Close the end with the finger, and place in 

 an upright position, with the open end immersed in a dish con- 

 taining a few centimeters of mercury.. Care must be taken that 

 the membrane is free from perforations and that no air is ad- 

 mitted into the tube. Water should evaporate from the mem- 



Fig. 48. Apparatus to demonstrate lifting 

 power of transpiration. After Oels. 



