TRANSPIRATION 201 



would lie within 2 or 3 mm. of the surface of a leaf on which 

 it might be placed. Note the position of the pointer on the scale 

 and' set the instrument on a leaf laid flat on a table. If the lower 

 surface of the leaf is uppermost, and the stomata are open, the 

 film begins to straighten within ten seconds and the amplitude of 

 the movement will correspond to the amount of watery vapor 

 thrown off by the leaf. If the upper surface is tested no move- 



FlG. 101. Arrangement of elements conducting water in a leaf with netted veins. 

 After Sachs. 



ment will result unless open stomata are present. This test is 

 a very delicate one, and vapor from the breath of the operator, 

 or moisture from the hands may interfere with results unless care 

 is exercised. Allow the index to return to zero after every test. 1 

 274. Transpiration. The walls of all cells contain more or less 

 water of imbibition, and when exposed to an atmosphere not sat- 

 urated with watery vapor some of the liquid evaporates, a process 

 which continues until checked by surface tension. Loss of water 



1 MacDougal. A new hygrometer suitablefor testing action of stomata. Torreya, 

 1 : 16. 1901. 



