RESISTANCE BY LEAVES TO "WATER LOSS 



7 



paper used in these experiments, placed upon such vials as the 

 one just described, loses its blue color in from 13 to 30 seconds, 

 depending of course on the temperature. Neither the water 

 tests nor the leaf tests may be carried out with direct sunshine 

 falling upon the apparatus. A small shade is sufficient. 



Placed against the moist air, which is continually supplied with 

 water vapor from the adjacent water surface, the paper begins 

 inmiediately to absorb moisture, the rate of this absorption de- 

 pending upon the rate at which aqueous vapor can reach the paper 

 surface, and upon that at which the moisture is fixed through the 

 hydration of the cobalt salt. The former of these rates should 

 be constant for any arrangement of vial and water surface, and 

 for any given temperature of the water. The latter rate falls 

 continuoush" as water enters the paper. Finally, when all blue 

 color has disappeared, a certain amount of water has been ab- 

 sorbed, which depends on the quantity of salt held in the paper, 

 the thickness of the latter, etc. It is not requisite, however, to 

 determine this amount, it being a constant which falls away 

 in the calculations. Of course absorption continues after the 

 color change has been produced, since the paper is by no means 

 saturated at the critical point of the indicator, but of this further 

 absorption there is no visible evidence. 



The use of the free water surface is attended with several diffi- 

 culties. To avoid some of these, ^Ir. E. ^I. Harvey de\'ised a 

 simple apparatus by which the hygrometric paper can be brought 

 to lie within about a millimeter of the surface of a piece of water- 

 saturated blotting paper. " The wet paper lies beneath a sheet of 

 vulcanite which is perforated b\^ two circular openings about 8 

 mm. in diameter and about a centimeter apart. A glass plate 



' A saturated paper of this sort evaporates water for a long time at the same rate 

 as does an equivalent area of free water surface. On this point see the following: 



Renner, O., Zur Physik der Transpiration. Ber. d. d. Bot. Ges., 29: -4:51-547, 

 1911. Reviewed in Plant World 14: 195-196, 1911. 



Livingston, B. E. and Brown, W. H., Relation of the daily march of transpira- 

 tion to variations in the water content of foliage leaves. Bot. Gaz. 53: 309-330, 

 1912. 



The paper in Mr. Harvey's apparatus is kept constantly saturated by dipping 

 below into a small reservoir of water. 



