Bva p oration . Evaporation was measured in thf^s^ sti;dies 

 by means of cylindrioal porous cup atmo/aeters located so as 

 to have the same local exposure as the plant cultures. The 

 atmometer mountings were provided with -mercury valves so 

 arranged as to prevent the entrance oi' rain. They were 

 read at intervals ifif' about two weeks, the da"*"es of reading 

 being the same as those on -''h'oh observations were made on 

 the plants. After every reading each atmometer cup was re- 

 moved and. replaced by another that had Just been standard- 

 ized. The use'i cup was subsequently restandardized so as 

 to detect any change in the coefficient of the cup conse- 

 quent upon its exposure. 'j7h.en the re standardization 

 ed a change in the coefficient, the m.ean of the original co- 

 efficient and the coefficient found upon restandardization 

 was employed to reduce the reading to the Livingston cylin- 

 drical standard. The evaoorati on readings should therefore 

 be directly comparable to other mei^surements related to the 

 same standard. 



As has been pointed out by Livingstonj the porous 

 cup atmometer is somewhat similar to plant foliage ih the 

 way in which its evaporati^ng surface is exposed to the sur- 

 roundings. It may therefore be supposed tha" the transpi- 

 ration from the plants for any period should be approximately 

 proportional to the evaporation from the atmometer, except 

 in so far as the transpiration rates may be influenced by 

 conditions within the plant. The work of Briggs and Shantz 

 indicates that evaporation from small open oans or porous 

 cups is inflnenced by the same external conditions, and in 



v'3. 







^ 3ee their paper, cited on p. -15 



