METHOD FOR DETERMINING TRANSPIRING POWER 309 



spiring power may be studied and compared for a wide range of 

 physiological types, by the method of cobalt chloride paper as 

 thus far developed. It must be remembered that the trans- 

 piring power of plant surfaces varies greatly, not only with the 

 species, but also (for the same species or variety) with the con- 

 ditions under which the plants have grown, with the conditions 

 immediately preceding the test, with the time of day, and prob- 

 ably with other conditions. 



SUMMARY 



The foregoing pages have presented improvements that ren- 

 der field tests of plant transpiring power much less arduous and 

 considerably more precise than has hitherto been the case. 

 The elimination of repeated field tests over the standard evapo- 

 rating surface, and the substitution therefor of a thermometer 

 reading and a simple arithmetical calculation, greatly increase 

 the ease with which determinations may be carried out upon 

 plants in the open. The introduction of composite paper slips, 

 including the two permanent color-standards> makes for much 

 greater precision than has been possible heretofore, and the use 

 of small desiccators for keeping the slips dry and approximately 

 at air temperatm-e, until a test is to be made, add greatly to 

 the ease of field manipulations. Improvements in the stand- 

 ard evaporating surface — now relegated to laboratory use — 

 make this apparatus much less susceptible of accidental altera- 

 tion, and render its use less troublesome, than has' been the 

 case in the past. Finally, the new cobalt-chloride paper is 

 decidedly more uniform than any we have previously used, 

 which is a very important consideration in the application of 

 this method. 



