METHOD FOR DETERMINING TRANSPIRING POWER 303 



tests on plant leaves show unexplained variations of several 

 per cent. On the whole, it may be concluded, that the time 

 period for a slip at any given temperature may be as satisfac- 

 torily obtained by calculation from the observed period at an- 

 other temperature, as by actual tests made at the given tem- 

 perature. Indeed, it is practically certain that calculations for 

 field temperatures, made from the average of a large number 

 of tests in the laboratory, will prove to be more nearly correct 

 than most single field tests, or even averages of a number of the 

 latter; these tests can be carried out more easily and with greater 

 precision in the laboratory than is usually possible in the open. 



PERMANENT STANDARDIZATION OF THE PAPER SLIPS 



Considering that the simple temperature relation dealt with 

 in the last section may be regarded as estabhshed, it follows that 

 the testing of the paper slips in the field may now be generally 

 dispensed wdth, thus doing away with the need of a convenient 

 field instrument for this test. Each composite sUp is to be 

 thoroughly tested in the laboratory, under known temperature 

 conditions, and from the average time period derived from a 

 number of such tests a correction coefficient is to be calculated 

 for each temperature at which field observations are to be car- 

 ried out. The standard evaporating surface is not to be taken 

 to the field at all. 



Using the porous cup apparatus first described in this paper, 

 it is not a very arduous task to make a large number of deter- 

 minations upon each single composite slip. The sfips are first 

 numbered on a corner of the under surface of the more intense 

 blue standard (that is, on the side which hes against the leaf 

 in actual use, the side on which no black bars are \dsible). Rec- 

 ords are kept by slip numbers, as the tests are made, and, finally 

 the average time period for color change of that particular sHp 

 at 20°C. is found by calculation, and is expressed as a single 

 number characteristic of that slip. Knowing this number and 

 also the air temperature at which any field test is being made, 

 the time period for this slip at the latter temperature (on the 



THE PLANT WORLD, VOL. 19, NO. 10, 1916 



