

19 20] FREEMAN— EVAPORATION AND TRANSPIRATION 199 



i . c(w) k + a 

 of the expression decreases, until when a is indefinitely 



large it vanishes; hence the value of y will equal / — / x , that is, the 



air will be saturated for the temperature /. At the same time the 



a 

 expression 7—- will approach 1; that is, as the size of the evapo- 

 rating area increases in relation to its environment, the more depend- 

 ent is the rate of evaporation upon the wind movement. On the 



c(w)k+ 

 other hand, if a decreases, the value of the expression — 



a 



increases and approaches infinity, at which point y would equal zero. 



a 



At the same time the expression 77- will approach zero. In 



other words, when the evaporating area decreases, the rate of 

 evaporation depends less and less upon the wind movement. As 

 w increases, the value of y will decrease, and vanish when w reaches 

 infinity. As w decreases, y will increase; and when w becomes zero, 

 the value of y will equal / — t T , that is, the air will become saturated 

 for temperature /. It thus appears that the formula remains 

 rational when any of its variables are projected to their extreme 

 limits. Moreover, it appears to correspond, within allowable 

 limits of experimental error, with the results obtained throughout 

 the range of conditions covered by the work of the writer. This 

 range does not include temperatures either above the boiling point 

 or below the freezing point of water. Disturbing factors which 

 might be introduced at those critical stages in the physical con- 

 dition of water were not investigated, inasmuch as they would 

 practically never be reached in investigations concerned with the 

 transpiration of living plants. 



Fig. 5 gives the results of an experiment made upon a potted 

 alfalfa plant. The temperature was raised artificially by means of 

 an electric current passed through a coil of nicrome wire. This 

 was wrapped around the cylinder into which the branches were 

 inserted from below through openings in the stopper. The opening 

 through the stopper was sealed around the stems with low 

 melting point paraffin. The temperature was controlled by regu- 

 lating the current passing through the resistance wire by means 



