318 L- A. GIDDINGS 



the rate of evaporation and the rate of transpiration, but it is 

 equally clear that there is a distinct difference also. The rate 

 of evaporation increased in an almost direct ratio with an in- 

 crease in wind velocity. This was not true for transpiration. 

 The rate of transpiration increased with an increase in wind 

 velocity up to a certain point after which it did not continue to 

 increase in proportion to the increase in wind velocity. This is 

 well brought out by the fact that as a rule the second leaf lest the 

 greatest amount of water, the leaf having the greatest wind 

 velocity being second. From these and similar experiments 

 it seem.s that the wind increased the activity of the leaves until 

 a velocity of about 8 miles per hour was reached, but that if the 

 velocity rose above this the activity of the leaves was impaired 

 and the rate of transpiration became less. 



In the experim^ent given in detail the evaporimeter farthest 

 from the fan lost more than the one next closest. This happened 

 in some of the other experiments, but may be accounted for 

 because of currents of air circulating through the room as the 

 doors and windows of the laboratory were open. Since the 

 results from the two instruments closest to the fan were con- 

 stant, these occasional variations would not invalidate the 

 results of the experiments. 



FIELD EXPERIMENTS 



The field experiments discussed in this paper were carried on 

 at the Macbride Lakeside Laboratory during the summ>.er of 1912. 

 The detached leaves of Silphium laciniatum were used as in the 

 laboratory experiments. The stations were set up in the open 

 field where the plants naturally grew so that the plants used in 

 the experiments were subjected to the same meteorological 

 conditions as were those growing in the prairies of the surround- 

 ing country. 



In these experiments two problems were studied. One was 

 the relation between evaporation and transpiration, and the 

 other was the rate of transpiration of leaves from different 

 heights when placed under similar conditions. To bring the 

 leaves from different heights under the same conditions, they were 



