486 BOTANICAL GAZETTE [December 



counteracted or even destroyed by convection currents in the 

 water surrounding the leaf. It is not thought, then, that the 

 increase in transpiration in the water lily is due to temperature 

 differences, but to other causes. The arrow-head is a transition 

 plant, growing, with some modifications, on land, as well as in water, 

 and it is interesting to note that this plant, even when growing in a 

 hydrophytic habitat, behaves more like a typical land plant in the 

 matter of its transpiration. 



(e) Relation of certain physical factors to transpiration 



An inquiry into the various external factors which exert an 

 influence on transpiration leads to some interesting conclusions. 

 It is not our purpose to consider all of the possible factors which 

 may or may not affect transpiration, nor to consider every factor 

 separately, but rather to consider the combined effect of the more 

 important factors as they occur under natural conditions in the 

 field. Relative humidity, temperature, and wind are quite gener- 

 ally recognized as the principal factors affecting evaporation from 

 an open water surface. In so far as plant transpiration is largely 

 a physical process, omitting, therefore, the phenomena of root 

 pressure, increased protoplasmic activity, and other purely physio- 

 logical mechanisms, it would be expected that these three factors 

 would be of principal importance in plant transpiration. 



i. Temperature. — The graphs of transpiration shown in 

 charts VIII-XIV show the effect of temperature on the rate of 

 transpiration. With few exceptions the temperature during the 

 night was much less than that during the day. The transpiration 

 graphs show this same relationship as between night and day, the 

 rate during the day being high and correspondingly low during 

 the night. That temperature is not the only factor is shown by 

 the fact that the highest rate of transpiration recorded for most 

 of the plants occurred during the day of September 4, at which 

 time the day temperature was somewhat low. Likewise, during the 

 periods of highest day temperature, that is, September 1,2, and 15, 

 the transpiration was only moderate. Moreover, on the nights of 

 August 31 and September 3 and 4 the wind velocity was very 

 nearly constant, from 3-5 miles for the period, and the percentage 



