48 MINNESOTA BOTANICAL STUDIES. 



TABLE VII. (Continued.) 



STATION I STATION II STATION III STATION IV 



Plants Day Night Day Night Day Night Day Night 



loss loss loss loss loss loss loss loss 



Apocynutii androsac- 



mifolium 9.2 7.6 16.2 12.2 19.9 8.5 19.9 14.8 



Wagner a 



leptosepala 12.3 8.2 18.3 9.5 17.5 10.9 16.7 9.6 



Wagnera 

 stcllata 14.2 y.y 16.2 9.6 16.4 11. 3 17.9 10.6 



Arctostaphylus 



iiva-ursi 13.6 8.8 18.6 12.5 14.7 9.1 23.0 12.5 



From this experiment it is shown still more conclusively that 

 water loss by transpiration increases with the intensity of light. 



It will be observed that Stations II and lY have the greatest 

 light intensity, the highest temperature, and the lowest humidity. 

 The plants in these stations transpired the most, showing conclu- 

 sively that this function varies with the physical factors. 



Effect of Altitude on Transpiration. 



The dwarfing of vegetation found in high alpine regions led to 

 an investigation of the effect of altitude on transpiration. Three 

 attempts were made to determine this by means of plants placed in 

 potometers, two of which were not satisfactory, owing to adverse 

 weather conditions. Simultaneous readings of physical factors were 

 taken at three different altitudes, 1,900 m., 2,600 m., and 3,800 m. 

 At the two latter, simultaneous measurements of transpiration of 

 Edwinia americana and Wagnera stcllata were taken, three plants 

 of each species being used lin each case. From an average of a 

 number of readings it was found that the light intensity was prac- 

 tically constant, that the relative humidity increased with the alti- 

 tude, and that the atmospheric pressure was decreased. The above 

 results are corroborated by Clements after investigations carried on 

 for several successive seasons. (Sci. 25:287, 1907). At 2,600 m. 

 Edivinia lost 4.4 mg. while at 3,800 m. it lost 5.6 mg. In the 

 former, Wagnera lost 3.9 mg. and in the latter, 4.3 mg. 



That reduced pressure leads to increased transpiration is demon- 

 strated in part by these data. In order to substantiate this conclu- 

 sion the following experiment was carried on : 



Six plants of Helianthus annuus were used, two under normal 

 pressure, 28.9 inches, two under a pressure of 20 inches, and two, 15 



