38 MINNESOTA BOTANICAL STUDIES. 



CHECK NO. I CHECK NO. II 



No. °fy g No. ~ 



Wagnera 



Icptoscpala i ig.8 9.2 i 18. 10. i 



2 jg.o 8.8 2 21.4 9.1 



3 18.8 8.2 3 19.3 8.8 



Apocynum 



androsaemifoUum i 16.4 7.8 i 18.6 10.3 



2 14.0 9.0 2 16. 1 lO.I 



3 140 8.6 3 15.6 9.3 



Fragaria 



bractcata I 19.4 12.0 i 19.8 10.6 



2 18.8 I0.8 2 19.3 10.8 



3 196 9-2 3 16.6 II. 3 



Popidus 



tremuloides i 14.4 9.4 i 15.8 10. 



2 14.2 7.84 2 15.2 13.3 



" 3 16.8 10.2 3 13.6 II .7 



Both in the main table and in the checks only slight variations 

 occur, not usually exceeding 3 mg. per square centimeter for a pe- 

 riod of 12 hours. Therefore it may be concluded that plants of the 

 tame species grown in the same habitat when tested under the 

 same physical conditions show but slight variation in transpiration 

 per unit of surface exposed. 



Periodicity in Transpiration. 



For the purpose of determining the time of maximum and min- 

 imum transpiration during a day of twenty-four hours, three series 

 of two plants in each were set up and tested. Series I were Hclian- 

 thus plants four weeks old. Series II Helianthus eight weeks old, 

 and Series III, barley plants three weeks old. 



Weighings were begun at six p. m., February 16, and made 

 hourly until six p. m., February 17. Physical factors, such as light, 

 temperature, and humidity, were made between the hours. The day 

 chosen for the experiment remained cloudy throughout, causing only 

 slight variations in physical factors. 



Curtis (Bull. Torrey Bot. Club, 1901, 28: 335) found that on 

 cloudy days when the intensity of the light was quite uniform, or at 

 least not in keeping with the time of day, the transpiration curve 

 shows a pronounced maximum near midday. It was also character- 

 ized by minor fluctuations that occurred independently of climatic 

 changes. An illumination of an electric light 900 candle power under 

 uniform external conditions demonstrated a periodicity in the tran- 

 spiration of several plants that correspond in the main with the 

 curves determined on cloudy days. 



The following table gives the loss for one square centimeter in 

 milligrams for twenty-four consecutive hours. 



