WATER RELATIONS OF SAND-DUNE PLANTS 



177 



The records of air temperature, relative humidity, Hght intensity and of 

 observations on wind indicate that the greatest opening of the stomata and 

 the highest rates of transpiration precede the peak evaporation rates by 



S-80- 



20 



15 



10 y 



5I 



10 



o 



75 



50" 



in 



z 



ui 

 25? 



I 

 O 



3a.m 



Fig. 3. The diurnal changes in transpiration rate, stomatal aperture and water deficit 

 in Seneciojacobaca. Some of the results of an experiment of 16 June 1959 are plotted. 



The upper part of the figure shows transpiration rate (• •), mean stomatal 



aperture (Q Q) and water deficit (A A)- Changes in some of the chief 



physical factors are illustrated in the lower part of the figure: air temperature 

 (A A), relative humidity (O O), light intensity (• '•). 



about 2 hours. The correspondence of the greatest transpiration rate with 

 the maximum stomatal opening is striking; the decrease of transpiration 

 rate at a time when environmental conditions are tending to increase 

 evaporation is strongly suggestive of stomatal regulation. It may be noted 



