WATER RELATIONS OF SAND-DUNE PLANTS 



179 



ment no large changes in stomatal apertures were observed; however, it 

 is of note that the widest apertures correspond with the greatest transpiration 

 rate, and the stomata are partly closed during the early morning and the 

 late afternoon when rates of loss of water vapour are low. The water 

 deficit is seen to be low at dawn, but to develop rapidly to a value of about 



Fig. 4. The diurnal changes in transpiration rate, stomatal aperture and water deficit 

 in Cynoglossiwi officinale. A selection of the results obtained on 21 June i960 is plotted. 



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



(A A), mean stomatal aperture (Q O) and water deficit(A A)- Below 



are given data for air temperature (A A), relative humidity (O O) and 



light intensity (• •). 



