WATER RELATIONS OF SAND-DUNE PLANTS 



181 



were wide open and. increased when the stomata were closing, there being 

 a negative correlation between starch content and pore width (cf Yemm 

 and Willis, 1954); similar observations were made on Senecio jacobaea and 

 Cynoglossum officinale. The changes in transpiration rate are clearly similar 

 to those of the evaporation rate, the relative transpiration (transpiration 

 rate/evaporation rate calculated on an area basis) being fairly constant, and 



a3 



11 Ip.m. 3 



G.M.T. 



Fig. 5. Diurnal changes in transpiration rate, stomatal aperture and water deficit in 

 Hydrocotyk vulgaris. Results obtained on 18 June 1958 (solid circles) and on 16 June 

 1959 (open circles) are shown. Values for stomatal aperture are given for 16 June 



1959 only (A A); records of temperature, relative humidity and light intensity 



for this date are plotted in Fig. 3. 



declining only a httle towards the end of the day. In another experiment, 

 extending from 10 a.m. to 5 p.m. on 15 June 1959, there was also a marked 

 parallehsm of the course of transpiration and of evaporation, the relative 

 transpiration varying only sHghtly ; it tended to fall from an initial value 

 of 60% to 50% by the end of the investigation. 



Some estimates of 'cuticular' transpiration gave values as high as i-8 

 mg/g/min (about 10% of the total transpiration) under conditions of high 

 evaporation. 



