170 A.J. WILLIS AND R.L.JEFFERIES 



ment of light intensity in absolute units in the field is of great complexity 

 (Evans, 1956). 



An indication of the rate of evaporation was obtained from the changes 

 in v^eight of saturated filter paper suspended from a torsion balance on the 

 dunes. Pieces of filter paper, usually of 12-65 sq. cm or 4 sq. cm area, were 

 saturated with water, weighed initially and again at appropriate intervals 

 (usually about i min) for some 10 min. In the experiments reported here 

 the rate of loss of water vapour was seen to be uniform over this period. 

 Estimates made in this way at every hour were found to give a satisfactory 

 indication of the diurnal changes in evaporation. 



(c) Estimation of Water Deficits 



The water deficits of leaves at different times of day were measured by the 

 method described by Yemm and WiUis (1954), which is similar in principle 

 to that of Weatherley (1950). After an initial weighing of the leaf, im- 

 mediately after excision, it was transferred to a saturated atmosphere, and 

 its cut petiole allowed to stand in distilled water. The leaf was then weighed 

 at appropriate intervals until it was fuUy turgid; the dry weight of the leaf 

 was fmally determined. The water deficit is here expressed as the amount 

 of water absorbed by the leaf to reach fuU turgidity, calculated as a per- 

 centage of the water content of the fuUy turgid leaf (Stocker, 1929). Under 

 field conditions on very hot dry days difficulties were encountered in 

 maintaining a saturated atmosphere, and it was found convenient, parti- 

 cularly when high deficits were involved, to allow the leaves to pick up 

 water overnight, a final weighing being made early the next morning. 

 However, many plants restored their deficit in less than 3 hours, the course 

 of water uptake being as illustrated by Yemm and WiUis (1954, Fig. 3). 



(d) Measurement of Transpiration Rates: Transpiration Decline 

 Curves 



Transpiration rates were estimated from weight losses of detached leaves. 

 The decline in transpiration of the cut leaves was determined from weigh- 

 ings at frequent intervals ; in this way transpiration dechne curves of the 

 type described by Hygen (195 1, 1953) were obtained. There were, how- 

 ever, several important differences from the procedure of Hygen ; the 

 leaves were not initially always completely turgid, nor were the stomata 

 always open. Furthermore, the conditions under which the determinations 

 were made were those of the variable environment. 



The method adopted in this investigation was to cut a leaf of convenient 

 size (100-500 mg) from the plant in its natural habitat, weigh the leaf on a 



