WATER RELATIONS OF SAND-DUNE PLANTS 185 



evaporation may increase later in the day there is no corresponding 

 increase in transpiration but usually a decline. This behaviour seems to be 

 representative of many of the plants of the dry slopes. On the other hand, 

 in plants of the 'slacks' such as Hydrocotyle vulgaris, there is frequently a 

 fairly high correlation of transpiration rate and evaporation rate through- 

 out the day, relative transpiration being rather high and varying only 

 httle. A similar contrast in the daily course of transpiration, related to 

 differences of water supply, was noted by Bosian (1933-34). 



In stomatal behaviour, most of the sand-dune plants investigated seem 

 to fall into the alfalfa type recognised by Loftfield (1921), the stomata 

 opening during the morning and closing at night. Under conditions of 

 high evaporation, and of water stress, early closure may, however, set in, 

 as observed for example in Senecio jacobaea and Cynoglossum officinale. 

 Midday closure of stomata has often been attributed to water stress (Loft- 

 field, 1921; Sayre, 1926), although Nutman (1937) has given evidence 

 that in Coffea arabica (a plant of shade) this closure is related to high light 

 intensity. More recently Heath and his collaborators (cf Heath, 1959) have 

 interpreted midday closure in onion as a high temperature effect, operating 

 through an increase in carbon dioxide in the intercellular spaces of the leaf 

 In the present studies midday closure has often been found to precede the 

 highest temperatures and hght intensities, and most frequently is correlated 

 with a high water deficit. Although no decisive information has been gained, 

 it seems probable that the early stomatal closure of the plants of the dune 

 slopes is largely attributable to water stress ; usually there is no re-opening 

 of the stomata of these plants until the following day although during the 

 afternoon hght intensity and temperature normally decrease. It is of 

 interest that on a few occasions at about or just after midday considerable 

 variabihty in stomatal behaviour from leaf to leaf has been observed; at 

 this time the stomata may well be in a very sensitive state, the dehcate 

 balance of turgor between the guard cells and other cells of the leaf being 

 easily disturbed. 



In many of the records there is a general parallelism of transpiration rate 

 and size of stomatal aperture. This is suggestive that transpiration rates are 

 influenced by the extent of stomatal opening over a wide range, as indicated 

 by the experimental studies of Milthorpe and Spencer (1957). Stalfelt (1932) 

 also showed that under conditions of high evaporation, especially in moving 

 air, stomata influence transpiration rate up to the widest apertures obtained. 

 Stomatal regulation of transpiration losses seen in the present studies in 

 plants of the dry dune slopes is doubtless of considerable ecological signifi- 

 cance. The diminished rate of transpiration after midday, associated with a 



