WATER RELATIONS OF SAND-DUNE PLANTS 187 



increased water deficit. Under conditions when the sand is of low moisture 

 content, deficits of more than 20% have been observed during the day in 

 some plants, with values of the order of 10% at sunrise. The studies of 

 Rutter and Sands (1958) and of Sands and Rutter (1958) with Pinus sylvestris 

 are of interest in this connection, as they give evidence that when deficits 

 of 17% or more develop in this plant the soil moisture has probably been 

 reduced to the permanent wilting point. Species differ in their capacity 

 to obtain water from the soil, but near wilting conditions are clearly often 

 experienced by plants on the dry dunes. On the other hand, in the 'slacks' 

 the plants are rarely under prolonged water stress and deficits developed 

 during the day are quickly restored at night. 



Although the leaves of plants of the dry dunes may have considerable 

 water deficits at dawn, the stomata nevertheless usually open widely during 

 the morning, as the experiments on Setieciojacobaea indicate. Stalfelt(i929, 

 1932, 1955) has, however, shown that in Vicia faba and Betida puhescens 

 'hydroactive closure' of stomata begins at deficits as low as 3 and 4^0 

 respectively. Similar findings on some other plants (many of them shade 

 forms) have been reported by Pisek and Winkler (1953), but these workers 

 (see also Pisek and Berger, 1938) have further demonstrated that in sun- 

 exposed herbs 'hydroactive closure' does not usually start, under natural 

 conditions, until deficits reach some 10% or more, particularly when light 

 intensities are high. The present investigation shows that plants of the dry 

 dunes fall into the latter category, the success of these plants in a dry 

 environment doubtless being in part due to this feature of stomatal 

 behaviour. 



The results of the present study indicate some of the important differences 

 in the water relations of plants of the dune slopes and of the 'slacks'. In 

 their differing microchmates and under the different conditions of water 

 availabihty, the plants show contrasted behaviour with respect to trans- 

 piration rates, stomatal changes and the development of water deficits. 

 Ecological differences and the distinctive distribution of these plants may 

 well in part have a physiological basis in terms of water relations; charac- 

 teristically, plants of the dune slopes have an efficient means of regulation 

 of transpiration, whereas this is not essential and may be lacking in plants of 

 the 'slacks'. 



ACKNOWLEDGMENTS 



We are indebted to past and present members of the Department of 

 Botany, University of Bristol, for assistance with field work and to 

 Professor E. W. Yemm for helpful discussion. 



