WATER RELATIONS OF SAND-DUNE PLANTS 169 



Despite the difficulties associated with field measurements and the 

 interacting complex of changing factors, these exploratory studies were 

 planned to provide some basic data concerning the diurnal changes of 

 water deficit, transpiration rate and stomatal behaviour in a range of 

 species, and thus elucidate characteristic differences in the water relations of 

 plants of different sites. 



EXPERIMENTAL METHODS 



The choice of plants for the experiments was made so as to include some 

 representatives of the vegetation of the dune slopes and of the 'slacks'. A 

 plentiful supply of leaves of suitable size and shape for weighing was 

 required and it was also desirable that the epidermis should strip readily 

 for stomatal studies. The most extensive investigations were with Senecio 

 jacohaea L., Cynoghssum officinale L. and Hydrocotyle vulgaris L. (all amphi- 

 stomatous species) ; a number of other plants was also studied. 



The various methods used to investigate features which affect the water 

 balance of the plants are considered in turn. 



(a) Measurement of the Water Content of the Soil 



Soil samples were collected from freshly excavated holes, at various sites, 

 in 2 in. X I in. specimen tubes, sealed with Vv^axed corks and tape. The 

 samples were dried to constant weight at 8o°C, and the water content 

 expressed as a percentage of the weight of the oven-dried soil. 



(b) Physical Factors of the Environment 



The chief physical features assessed were temperature, relative humidity, 

 light intensity and the rate of evaporation ; measurements were made at 

 frequent intervals. 



Air temperature was taken as the dr)'-bulb reading of a whirhng psychro- 

 meter, which was swung rapidly at approximately i m above ground 

 level for about 20 seconds. The relative humidity was estimated from the 

 depression of the wet bulb. 



Light intensity was measured by means of an EEL Lightmaster photo- 

 meter, graduated in three ranges — o-io, o-ioo and o-iooo ft candles 

 (lumens/sq. ft). The sensitivity of the cell to different wavelengths is 

 recorded by Ashton (1958). To extend the range of the photometer to 

 approximately 10,000 ft candles, a cHp-on neutral density filter was 

 employed. Although Hght intensities are expressed in this study in terms of 

 foot candles, it is clear that these units are arbitrary, and that the measure- 



