i84 A.J. WILLIS AND R.L.JEFFERIES 



decline curves, the results of the present study appear to give a fairly good 

 indication of the daily march of transpiration in a number of sand-dune 

 plants. However, a full analysis and interpretation of the course of trans- 

 piration is difficult in view of the many varying physical factors involved 

 and of changes in certain features (especially stomatal apertures and leaf 

 water content) of the plants themselves. When rates of transpiration are 

 compared it must be reahsed that here the rates are expressed on a weight 

 and not an area basis; as discussed by Kramer (1959), the relative rates of 

 different species may be quite different when expressed on a different basis. 

 However, many of the plants of this investigation have somewhat similar 

 surface/ weight ratios. 



Under conditions of high evaporation in summer, plants of the dune 

 slopes and of the low-lying 'slacks' often show high transpiration rates 

 (15-20 mg/g/min). In Senecio jacobaea, for example, the water content of 

 the leaf may be completely replaced in 45 minutes. These rates are con- 

 siderably greater than the maximum rates obtained by Hygen (1953) under 

 standard conditions, but fairly similar to those found by Bosian (1933-34) 

 whose methods of investigation were in many respects Hke those employed 

 in the present study, transpiration being estimated from quick weighings 

 of cut shoots. For example, Bosian (1933-34) records rates of about 15 

 mg/g/hr or more for the 'xerothermic' species Artemisia campestris, Teucrium 

 chamaedrys and Helianthemum chamaecistus in summer (late May) on the 

 Kaiserstuhl near Freiburg. However, the highest rates of transpiration 

 recorded on Braunton Burrows are considerably lower than some of the 

 highest reported, under summer conditions, by Evenari and Richter (1937) 

 for plants in the Wddemess of Judaea, and by Oppenheimer (1951) for 

 plants of other hot, semi-arid countries. 



It seems likely that the transpiration rates reported in the present study 

 may provide a better indication of the rates of the plants of the dune 

 slopes with regard to their own microcHmate than of the plants of the 

 'slacks'. This is because the detached leaves o£ Senecio jacobaea are, between 

 weighings, supported on the balance in conditions similar to those normally 

 experienced by the leaves on the plants, whereas the low-growing leaves 

 of Hydrocotyle vulgaris are removed from their microclimate of rather 

 moist, still air of the floor of 'slacks' to somewhat drier conditions for 

 weighing. The rates given for H. vulgaris may, therefore, tend to be too 

 high. These difficulties do not, however, obscure the important differences 

 in the course of transpiration of the plants of the dune slopes and of the 

 'slacks'. As shown for Senecio jacobaea and Cynoglossum officinale, the trans- 

 piration rate reaches a peak in the morning, and although the rate of 



