WATER RELATIONS OF SAND-DUNE PLANTS 



183 



Deficits of some 10% or more have been recorded in, for example, Lycopsis 

 arvensis L., Potentilla anserina L. and Ammophila arenaria (L.) Link. On the 

 other hand the leaves oiCarex arenaria L. from dune slopes do not usually 

 show very high deficits; an average figure for midday in June is 5%. 



Transpiration rates greater than 12 mg/g/min have been observed in 

 Potentilla anserina and Filipendula ulmaria, with 'cuticular' rates of about 10% 

 of these values. Losses from, for example, Viola hirta L. and Carex arenaria 

 (midday values of c. 5 mg/g/min) are, however, somewhat lower; several 

 experiments indicate that in C. arenaria 'cuticular' transpiration may be 

 more than 10% of the total. Low transpiration rates have also been found in 



Fig. 7. Changes in transpiration rate of Ammophila arenaria. Transpiration rates are 



plotted on a weight (• •) and an area basis (0---0); rates of evaporation 



are also shown (A A). The results were obtained on 24 June i960. 



Salix atrocinerea (3-7 mg/g/min during the middle of the day). In Ammophila 

 arenaria rates of transpiration expressed on a weight basis are low, as shown 

 in Fig. 7. The leaves of marram have a high proportion of dry weight to 

 fresh weight; however, the trends of transpiration are similar when the 

 rates are expressed on an area basis. In this experiment inrolling of the leaf 

 was noticeable from 9 a.m. onwards; it is of interest in this connection that 

 although rates of evaporation increased substantially after 9 a.m. there was 

 no corresponding increase in transpiration rate (see Fig. 7). 



DISCUSSION 

 In spite of the difficulties involved in weighing leaves under the natural 

 conditions of plant habitats, and in calculating transpiration rates from 

 13 



