l82 



A.J. WILLIS AND R. L.JEFFERIES 



9 11 1pr 



G.M.T. 



Fig. 6. Transpiration rate and stoniatal behaviour o£ Hyclrocotylc vulgaris in relation to 

 evaporation rate. The experiment was carried out on 21 June i960. Upper part of 



figure: transpiration rate (• •), evaporation rate (A A); lovv'cr part of 



figure: mean stomatal aperture (O O), starch content of guard cells (• •). 



Records of temperature, relative humidity and light intensity for this date are shown 

 in Fig. 4. 



(e) Studies on Other Plants 



The chief results of investigations on the water relations of various other 

 sand-dune plants are outlined below. 



Plants of the 'slacks' generally have httle or no water deficit. Even on 

 hot days in summer, deficits exceeding 10% are unusual for the majority 

 of the species. For example, low deficits are normally found in the leaves 

 of plants of such different growth form as Epipactis pahistris (L.) Crantz, 

 Scirptis holoschoemis L. and Salix atrocinerea Brot. On the other hand, some 

 plants characteristic of low-lying habitats on the dunes such as Orchis 

 praetermissa Druce and Filipendula uhnaria (L.) Maxim, have been known on 

 some occasions to develop deficits of the order of 10% and 1 5 % respectively. 



On the dry dune slopes considerable water deficits are by no means 

 uncommon; under extreme conditions leaves may wilt on the plants here. 



