i86 A.J. WILLIS AND R. L.JEFFERIES 



reduction of pore size, is a feature characteristic of these but not of 'slack' 

 plants, in which the transpiration losses and the stoniatal apertures are more 

 uniform for much of the day. Survival of plants on the dune slopes may in 

 part depend on efficient regulation of transpiration under desiccating 

 conditions, especially in plants where losses may potentially be very 

 considerable. In some plants, for example Carex arenaria, on the other 

 hand losses may never be very great, whatever the conditions of evapora- 

 tion. Sometimes special features, such as the inrolling of the leaves of 

 marram, may to some extent regulate transpiration losses. 



In plants of the dune slopes complete stomatal closure leads to con- 

 siderably diminished transpiration losses. Estimates of 'cuticular' transpira- 

 tion during conditions of fairly rapid evaporation usually range between 

 0-6-0-9 mg/g/min for plants such as Senecio jacohaea and Cynoglossiim 

 officinale. These represent rates of less than io% of the total transpiration. 

 Similar reductions of transpiration losses by stomatal closure were recorded 

 by Hygen (1953) for plants of dry situations in Central Norway. On the 

 other hand, he showed that for mesophytic species 'cuticular' losses were in 

 general between 10-20% of the total; corresponding values were observed 

 in the present study for a number of plants of the 'slacks'. 



Ecological characterisation of plants by means of transpiration rates 

 derived from decline curves has been attempted by Hygen (195 1, 1953). 

 who indicated that the 'standard product' {Eg.Ec, where Eg is the transpira- 

 tion rate under standard conditions in water-saturated leaves with stomata 

 open, andEc with stomata closed) may be used as an index of xerophytism. 

 He demonstrated the usefulness of this parameter with studies on species of 

 Vaccinium, the xerophytic V. vitis-idaea having the lowest values. In the 

 present investigations, calculation of the 'standard product' under compar- 

 able conditions of high evaporation for Senecio jacohaea and Cynoglossum 

 officinale gave values of about 14 in both of these plants, whereas for 

 Hydrocotyle vulgaris it was almost double this (about 27). If (as other observa- 

 tions of the present study suggest) the results for these species may be taken 

 as representative, values of the 'standard product' appear to be a useful 

 means of characterising the plants of the dry dune pasture and of the 

 'slacks' in terms of their transpiration rates, the more xerophytic nature 

 of the plants of the dry dune slopes being indicated on a quantitative basis. 



As might be expected, water deficits in plants of the dry dunes are often 

 large, and although this deficit is frequently to some extent reduced over- 

 night, full turgidity may not be regained by the next morning. In a period 

 of drought, there may be, as discussed by Slatyer (1957). a progressive 

 increase from day to day in total soil moisture stress, and an associated 



