WATER RELATIONS OF SAND-DUNE PLANTS 



175 



culty is that the volatile solutions may quickly change in composition 

 by evaporation, especially during a hot dry day. 



2 3 A 5 



INFILTRATION INDEX 



Fig. 2. The relation between infiltration index and mean stomatal aperture in Senecio 



jacobaea. 



(f ) General Procedure 



As far as was practical, measurements of physical factors, of water deficits, 

 transpiration rates and stomatal apertures of particular species were made 

 at intervals of about one hour from dawn to sunset. These data were 

 augmented by information from many shorter-term experiments. In any 

 experiment, care was taken to ensure that the leaves taken for the various 

 assessments were as far as possible comparable. The leaves used were 

 fuUy expanded, but not senescent. 

 AU times are given in G.M.T. 



EXPERIMENTAL RESULTS 



For convenience, data relating to the water content of the dune soils are 

 given first, and then the results of investigations made on particular species. 



(a) The Water Content of the Soils 



Especially during the summer, there is a sharp contrast in the water content 

 of the sand of the dune slopes and of the 'slacks'. Even in dry years the water 

 content of the soil of the low-lying parts is high, whereas on the dunes 

 themselves it is more variable, being very low in times of drought and 

 moderate in wet periods. 



It has already been shown (WiUis et al, 1959, Fig. 14) that in dry dune 

 pasture the layers of the sand above the reach of the capillary rise may be of 

 low water content in summer. In some years, however, moister conditions 



