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



prevail; for example the data given in Table i indicate that in June 1956 

 the sand of the dune slopes had a water content of some 4-8% in the top 

 90 cm. On the other hand it is clear that in June 1957 the surface layers of 

 this site were very dry, plants rooted in the top 15 cm being extremely 

 limited in their water supply. 



Table i 



The water content of the soils at Braunton Burrows. Water contents are expressed as 

 percentages of the weight of oven-dried soil 



Plants of the 'slacks', being always near to the water table, have a much 

 more constant and plentiful water supply. Only rarely does the water 

 content of the soil fall appreciably below 10% even at the surface, and for 

 much of the year the soil is saturated throughout. 



(b) The Water Relations of Senecio jacobaea 



This study was carried out on plants in their second year of development 

 growing on the dune slopes well above the level of the water table. 

 Generally the 6th-8th expanded leaves (counting from the apex down- 

 wards) were used experimentally. 



The results of a typical investigation are shown in Fig. 3 . The transpiration 

 rate was very low at dawn, and rose during the mornhig to reach a peak 

 value of almost 20 mg/g fresh wt/min before midday. Subsequently the 

 rate decreased fairly uniformly and by the end of the day was similar to 

 that at dawn (c. 2 mg/g/min). The changes in stomatal apertures in general 

 paralleled those of transpiration rate. The stomata opened quickly after 

 6 a.m. and were of greatest mean aperture at about 1 1 a.m. After this time 

 there was a progressive closure. The water deficit of the plants was con- 

 siderable {c. 8%) at dawn, and increased during the morning. There was, 

 however, some restoration of the deficit in the early evening. 



