WATER RELATIONS OF SAND-DUNE PLANTS 171 



torsion balance as soon as possible after cutting (usually within i min), 

 and make a series of subsequent weighings at known times. When there 

 had been a heavy dew, the leaves used in transpiration studies in the early 

 morning were blotted dry before the initial weighing. It was necessary to 

 shield the balance from wind; for this a box was used with a perspex front, 

 which was removed between readings. Trials showed that the results 

 obtained when the leaves remained on the hook of the balance were usually 

 fairly similar to those obtained when the leaves were removed between 

 weighings and suspended in the open. Only on very hot days and under 

 windy conditions were the differences considerable (most of the experi- 

 ments of this study were carried out when there was httle wind). By using 

 a torsion balance with a scale of o-ioo mg, and by employing appropriate 

 counterbalance weights, it was possible to weigh to an accuracy of o-i mg. 

 The timing was accurate to one second. 



The form of the transpiration dechne curve of leaves investigated usually 

 conformed to that described by Hygen (195 1). At first there is a rapid, 

 fairly uniform transpiration rate (the 'stomatal phase'), the decline follow- 

 ing a rectilinear course; this is succeeded by a phase of decreasing trans- 

 piration rate (the 'closing phase'), which is followed by a fmal rectilinear 

 portion of the decline curve (the 'cuticular phase') when the rate of loss of 

 water vapour is low. In the experiments of Hygen the leaves were brought 

 to a condition of full turgidity initially, the stomata being assumed fully 

 open; by the end of the second ('closing') phase the stomata were regarded 

 as completely shut. Although in many of the experiments of the present 

 study carried out during the day the leaves were not fully turgid, and the 

 stomata not fully open, decline curves showing initial and fmal portions 

 approximating to straight lines were often obtained; however, the initial 

 rectilinear portion was frequently short. On the other hand, the decline 

 curves for leaves detached in the early morning or during the evening are 

 not so readily interpreted in terms of three phases, as these are not sharply 

 defmed. Some examples of curves obtained at different times of day are 

 given for leaves o{ Senecio jacobaea in Fig. i. The decline curve for the leaf 

 detached at about 1 1 a.m. indicates three phases, but these are less clear for 

 the leaf detached in the evening, when the initial rectilinear phase is short. 

 Very different is the curve for the leaf excised at about 6.30 a.m., as the 

 transpiration rate is rather low initially and fairly uniform for a consider- 

 able time (a prolonged steady rate of transpiration of leaves excised in the 

 early morning has been often observed in the present study). The major 

 differences in the curves are clearly in part referable to different physical 

 conditions at different times of day and in part to differences with respect 



