WATER RELATIONS OF SAND-DUNE PLANTS 173 



of the decline curves appear to fall closely on a straight line for the first 

 few minutes and to form the basis for a useful comparative estimate of 

 transpiration rate. In practice, the estimate of the rate of transpiration at the 

 time of excision is derived by extrapolation from the values for the first 

 three minutes after cutting the leaf Values based on a rather longer period 

 give similar results ; detailed examination of records for Hydrocotyle vulgaris 

 showed that the estimates of rate based on the first four minutes after 

 excision are not appreciably different from those based on the first three 

 minutes. The estimates by Hygen (195 1, I953) of initial transpiration rate 

 are based on semi-logarithmic plots of weight against time, but, as discussed 

 by Williams and Amer (1957), it is not essential to take logarithms (or 

 compute a 'reduced fresh weight'), there being a prolonged constant rate 

 phase during which transpiration is independent of water content. Wilhams 

 and Amer (1957) further point out that Hygen's 'standard product' may 

 be calculated directly from the slopes of the first and second straight 

 portions of a plot of instantaneous weight of the leaf against time. 



Transpiration rates are usually expressed on either an area or a weight 

 basis. In the present investigations the most convenient basis of expression, 

 in view of the methods used, is mg transpiration/g initial fresh weight/ 

 minute, the initial fresh weight at the time of excision being estimated by 

 extrapolation. It must, however, be noted that on some occasions the leaves 

 were essentially fully turgid but at other times had an appreciable water 

 deficit, which in extreme instances was of the order of 20%. Nevertheless, 

 comparison of results for Hydrocotyle vulgaris showed that the diurnal 

 changes of transpiration rate expressed on a fresh weight basis followed a 

 very similar course to that of rates expressed on the basis of dry weight of 

 the leaves. In a few experiments leaf areas were determined, and the results 

 can be expressed in terms of both weight and of area. 



(e) Investigation of Stomatal Behaviour 



Although estimations of stomatal changes have been attempted with 

 various types of porometer at Braunton Burrows, the two most useful 

 methods of investigating the behaviour of stomata under natural conditions 

 were found to be the examination of strips of epidermis and the infJtration 

 technique. 



Epidermal strips, fixed by the method of Lloyd (1908), were examined in 

 iodine-phenol-KI reagent (Heath, 1947) and the stomatal apertures 

 measured as described by Yemm and WiUis (1954) • Although there is 

 evidence that in Pelargonium (Heath, 1950) the stomata may open during 

 stripping and fixing, measurements made from epidermal strips do never- 



