WATER RELATIONS OF SOME PSAMMOPHYTES 191 



with water, up to 8-10% of soil moisture, while others were observed on 

 arid substratum, with 2-4% of humidity. Every 60 minutes transpiration 

 measurements were carried out by a rapid weighing followed by a 3 minutes 

 exposure in the original stand and another weighing. Light showers of 

 rain occurred between the 41st and 45th hour of the experiment. During 

 this period the plants, after being picked up, were cursorily dried with 

 blotting paper and exposed, for 3 minutes, in their original stand under a 

 small cover of plastic foil. Weighing was performed on a torsion balance, 

 the scale of which was divided into sections of i mg. Each sample consisted 

 of a bunch of leaves collected from the same tuft. When the whole tuft 

 had been used up, samples were taken from the neighbouring tuft. The 

 change of tuft is always noted in the appended figures. The base of the 

 bunch of leaves was tied up with a narrow strip of Cellux plastic band, which 

 greatly facilitated further operations with the grass. Each sample of 

 Helichrysum arenarium consisted of one terminal portion of the stem 

 bearing several leaves. Variations in the temperature of air and of soil at 

 a depth off cm, and air humidity and evaporation from the disc of the Piche 

 evaporimeter (5-2 cm in diameter, 5 cm above ground) were measured 

 every hour. At 4-hour intervals a soil sample from the root system of each 

 plant under study was taken with a narrow probe trowel and its moisture, 

 expressed as percentage of dry weight, was deterinined. When the field 

 experiment had been concluded, the plant samples were oven-dried at 

 I05°C and their water content was established. Transpiration was expressed 

 in miUigrams water loss per i g of the water content in i minute. The water 

 content was expressed as percentage of fresh weight. 



RESULTS 



Variations in the rate of transpiration and the water content in the tissues 

 of the plants, microclimatic data and soil moisture data, during the period 

 of 50 hours, are expressed by graphic methods in the appended figures. 

 Variations in the values measured, irrespective of the changes due to 

 accidental and unknown factors, were illustrated by a curve plotted from 

 moving averages of five successive measurements. 



As shown in Fig. i , chmatic conditions during the 50 hours of the experi- 

 ment varied considerably. The weather in the afternoon of the first day, 

 during the following night and the whole of the second day was of an 

 anticy clonic type, with a high maximum temperature of 32-4°C during 

 the dayhght hours and with relatively low night temperature of io-3°C. 

 During the second night, however, the sky became overcast with clouds 



