220 G.ORSHAN 



existence in a few Mediterranean ones at least. In the Mediterranean 

 territory, however, partial death correlated with radial sphtting of the 

 plant is not common. 



Although the cut leaf quick-weighing method has many defects it still 

 seems to be the only practical method yet devised to measure transpiration 

 rates of intact plants in the fields. An analysis made on transpiration rate 

 values of a few desert plants showed that the coefficients of variance for the 

 daily averages were about io% and when parallel measurements were 

 made on 2 plants in the same stand the values were of the same order of 

 magnitude (Orshan, 1961; Zohary and Orshan, 1954). 



These data support, although do not prove, the assumption that the 

 curves in Figs. 6 and 7 represent actual transpiration rates and that both 

 the desert and Mediterranean chamaephytes markedly reduce their trans- 

 piration rates during the summer. The fact that no clear-cut reduction in 

 the transpiration-rate values was foimd for evergreen Mediterranean trees 

 and shrubs while a marked reduction in the values for the deciduous ones 

 was observed (Poljakoff, 1945 ; Oppenheimer, 1953) is supporting evidence. 



The means by which the reduction in transpiration rate during the 

 summer is brought about is not clearly understood. One has to bear in 

 mind that in the chamaephytes different leaves or stems make up the 

 transpiring body of the plant in different seasons. When the transpiration 

 rates of different leaf types were measured on the same plant in a few 

 chamaephytes during periods when both could be found intact, no 

 significant differences were found between the daily averages, although 

 differences in the daily march of the curves were prominent (Orshansky, 

 1952). This fact suggests that, while transpiration by different leaf types of 

 the same plant may be affected in different ways by the same factor, it is 

 doubtful whether these differences are sufficient to account for the striking 

 reduction in the transpiration rate of the chamaephytes during the summer. 

 It seems more probable that this reduction is simply caused by the decreas- 

 ing availabiUty of water in the soil. 



Although it is not intended to enter in this paper into a full discussion of 

 the nature of the differences in the transpiration rates between the Mediter- 

 ranean and desert plants the much higher values of the former are worth- 

 while notmg. Zohary (1959) has already pointed out that our results do not 

 agree with the views of Maximov (1929) and others that higher transpira- 

 tion rates are characteristic of desert plants. The fact that most of the plants 

 measured by us are either stem succulents or bear more or less succulent 

 leaves may account in part for this disagreement. 



The absence of differences between the relative seasonal reduction in the 



