2i6 G. ORSHAN 



Table 2 



Dry weight of the transpiring body in the spring of 1958 expressed as a percentage of 



the weight in the spring of 1957 



plants examined were either niicrophyllons or articulate, small branchlets 

 were weighed instead of single leaves. From each plant a branchlet was cut 

 from the southern side of the plant at a definite height with a razor blade, 

 tied with a silver wire, weighed and hung back on the plant for three 

 minutes at approximately the place from which it was detached and 

 weighed again. Two branchlets were weighed every hour and the trans- 

 piration rates calculated on a basis of fresh weight. The measurements were 

 started at about sunrise and continued until sunset. The average values for 

 each day were calculated from all the measurements carried out during the 

 day. The annual march of the transpiration rates is presented in Figs. 6 

 and 7. 



The most striking fact emerging from these figures is that the curves for 

 practically all the plants run more or less parallel. The maximal values 

 occur during April and May when there is stiU a considerable amount of 

 available water in the upper soil layers and the saturation deficit of the 

 atmosphere is high. The decrease in the transpiration rates during the 

 summer months may be attributed to the decrease in the amount of 

 available water in the soil and the increase of soil water tension. The low 

 values for the transpiration rates during the winter are caused by the low 

 saturation deficit of the atmosphere. Another striking fact is the marked 

 difference between the values for the Mediterranean and desert plants. The 

 transpiration rates for the former are far higher than those for the latter. 



When the relative seasonal reductions in transpiration rates, i.e. the drop 

 in the transpiration rates during the summer expressed as a percentage 



