192 



MILENA RYCHNOVSKA AND JAN KVET 



and the thermometer stood rather high at I4°C. A drizzle began at about 

 6 o'clock in the morning of the third day and developed to a mild shower 

 lasting up to 9 o'clock. The air humidity at the beginning of the second 

 night was also higher than that of the first. The variations in soil tempera- 

 ture are also in harmony with this pattern, although the extreme values 

 of soil temperature were recorded 1-2 hours later than those of air. 

 Evaporation values obtained from the Piche evaporimeter present only a 

 crude picture of the pattern of atmospheric conditions by which the 

 evaporation rate is influenced. This rate is higher on the first day than on the 



Fig. I. Microclimatic data in the period between i June to 3 June, i960. Upper graph: 

 Variations in temperature of air 5 cm above ground (full line) and of soil at the depth 

 of 5 cm (dotted line). Middle graph: Variations in relative air humidity. Lower graph: 

 Water evaporation from the disc of Piche evaporimeter (cm^). The fifty hours of 

 experiment are marked on the .v-axis. 



second and its values are on the whole inverse to those obtained from the 

 psychrometer. 



The daily variations in transpiration among the control group o£Festuca 

 dominii (Fig. 2) were in almost perfect harmony with the physical conditions 

 of evaporation. The noon maximum of the second day was higher than 

 that of the third day, and the night transpiration of the first night was 

 higher than that of the following night, in full accordance with lower air 

 humidity at the beginning of the night. Except for a depression in the first 



