WATER RELATIONS OF SOME PSAMMOPHYTES 195 



transpiration is several hours late. The rate of transpiration during the 

 second night somewhat exceeds that of the first night. 



The curve of the third plant under study -HelichrYsum arenarium (Fig. 4) - 

 reveals two distinct maxima during the dayhght hours and a depression at 

 noon. The rate of transpiration fell to zero during the first night while in 

 the second night noticeable evaporation of water was observed. The 

 intensity of transpiration among plants supplied with water was enhanced, 

 but the pattern of the curve is similar. 



DISCUSSION 



A comparison between the two transpiration curves oiFestuca dominii and 

 the climatic factors points to the unmistakable fact that this plant has lost . 

 its abihty actively to control its transpiration. As reflected in the morpho- 

 logical and physiological properties of the plant, its water turnover is 

 adapted to permanent water deficiency. The rate of transpiration is hmited, 

 that is to say, under favourable conditions it reaches definite maximum 

 values and is not able to surpass them. The transpiration curve runs parallel 

 to that of evaporation. This is exemphfied by the two curves in Fig. 2, 

 showing the same pattern of transpiration, whether the plants were suppHed 

 with water or not. No increase in the rate of transpiration was observed 

 after the plants were supphed with water. The rate of transpiration during 

 the night was also in agreement with the physical conditions, especially 

 with air humidity, during our experiment. Dry periods more pronounced 

 than those recorded during our experiment would probably result in 

 perceptibly reduced loss of water. Corynephoms canescens, on the other 

 hand, is far more active in controlling its rate of transpiration. Although 

 its transpiratory system does not provide for extensive variations, a 

 depression in the rate of transpiration takes place during hot midday hours 

 of a dry 'continental' day, when the plant suffers from water deficiency. 

 No depression, however, occurs during a wet 'oceanic' day. If the rhizo- 

 sphere of the plant holds enough water a definite increase in the rate of 

 transpiration takes place and no noon depression can be observed. During 

 the night hours, the water loss of plants growing on a dry substratum is 

 probably hmited to cuticular transpiration; if, however, the plant is 

 supphed with a sufficient amount of water, the vital factor of transpiration, 

 which depends on temperature and not on relative air humidity, plays a 

 more active role than the physical factor. The second night was warmer 

 than the first, and thus the rate of transpiration of the plant was somewhat 

 higher. 



