196 MILENARYCHNOVSKA ANDJANKVET 



Helichrysum arenarintn proved to be the most adaptable plant of the 

 three. As shown by the noon depression, the plant regulates its water 

 turnover by stomatal action. Its night transpiration falls to zero, which is 

 apparently due to the closing of the stomata. When the plant is supplied 

 with water, its rate of transpiration is enhanced and the noon depression 

 is not so clearly pronounced. The rate of transpiration during the night is 

 probably also controlled by vital (rather than physical) factors. The plant, 

 enveloped in a warm environment both above and below ground, loses 

 water by evaporation, even if the atmosphere is saturated with water 

 vapour to a high degree. We did not take note of the state of the stomata 

 during our experiment. The stomata of xerophilous grasses are mostly 

 found on the upper, i.e. inner, side of the tube-shaped leaves, and thus are 

 difficult to observe by ordinary methods. This observation was not con- 

 sidered necessary for our study of the character of transpiration. 



All these facts are in agreement with phy togeographical conditions which 

 have impressed their mark on the plants during their development. 

 Festiica dominii, a 'continental' plant, one of the group of xerophytic 

 Festucae inhabiting the deserts and semi-deserts of Central Asia, has become 

 adapted to permanent drought. As permanent closing of the stomata 

 would make photosynthesis impossible, the plant appears to restrict loss of 

 water by substantially diminishing the rate of transpiration while its 

 stomata remain half open. Its rate of transpiration is thus low and depends 

 on physical factors. Stocker, (1956), reports similar conditions among desert 

 plants. As the transpiratory system of Festiica dominii adapted to arid 

 situations does not provide for an enhanced rate of transpiration, no 

 increase in the rate of transpiration occurs after supplying the plant with 

 water. Water-retention of the soil under the dense tufts of Festiica dominii is 

 also perceptibly higher than under those of Corynephorus canescens, which 

 points to the highly economical water turnover of the former plant in dry 

 periods. I have reported in another paper(Rychnovska, 1961) that supplying 

 an experimental tuft with water does not enhance the rate of sugar produc- 

 tion. That again fully conforms with our hypothesis. Although the above- 

 mentioned water relations are highly beneficial in arid situations, they 

 constitute an obstacle for the vital activities of the plant in more favourable 

 circumstances. Thus we cannot be surprised that-as the continental 

 chmate gives way to the cHmate of the oceanic type in Central Europe and 

 competition with plants of more pronounced vital activities becomes more 

 intensified-Fe5f;/cfl dominii appears more sporadically. Its area of distribu- 

 tion does not extend further westward into regions characterised by still 

 fewer climatic extremities. 



