WATER RELATIONS OF SOME PSAMMOPHYTES 197 



Corynephonis canescens, on the other hand, is an 'atlantic' plant adapted 

 to the water conditions of maritime dunes. Long periods of drought are 

 quite unknown there, and thus no adaptation to permanent drought could 

 develop among the plants in that area. Although the anatomy of Coryne- 

 phonis canescens has adapted itself to sandy soils, the plant has not lost its 

 ability actively to regulate its water turnover. In the more arid conditions 

 of Central Europe it usually restricts its water output, as shown by the 

 depression in the midday and afternoon hours. This controlling of the loss 

 of water is probably due to the closing of the stomata. If it is so, it must 

 result in reduced gas exchange and dimnished rate of photosynthesis. That 

 would constitute a serious drawback for the plant during prolonged 

 periods of drought, since it would reduce its vital activity, and the plant 

 would not be able to survive in competition with others. Indeed its area of 

 distribution in Central and Eastern Europe does not extend further into the 

 interior of the continent. The daily variations in transpiration of plants 

 supphed with water, without any noon depression, and the enhanced rate 

 of transpiration of the control samples during a rainy day conclusively 

 indicate that, only when sufficiently supphed with water, Corynephorus 

 canescens is able to retain its full vital activity. Direct evidence for the 

 diminished rate of photosynthesis of plants growing on dry soils and for 

 the enhanced rate of sugar production, after the plants were supphed with 

 water, was submitted in another paper (Rychnovska, 1961). 



From the phytogeographical point of view, Helichrysum arenarium 

 occupied an intermediate position between the two above-mentioned 

 species. It is a continental boreomeridional European plant whose area of 

 distribution extends westward up to Belgium and the Netherlands and 

 northward to Denmark and the southern part of Sweden (Meusel, 1943). 

 It is highly probable that the water relations of this plant are adaptive to a 

 considerable degree. As shown by its transpiration curve, the intensity of 

 transpiration is relatively high and actively controlled. During the hot 

 midday hours the rate of transpiration is temporarily diminished, but in 

 the early afternoon it again reaches a high level. As the stomata were 

 apparently closed during the first night, the rate of transpiration fell to 

 zero in both groups. The second night, characterised by higher tempera- 

 ture, provided better conditions for active transpiration, although the 

 atmosphere was saturated with water vapour to a considerable degree. The 

 rate of transpiration of the samples supplied with water is higher than that 

 of the control group, even though the noon depression is also present. The 

 plant is able to endure prolonged periods of drought, because its rate of 

 photosynthesis is comparatively low and not immediately affected by 



