198 MILENARYCHNOVSKA ANDJANKVET 



variations of humidity (Rychnovska, 1961). The water turnover is intensi- 

 fied when the soil contains a sufficient amount of water. An interesting 

 fact should, however, be noted in this connection. If the plant is supplied 

 with a large amount of water, the water content of its tissues is smaller 

 than that in the control group of plants. The question here is, whether the 

 regulating mechanism does not partially fail in plants adapted to dry 

 habitats when supphed with an excessive amount of water and whether a 

 paradoxical situation of disproportionate water loss does not arise in this 

 case. All this wiU have to be verified by further research. 



The present paper lends further support to the idea that many phyto- 

 geographical problems can be solved on the basis of physiological and 

 ecological analysis, and that, although the ecological factors are of only 

 local apphcabihty and complex in character and although the plant species 

 have a wide range of adaptabihty (Meusel, 1943), it is possible to ascertain 

 the causal fundamentals responsible for the distribution of the various 

 species of plants. On the other hand, it would be a mistake to limit the 

 explanation of the causes of plant distribution to one factor only ; it is 

 necessary to investigate the selected species of plants in relation to all the 

 important factors (Boy sen-Jensen, 1949). In our case, however, the other 

 factors have already been analysed and the results reported in another 

 paper of mine already quoted. We may thus be allowed to conclude that, 

 with respect to specific edaphic conditions, it is the water relations of these 

 three plants, those water relations which resulted from the plants' adapta- 

 tion to the climatic conditions in the centre of their area of distribution 

 and which became established during the phylogenesis of these plants, 

 that on the boundary of their area of distribution constitute the decisive 

 phytogeographical factor. 



REFERENCES 



Boysen-Jensen, p. (1949) Causal Plant Geography. Kgl. Danske Videnskabernes 



Selskab, Biologiske Neddelelser , B. XXI, Nr. 3. Kobenhavn. 

 Meusel, H. (1943) Vergleichende Arealkimde. Berlin. 

 Rychnovska-SoudkovA, M. (1961) Corynephorus cancscens{L.) P. Beauv. Physiologisch- 



dkologische Studie einer Pfianzenart. Rozpravy CSAV. 71. Praha. 

 Stocker, O. (1956) Die Abhangigheit der Transpiration von den Umweltfaktoren. 



Handbuch der Pfianzenphysiologie, Bd. III. Berlin. 



