Discussion 



A glance at the literature on this subject published during the last three de- 

 cades shows how inadequate and contradictory is our knowledge of the water eco- 

 logy of desert plants. From the results obtained one becomes doubtful whether the 

 older view on xerophytes, as expressed by Pfeffer (1897), Schimper (1898), 'farming 

 (I9I8) and others, and so strongly condemned by Maximov (1929) and his associates, 

 is to be rejected altogether. In their effort to bring evidence for the assumption 

 that xerophytes possess a higher intensity of transpiration than mesophytes, Maxi- 

 mov and his associates used a series of plants not critically chosen as xerophytes. 

 Among other plants that Maximov considered as xerophytes were the exceedingly 

 deep rooting Alhagi and Haloxylon ammodendron, the clearly mesophytic Portulaca 

 and Zygophyllum fabago, etc. 



Our examinations clearly show that plants growing under extreme drought not 

 only show a very low transpiration intensity as compared with the less xerophytic 

 Mediterranean plants but also use pheno- ecological and pheno- morphological pro- 

 perties for further reduction of water loss. This is far from agreeing with the view 

 of Maximov on xerophytes. 



Not less contradictory to Maximov's view is the fact that among the permanent 

 desert vegetation not a single plant has been found 'with a capacity of enduring 

 wilting without injury', a character so strongly assigned by Maximov to xerophytes. 

 \^hat is very striking in desert plant life is that plants of the permanent vegetation 

 are physiologically active the whole year round; some of them flower just at the 

 end of the dry season, some set fruits. None of them are so-called 'stop and wait' 

 plants. 



The present study may thus supply substantial data for the reconsideration of 

 some aspects of the older view on the water ecology of desert plants. 'iXhere the 

 habitat is exposed to permanent or seasonal drought, the biseasonal vegetation is 

 exceedingly thrifty in its water expenditure, both in the rainy and the dry season. 

 This is well shown by the transpiration intensity values, in the phenological events 

 of the plants falling in the 'right time', and in their morpho- ecological behaviour 

 resulting in a considerable reduction of the transpiring body during the drought 

 period. The permanent vegetation is active the whole year round. 



In the permanent desert vegetation of the area concerned the following hydro - 

 economical combined types may be distinguished: 



(a) High transpiring, evergreen deep-rooters with transpiration increasing in sum- 

 mer (e.g. Acacia spp.). 



(b) High transpiring, evergreen deep-rooters with transpiration decreasing in sum- 

 mer (e.g. Tamarix spp.). 



(c) Low transpiring, biseasonal and surface reducing deep-rooters with transpira- 

 tion increasing in summer (e.g. Atriplex halimus). 



(d) Low and medium transpiring, biseasonal and surface reducing spartoid deep- 

 rooters with transpiration decreasing in summer (e.g. Retama spp.). 



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