(3) In regard to transpiration ranges, two groups can be distinguished: steno- 

 hydric plants with a rather narrow range of transpiration intensity (e.g. Haloxylon 

 salicornicum, Zygophyllum dumosum, etc.) and euryhydric plants with a wide range 

 of transpiration intensity (e.g. Artemisia monosperma, Zilla spinosa, Retama duriaei, 

 Calligonum comosum), 



(4) Fig. 3 indicates the distinction of three main categories of plants according 

 to the summer values of the transpiration rate. These are megahydrics (high trans - 

 pirers) from 500mg/g.h. upwards, microhydrics (low transpirers) showing values up 

 to 350mg/g.h. and mesohydrics with values intermediate between both. It is clearly 

 shown that the micro- and mesohydric types are dominant among the permanent 

 desert vegetation while the megahydrics are rather exceptional. Indeed, only ex - 

 ceedingly deep rooting plants belong to the latter category. 



(5) I do not agree with Stocker (1933) that there is no relation between habitat 

 and transpiration. Fig. 4 shows clear differences in the summer transpiration rates 

 between various plant communities. This difference is particularly striking when 

 for each plant community one or two dominants are chosen that display the highest 

 percentage of the permanent plant coverage, as shown in Fig. 4 (broad column). 

 Comparing various plant communities of the desert with those of the Mediterranean 

 region one finds striking differences between the two in late summer transpiration 

 intensities. 



Osmotic Pressure of Cell Sap 



As in transpiration so also in osmotic pressure desert plants are greatly hetero- 

 geneous. 



Taking the data presented in Fig. 5 as a basis, at least three groups of plants 

 can be distinguished: 



(a) Plants of hydro- or automorphous salines, distinguished by their high osmotic 

 pressure caused by the accumulation of soluble salts in the cell vacuoles. In 

 this group of plants the values range between 40 and 150 atm. In spite of the 

 high pressure they are all low transpirers. 



(b) Plants with low or medium summer values distinguished by their high trans- 

 piration rate and their exceedingly deep roots reaching permanent sources of 

 soil moisture. These include Acacia spp. and Tamarix spp. 



(c) The rest of the plants are true desert plants showing maxima of osmotic values 

 between 16 and 72 atm. Comparing these values with those available for Medi- 

 terranean maquis one finds no marked differences between these and the desert 

 plants. The following plants are particularly worthwhile mentioning: Medi- 

 terranean, (from Walter, 1951), Olea europaea (52 atm.), Rhamnus alaternus (37 

 atm.), Phillyrea angustifolia (60 atm.), Lonicera etrusca (53 atm.), Pistacia tere- 

 binthus (42 atm.). Desert, (my data), Artemisia monosperma (16 atm.), A Herba 

 alba (29 atm.), Zilla spinosa (17 atm.). Anabasis articulata (58 atm.), Haloxylon 

 persicum (56 atm.), Calligonum comosum (17 atm.), Retama roetam (26.atm.). 



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