OSMOTIC PRESSURE IN RELATION TO PLANT DISTRIBUTION 173 



The behaviour of plants growing in salt swamps in the 

 desert was also examined by Fitting. In a comparison 

 between the pressures found in the same plants growing 

 on cultivated land, on salt marshes, and on rocky or 

 sandy desert, he clearly shows how closely the two latter 

 habitats resemble one another as regards a physiological 

 scarcity of water. 



It has been supposed that the diminution of loss by 

 evaporation is the principal benefit derived by such plants 

 from their high pressure. This effect, while undoubtedly 

 of some importance, does not, however, seem to be the 

 principal one. It appears more probable that such high 

 pressures enable the roots to abstract water from the soil 

 against the action of its forces of imbibition and capillarity. 

 Pringsheim, too (1906), has drawn attention to this role of 

 osmotic pressure. 



FUNCTION OF ESSENTIAL OILS. 



Many desert plants are highly odoriferous, and it has 

 been pointed out by Dixon (1898) that the accumulation 

 in the intercellular spaces of the vapour of essential oils 

 of high molecular weight forms a very effective check to 

 evaporation. His experiments on " specific evaporation " 

 confirm the above suggestion. 



Giglioli (1911) has sought an explanation of the function 

 of these oils in the alterations produced by them in the 

 permeability of the protoplasm, and Osterhout (1913) has 

 shown that the first action of such substances as anaesthetics 

 is to lower the permeability of the protoplasm to salts. 

 Thus it is quite possible that evaporation may be hindered 

 both by a physiological alteration in the protoplasmic 

 surface and by a purely physical retardation of the water 

 molecules by the heavy vapours. 



Accordingly, the power to prevent excessive loss of 



