156 The Plant World. ^ 



taste, but it is clear that high osmotic pressure is not necessarilly 

 correlated with the presence of sodium chloride nor with acidity. 

 To determie the nature of the solutes to which the osmotic 

 pressure is due must remain a problem for the laboratory, not 

 for the field. 



Turning our attention now to the influence of the sub- 

 stratum upon the magnitude of the foliar pressure, it appears 

 that the highest pressures are usually manifest in plants grow- 

 ing in dry soil, and that extremely high pressures are not met 

 wnth in moist, non-saline soils. The pressures exhibited by the 

 plants growing in irrigated land are, however, usually somewhat 

 higher than those met with in the humid regions. In this 

 connection Fitting calls attention to Stange's experiments with 

 water cultures, *which showed that with nutrient solutions 

 of high concentration the osmotic pressure of leaves and roots 

 is much higher than otherwise. Thus non-halophytes like wheat, 

 bean, pea, etc., (which in weak solutions or on ordinary moist 

 soil have pressures about isosmotic with a 0.25 normal solution 

 of potassium nitrate) develop pressures about equivalent to 

 that of a 6 normal solution when grown in concentrated 

 liquid media or saline soil. Salt-accumulating forms, such 

 as Plantago maritima, Sa/sola kali, etc., (usual pressure, in non- 

 saline soil, about equivalent to that of 0.25 normal sodium 

 chloride solution) exhibit pressures about isosmotic with a 0.76 

 normal solution of the salt, of even higher, when grown in con- 

 centrated media. The irrigated soil of Biskra is much more 

 highly charged with salts than the cultivated soils of the humid 

 regions, and our author believes that this fact accounts for 

 the occurrence of rather high pressures in the fields of the des- 

 ert oasis. While wheat has a usual pressure balancing about 

 0.25 normal potassium nitrate solution, and Stange succeeded 

 (through the use of a concentrated substratum) in increasing 

 this to that of a 0.6 normal solution, the pressure of this plant 

 as grown at Biskra is found by Fitting to be equivalent to that 

 of a 0.6 to 0.8 normal solution of the same salt. It is i .ter- 

 esting to note, as does Fittin^, that the litt'e annual AnagaUis 

 coerulea exhibits pressures little if at all higher in the most 



♦Stange, B., Beziehungen zwischen Substratkonzentration, Turgor iind Wachstum 

 bci einigen phanerogamen Pflanzen, Bot. Zeitg, 50. 253. 1892. 



