158 The Plant World 



tion is seen to affect the organism in the same manner as does 

 actual dr}ness of the soil. 



Several salt-accumulating forms, growing in wet salt-spots 

 showed pressures equivalent to that of an 0.8 to 1.5 normal 

 solution of potassium nitrate, while the same forms in the 

 much less saline and at the same time much drier desert soil 

 exhibited preystircs isotonic with a 1.5 to 3.0 normal solution. 

 "Thus the pressure is the higjiest, not where salt may be most 

 easily accumulated, but rather in the driest positions, where 

 salt is nmch less easily available.'' (p. 249). 



This must not Ije taken to mean (a point which our author 

 does not mention) that actual dryness is more potent in increas- 

 ing the pressure of the call-sap than v:, that form of physiological 

 dryness which is due to high concentration of the soil solution ; 

 it seems to denote simply that the magnitude of physiological 

 dryness in the saline swamps does not equal that of the actual 

 dryness in the desert soil. If it were possible to obtain a non- 

 toxic solution sufiliciently concentrated, we might, on physical 

 grounds, expect to obtain the same sort of response it plants 

 growing therein as occurs in the nearl}- dry, non-saline soil. 



Forms in which sodium cholride does not accumulate 

 thrive likewise in the salt swamps, a good example being the 

 date palm {Phoenix), which there develops nuich higher pressure 

 than in the non- saline irrigated land. 



Uur author brings out clearly the point (of which I was 

 long ago convinced by superficial observations on the dunes 

 of Lake Michigan) that the sand dune is to be considered as a 

 relatively moist substratum. The dunes of Biskra possess a 

 more luxriant vegetation than that of the surrounding deiert 

 and upon them occur, together with generally distributed species 

 many forms which are rare or wanting elsewhere. I have re- 

 peatedly made a similar observation on the sand dunes of the 

 Salton Basin (^California), a region which seems quite closely 

 to resemble that of Biskra. I have also noted that generally 

 distributed desert plants often attain a more luxriant growth 

 upon some of these Salton dunes than upon the stony loam of 

 the basin floor. * 



*It must be remarked in this connection that such detert dunes sometimes owe their 

 origin to springs or moist soil. In such cases the blowing sand is more or less permanently 



