174 SOME RECENT BESE ARCHES IN PLANT PHYSIOLOGY 



water by transpiration, and that of elaborating or per- 

 mitting the penetration of substances suitable for main- 

 taining high osmotic pressures, should be considered as 

 factors governing the distribution of desert plants. 



SALT-MARSH, SAND-DUNE, AND ESTUARINE PLANTS. 



Much precision has been lent to the ecological studies 

 of Harshberger (1911) by his use of a hydrometer, with 

 thermometer attached, in mapping out the distribution of 

 plants in marshes and estuaries. The instrument is port- 

 able and readily used, and in this respect is greatly superior 

 to chloride titrations, freezing-point, or electrical con- 

 ductivity measurements, as a means of making comparative 

 measurements of salinity. 



The hydrometer can also be used in rough estimations 

 of the salinity of soil water, being very suitable for noting 

 the progress of attempts to reclaim salt marshes. 



Harshberger has proved that the degree of salinity is 

 the determining factor in the distribution of salt-marsh 

 plants, although the texture of the soil, its aeration, and the 

 lines of drainage, are also of importance. 



By means of conductivity determinations, Scofield (1905) 

 traced the salt-water limits of wild-rice, a grass of much 

 economic importance. The maximum salinity which it 

 was capable of resisting in the field was that represented 

 by an approximately 0-03 normal solution of sodium 

 chloride, the water of the sea in the same region having 

 a conductivity equal to that of a 0-28 normal solution. 

 Thus it was shown to be mere waste of time to attempt 

 to sow the wild-rice in many tracts of land which were, to 

 all appearances, quite suitable. 



It was demonstrated by Drabble and Lake (1905, 1907) 

 that salt-marsh and sand-dune plants are physiologically 

 xerophytes, and possess relatively high osmotic pressures. 



