158 



MINERAL SALTS ABSORPTION IN PLANTS 



40 



30 



I - 



10 



(a) 



200 



150 



100 



50 



c 

 o 



o 

 5 



May June July 



Aug. 



E 

 o 



24 r 



22 



20 



I 8 



1 6 



14 



I 2 



10 



/^' 



7 



A-' 





y 



-'n. 



P-, 



-i650 



-600 



550 



500 ■£• 



4) 



C 



o 

 o 



--o—. 



xr' 



(b) 



NT 



_L 



_L 



450 



-400 S 



350 



300 

 250 



o 

 5 



01 23456789 



Leaf 



Fig. 49. Regulation of salt concentration in halophytes 



Total osmotic pressure (O— O); osmotic pressure of cell sap due to 



chlorides (D D) ; osmotic pressure of cell sap due to sodium salts ( A - - - A ) 



and percentage water content ( •— • ) of a. Jimciis gerardi during a growing 

 season. Redrawn from Steiner (1934), and b. leaves of Iva oraria of different 

 ages (leaf age increases in the series 1-9) (redrawn from Steiner, 1939). 



their rate of transpiration is reduced. This led to the suggestion 

 that halophytes have characteristically a low rate of transpiration, 

 and that their structure is adapted for this purpose (cf the structure 

 of xerophytes). However, Stahl (1894) demonstrated by the cobalt 

 chloride paper method that halophytes can transpire surprisingly 

 rapidly under favourable conditions, and Delf (1911) confirmed that 

 in Salicornia sp. and Sueda maritima, the rate of transpiration from 

 detached shoots can be as high, or even higher per unit of surface 

 area than that of a typical glycophyte such as Vicia faba. Delf 



