SALT TOLERANCE 157 



showed that when plants of Limonium (Statice) gmelinii were grown 

 in solutions containing 10 per cent sodium chloride, the osmotic 

 pressure of cell sap rose to 165 atm, and a plant of Atriplex sp, 

 growing on a salt rich soil in Utah is said to have had sap with 

 osmotic pressure exceeding 200 atm (Harris 1934). The growth rate 

 of most halophytes is, however, retarded when the concentration of 

 salt in the soil solution exceeds about 3 per cent. 



Plants absorb relatively less salt from concentrated solutions 

 than from dilute ones, so that the external osmotic pressure tends to 

 increase relative to that of the cell sap as the concentration of salt 

 in the medium increases. Theoretically, as external concentration 

 is increased, a point X (Fig. 48) is reached at which the DPD 

 of the plant and of the medium become equal, and water absorption 

 then stops. 



In the case of some halophytes, the salt concentration of the 

 cell sap and its osmotic pressure continue to increase throughout the 

 growing season. This group may be termed "accumulators" and 

 Juncus gerardi can be cited as an example (Fig. 49a). Salt is 

 absorbed throughout the vegetative period, and since the amount 

 of water does not increase appreciably the osmotic pressure rises to 

 a high value, but it does not apparently injure the plant. In other 

 halophytes ("regulators") the osmotic pressure of the cell sap does 

 not increase progressively with age even though salts are all the time 

 being absorbed. The regulatory mechanism was investigated in 

 leaves of /va oraria by Steiner (1939), who showed that with increas- 

 ing age there is a comparable increase in both salt and water content, 

 so that the osmotic pressure of the cell sap remains approximately 

 the same (Fig. 49b). Water absorption leads to swelling of the 

 leaves and to a decrease in the surface area to volume ratio, i.e. there 

 is the development of succulence. The degree of succulence in 

 Salicornia plants has been shown to be related to the concentration 

 of salts in the nutrient medium (Table 18). Salt-tolerant plants 

 including the littoral species. Beta maritima, Cakile maritima and 

 Honkenya peploides, as well as more ubiquitous species, such as 

 Solanwn dulcamara and Plantago coronopus, all have less fleshy leaves 

 when growing inland than near the sea (Lesage, 1890). 



The idea that halophytes are adapted to conditions of "physiolog- 

 ical drought" was advanced by Schimper (1891) following his 

 observation that when glycophytes are watered with saline solutions, 



