SALT TOLERANCE 161 



Ruhland estimated that there about 700 glands/mm^ of leaf 

 surface in Limonium gmelinii and these may excrete up to 1 ml of 

 liquid per hour, containing perhaps 0-05 mg of sodium chloride. 

 He found that the concentration of salt in the fluid excreted was 

 about equal to that in the leaf tissue as a whole, and concluded 

 that the glands do not necessarily perform osmotic work. His 

 concept was that accumulation of salt takes place in the mesophyll of 

 the leaf, and that a liquid rich in salt is squeezed out via the gland in 

 some way under pressure (cf. guttation). 



More recently, Arisz et al. (1955) have examined the mechanism 

 of excretion by salt glands of another plant, Limonium latifolium. 

 Like Ruhland, they made use of the earlier observation of Schtscher- 

 bak (1910) that glands attached to isolated fragments of leaf will 

 continue to function. Leaf disks were rested on filter papers soaked 

 in salt solutions, and kept in a water-saturated atmosphere to 

 prevent evaporation of the excreted fluid. At the end of an experi- 

 ment, the liquid accumulated on the upper surface of the disk was 

 collected and analysed. By measuring also the volume of liquid 

 excreted it was possible to calculate the concentration of salt. Under 

 optimum conditions, the concentration of salt in the excreted fluid 

 proved to be higher than the mean salt concentration of the leaf 

 tissue, leading Arisz to the conclusion that the glands are capable of 

 performing osmotic work. There is no reason to doubt that this is 

 the case. Illumination of the leaf disk and increase of temperature 

 increased the amount of fluid excreted, and metabolic inhibitors 

 reduced it, but none of these treatments aff'ected the concentration 

 of salt in the exudate. Very little liquid was excreted under anaerobic 

 conditions, and no chloride could be detected in it. When the 

 osmotic pressure of the medium upon which the disks were placed 

 was increased by addition of sucrose or magnesium sulphate, the 

 amount of liquid excreted was reduced, but its concentration 

 increased so that there was little effect on the total amount of salt 

 present. 



Arisz suggested that there is active accumulation of salts in the 

 excretory cells of the gland. This tends to cause uptake of water 

 from surrounding cells, and since the gland cannot swell appreciably 

 owing to the enclosing cuticle, fluid is exuded under pressure through 

 the pores. Since, on the basis of this hypothesis, the amount of liquid 

 excreted depends on the pressure built up inside the gland, and this 



