SALT TOLERANCE 



159 



(1912) pointed out that in the majority of halophytes, the transpiring 

 surface is little protected from water loss, as there is often a thin 

 cuticle and numerous, unsunken stomata. 



In spite of these observations, it remains true that the ^x).q of 

 water loss by evaporation from succulent leaves is less than that 

 from thin leaves on a fresh weight basis. Thus by the development of 

 succulence, maximum storage of water and lowering of salt con- 

 centration is possible, with minimum increase in the rate of trans- 



Table 18. Succulence (total water content/surface area) in Salicornia 



striata (herbacea) Grown in Various Salt Solutions. 



(Van Eijk, 1939). 



piration. Delf suggested that halophytes may supplement the water 

 taken up by roots by absorption through the shoot from the damp 

 atmosphere in which the plants often grow. Conditions wliich favour 

 this process would of course, tend to reduce transpiration. 



D. Salt Glands 



Another mechanism by which salt concentration is regulated in 

 halophytes is by elimination of ions from the shoot. To some 

 extent, this occurs passively in guttation fluids which emerge from 

 hydathodes under the influence of root pressure, and are dispersed 

 by wind and rain. In addition, salt is actively excreted through the 

 cuticle from epidermal cells, or by means of specialized "salt glands". 



Salt glands vary in structure from the rather simple two-celled 

 gland found in Spartina townsendii (Fig. 50b) and in some Indian 

 halophytes (Mullan, 1931), to the intricate multicellular organs 

 described by Ruhland (1912) on the leaves of Limonium gmelinii 

 (Fig. 50a). In the latter, a single gland consists typically of a group 



