880 



PLANT PHYSIOLOGY 



are distinguished by a number of anatomical and physiological 

 peculiarities. These plants often possess fleshy stems and leaves 

 and resemble the cacti (Fig. 119). This characteristic structure 

 is hereditary, for it is reproduced by the plants when grown on 

 soils where no salts are present. With an increase of the salt 



concentration in the soil, succulence 

 increases, which after all may be 

 regarded as a measure, or as one of the 

 direct results, of high soil salinity. 

 V yj / Because of the abundance of sodium 



in/ chloride in some saline soils, large 



quantities of this salt are found also in 

 the ash of plants native to such soils. 

 Sodium chloride is not an absolute 

 physiological necessity for saline plants. 

 They can grow on soils that are not 

 saline. The experiments by Keller, 

 however, have shown that the addition 

 of this salt improves their development 

 (Fig. 120). This is one of the reasons 

 why they are able to displace from 

 alkali soils all the other plants, most of 

 which are injured by a certain concen- 

 tration of salt. Excessive amounts of 

 sodium chloride, however, can injure 

 even the most tolerant of saline plants. 

 Another group of halophytes is 

 formed of plants for which accumula- 

 tion of salts in the cell is almost as toxic 

 as for common mesophytes and the 

 roots of which possess a low permeabil- 

 ity to salts. They attain the suction 

 tension necessary for water supply by 

 means of increasing osmotically active organic substances such 

 as organic acids and sugars. This group includes various species 

 of the sagebrush, which covers large tracts of the saline semi- 

 deserts, and a number of other plants. This group grows on less 

 saline soils than Salicornia does. 



The third group is made up of plants whose root cells, like 

 those of the Salicorniaceae, are permeable to salts but which do 



Fig. 119. — Salicornia, a 

 fleshy plant from saline soil 

 {after Schimper). 



