194 PLANT SOCIOLOGY 



led to the genotypical fixation of the ecologically advantageous struc- 

 ture of succulents, favored by the salt soil. Succulence, however, need 

 not necessarily be xeromorphic, even though it actually is so in 

 numerous cases, as was demonstrated by Duval- Jouve (1868) in the 

 case of S. macrostachya. 



Salt Constancy. — As in the case of lime plants, so also for the 

 halophytes, a gradation may be shown from the absolute salt-avoiding, 

 through the salt-enduring, to the salt-obligate species. In the last 

 group are numerous Gramineae of such genera as Spariina, Glycerin, 

 Atropis, Distichlis, and Sphenopus and the succulents of the genera 

 Salicornia, Arthrocnemum, Salsola, Suaeda, and Atriplex. There are 

 also the sea plankton and the coast vegetation sprayed with sea water. 

 True halophytes may, however, be grown artificially without salt, 

 according to Contejean, Grabner, and others. Their growth remains 

 dwarfed, however (c/. Keller, 1925), and only such species as Salsola 

 kali, Suaeda fruticosa, Hordeum maritimum, and Aster tripolium, 

 which are not to be regarded as absolute halophytes, are able to persist 

 for any considerable time. Nevertheless, the optimum for the growth 

 of A. tripolium seems to be at a very low salt concentration. The 

 distribution of such species upon places of low salt content is chiefly a 

 question of competition. 



The experiments of Paris with Atriplex halimus (Bequinot, 1913, 

 p. 101) indicate that there are species which thrive normally and 

 continuously only upon sodium chloride soils. Such species have not 

 only a high suction force but require also the specific ionic effects of the 

 components of the salt. From the investigations of Iljin (1925) 

 this fact may now be accepted for Na ions as well as for Ca ions. 



It has been known for a long time that some marine algae and such 

 seed plants as Zostera and Posidonia can live only in highly concen- 

 trated salt water. They show toxic symptoms following dilution. 

 The accompanying curves show the NaCl optima for some marine and 

 terrestrial halophytes (Fig. 103). The optima for the narrow curves 

 agree with the concentration of sea water (^ to }{). The terrestrial 

 halophytes show optima at lower concentrations but with wider 

 tolerance. For further details concerning the present status of the 

 halophyte problem see the work of Montfort and Brandup (1927) and 

 of Stocker (1928, 1930). 



According to this interpretation, the obligate halophytes are plants 

 which for their normal development need certain ions of the alkali 

 metals and halogens, and which, therefore, can exist and bear seed only 

 in soils containing salt. They also can live in soils where the respec- 

 tive salts have accumulated in such quantities as to be fatal to most 



