192 PLANT SOCIOLOGY 



are distinct differences, even though it is customary to lump the 

 vegetations of all three together under the term "halophyte vegeta- 

 tion" because of their similar external appearance. 



The Halophyte Problem. — A fierce feud continuing through many 

 decades concerning the relation of habitat, structure, and manner of 

 hfe of "salt plants" is summed up in the expression "the halophyte 

 problem. " 



As is commonly known, the majority of the "salt plants" are suc- 

 culents, distinguished by their water-storage tissues. The classical 

 theory of Schimper sees herein a protection against transpiration, that 

 is, a xeromorphic adaptation to the so-called "physiological dryness" of 

 the saline habitats. Schimper assumes that salt plants are able to 

 meet their water requirements only with great difficulty on account of 

 the highly concentrated soil solution and that for that reason they are 

 dependent upon mechanisms for the decrease of water loss. 



This attractive idea found considerable favor for a time, but doubts 

 soon arose, for it was demonstrated that many halophytes lack xero- 

 morphic adaptations and that many of them not only have a high 

 absorptive power but also transpire vigorously. According to Stocker 

 (1925, p. 13), the loss of water from typical halophytes, such as Aster 

 tripolium and Salicornia herhacea, calculated for equal time and surface 

 units, amounts to twice that of Fragaria vesca and from four to five 

 times that of Erica tetralix. At an earher date Delf (1912) showed that 

 the transpiration from Salicornia per unit of area exceeds that from 

 Vicia cracca. Thus the cause for the high resistance of S. herhacea (and 

 of several other halophytes) against desiccation does not rest upon a 

 decrease of transpiration ; rather, it is to be looked for in the increased 

 suction force of the root cells. ^ Keller (1925, p. 234) has further 

 proved that the osmotic concentration of S. herhacea rises sharply with 



1 Recent investigations by Braun-Blanquet, Bharucha, and Meier show the 

 transpiration rate of most of the obligate halophytes of the Salicornion of the 

 lagoons near Montpellier, France, to be excessively low. Among the many plants 

 of the garigues, dunes, and saline soils which were examined, in their own habitats, 

 S. macrostachya exhibited the lowest transpiration rate, but S. fruiicosa, 

 Inula crithmoides, Atriplex poriulacoides, and Juncus maritimus showed similar 

 daily transpiration curves. It would be wrong, however, to venture any 

 broad generalizations, since in the same habitat and at the same time Tamarix 

 gallica behaved very differently and showed a high rate of transpiration, higher 

 than most of the plants of the dunes and higher than Quercus ilex. It has been 

 held that Schimper 's hypothesis of "physiological dryness" of saline soil has no 

 general validity, but the studies mentioned above indicate that much caution 

 must be exercised in criticizing this hypothesis. Saline soil may not be "physio- 

 logically dry" for every species. Further investigations of the phenomena of 

 halophytic vegetation are necessary before any broad generalizations can be made. 



