ADAPTATIONS OF THE SALT MARSH VEGETATION. 365 



The modifications of structure which can be attributed with some 

 degree of assurance to the action of these and other factors may be 

 referred to three principal categories: 



1. Structures preventing free access of water to submerged parts. 

 The most striking adaptation of this class is the persistence* of the 

 basal sheaths in grasses and grass-like plants. This is beautifully 

 exemplified in Spartina stricta maritirna, the bases of whose culms 

 are tightly enveloped by the closely imbricated, large papery sheaths. 



2. Structures serving to hold the plant fast in the watery, incoher- 

 ent soil. These take the form in most cases of long rootstoeks, 

 creeping through the mud and sending up erect leafy and flowering 

 branches at frequent intervals. Of such character are the under- 

 ground parts of the great majority of salt-marsh plants. Often, as 

 in Typha, Spartina polystachya, Phragmites, and other large plants 

 with a considerable weight to be supported, the rhizomes are very 

 long and large. Annual plants, which are few in the salt marshes 

 (Aster subulatus and Salicornm lierbaeea being the only common 

 species), have less need of firm anchorage in the soil. 



3. Structures serving to reduce the evaporation of water from the 

 leaves, which would otherwise be excessive, as the chiefly herbaceous 

 salt-marsh vegetation, unsheltered by large, woody plants, is directly 

 exposed to the drying effect of the wind and to the strong light and 

 heat of the sun. The necessity for such protection is the greater 

 because, as is well known, roots absorb water with difficulty when it 

 contains any considerable percentage of salts in solution, owing to the 

 decreasing force of endosmosis when the degree of concentration of the 

 external water approaches that of the cell sap. In order to compensate 

 the reduced absorbing activity of the roots it is obvious that the escape 

 of water from the upper part of the plant, especially from the leaves, 

 where it is normally greatest, must be correspondingly checked. 1 



1 To the plants of the salt marshes, growing in a soil that is impregnated with 

 salt (in solution) and subject to partial or total inundation twice a day by brackish 

 water, it is of the utmost importance that the supply of water grudgingly yielded 

 to them by the substratum should be guarded in every possible way. But even 

 were it easier for salt-marsh plants to absorb water, it would not be to their 

 benefit to take it up in great quantities, for this would result in an increased 

 accumulation in the cells of sodium chloride, which would tend to exert a disturb- 

 ing and even harmful intiuenc j upon the processes of assimilation and metabolism. 

 Some plants endure the presence of greater quantities of salt than do others, but 

 none can continue to live after a certain limit of accumulation has been reached. 

 Even if salt-marsh plants can, as has been suggested (Diels, in Jahrb. f. Wiss. 

 Bot.. vol. 2'S, p. 316. For a criticism of Diels's experiments and conclusions, see 

 W. Benecke. Jahrb. f. Wiss. Bot. 36, 179 to 196. 1901), decompose a considerable 

 quantity of sodium chloride by means of the organic acids in their cells and 

 reunite its elements into leas harmful combinations, a limit to this process would 

 soon be reached if the transpiration current were as unimpeded as in most plants 

 of ordinary, moderately moist inland soils. Consequently the difficult absorption 

 of water, which is usually regarded as an adverse condition, would appear to be 

 positively beneficial in the case of salt-marsh plants. 



