178 THE PLANT "LIFE OF MARYLAND 



of these are species which will bear a large proportion of salt in 

 the substratum, and the contrast between halophytic and mesophytic 

 species belonging to the same genus (e. g. Solidago) is striking. 



The absence of Salicornia from this and other associations of this 

 region is worthy of note, since in many maritime estuaries it is ex- 

 tremely common. Perhaps the explanation lies in the moderate per- 

 centage of salt present in the Chesapeake Bay and the Potomac, 

 which is 1.638 as compared with 3.631 for the Atlantic Ocean, 

 and this explanation is borne out by the observation that Sali- 

 cornia occurs in the marshes at Ocean City. It is certain that 

 Salicornia can grow in water which is even more salt than the sea, 

 as in places where sea water is exposed to evaporation. Probably 

 the competition of other plants has driven it out of the less salt 

 marshes. 



Spartina polystachya association. — This species is found in more 

 or less pure stands as a zone bordering most of the tidal streams near 

 their mouths. It prefers less salt than the preceding species, and 

 grows with its roots and the lower part of its stems between tide. 

 Its tall robust habit, and the prominent clusters of spikes are highly 

 distinctive, and are shown in Plate XVII., Fig. 2, from the 

 border of Mcintosh Run, St. Mary's County, on which stream it 

 stretches for about half a mile, replacing Spartina stricta at a short 

 distance from the mouth of the stream. The growth here is so 

 dense that other plants are excluded, but frequently Scirpus robus- 

 lus, an extremely coarse looking sedge, is present in small numbers. 

 When this association occurs somewhat farther up a stream, in 

 places where a smaller proportion of salt is present, several other 

 plants may accompany the Spartina as in the marsh on Chaptico 

 Creek, St. Mary's County, represented in Plate XVIIL, Fig. 1. 

 In this figure a conspicuous plant of Hibiscus moscheutos may be 

 seen, to the left of which are several specimens of Asclepias incarnata 

 and Strophostyles uvibellata; these together with the Spartina grow 

 on a point between the main stream and a tributary. 



Scirpus olneyi association.- — In much the same habitat as the pre- 

 ceding occurs the sedge known locally as "three-square grass." 

 Though it frequently grows on the border of a stream and thus fills 



