40 PLANT ASSOCIATIONS 
Sea Whin (Salsola kali), and others are very fleshy 
plants; their leaves are small, with an impervious skin, 
their root-systems are better developed than in most 
annuals. The grasses and sedges of these places, 
such as the Bent (Ammophila arenaria), Sea Wheat- 
grass (Triticum junceum), Sea Sedge (Carex arenaria) 
have underground stems which burrow widely through 
the sand, with an extensive root-system and tufts of 
inrolled leaves beautifully protected against over- 
transpiration, and well worth microscopical exam- 
ination. . 
If we turn from the shingle beach to the salt-marsh, 
where water is very abundant, we shall be struck by 
the peculiar fact that its vegetation displays characters 
quite similar to those we have just been studying. 
How can we reconcile this with the theory that the 
peculiar characters of the shingle-beach plants are 
correlated with lack of moisture? The explanation is 
to be found in the fact that plants have difficulty in 
absorbing water if it is highly charged with mineral 
substances in solution. In the salt-marsh the heavy 
muddy soil is impregnated with common salt (chloride 
of sodium): the plants absorb it with difficulty; and 
in consequence they are faced with the same main 
problem which confronts the Sea Holly and Sea Whin, 
and they meet it in the same way. Indeed, the salt- 
marsh plants appear to be more highly specialized, 
for very few intruders from outside can venture in, 
while on the beach we may meet with many plants 
which belong to other formations growing success- 
fully, at least for a time. The salt-marsh flora is very 
exclusive, and contains but few species which we en- 
counter in other situations. Some of them are also 
