202 THE COAST-FLORA OF THE CLACTON DISTRICT.
with different places, but that in the same place it is by no means
stationary.
Most plants growing in the salt-area show definite alterations
in their tissues, notably succulence, which may be either of the
leaves or of the stem. Succulence must be regarded as a sign
of lack of water : the saline soil, even when wet physically, is
physiologically dry. The plants cannot absorb the sea-water
plentifully, because they have no machinery to get rid of the
salt, which has once entered their tissues, and must, therefore,
render transpiration as slow as possible, which is precisely the
case of xerophytism. This is the usual explanation of succulence :
Halophytes are a type of Xerophytes. Against it it may be
said that the Halophytes have neither fewer stomata than other
plants, nor are those in any way protected. Diels asserted that
succulence was a means by which the plant freed itself from an
excess of salt ; he held that, favoured by the frequently red-
coloured cell-sap, certain organic acids were formed in the tissues
which entered into volatile compounds with the chlorine. His
analyses seemed to show that, when supplied with pure water
only, a diminution of chlorine in the leaves actually occurred, no
trace of salt being found in the water itself. However, later
research appears to have disproved Diels' theory. What happens
is that the plant withdraws the salt from the actively growing
shoots and stores it up in the older leaves and in the stem.
Lesage made a series of very interesting culture experiments
which established that increase in salt tended (1) to promote
succulence (the palisade cell-layer becomes thicker, salt acting
morphologically like sunlight), (2) to diminish the amount of
chlorophyll (which accounts for the translucence of the leaves),
(3) to decrease the size of the leaves, and (4) to dwarf the size
of the whole plant. Reversing the argument, the degree of
succulence, etc., may be said to be a good test of the degree of
salinity of the soil.
The question whether the physical or chemical nature of the
soil, including moisture, is of primary importance to the plant
is the subject of a long-standing controversy among botanists.
In the psammophilous shore vegetation, the physical properties
of dryness, and looseness of particles, evidently co-operate with
the chemical action of the salt and the poverty of nutritive
substances in producing an extremely Xerophytic vegetation.