SECTION X 



CLASS VII. PSAMMOPHYTES. FORMATIONS ON SAND 



AND GRAVEL 



CHAPTER LXVIII. OECOLOGICAL FACTORS. FORMATIONS 



Sandy soil and its qualities have already been discussed on p. 59. 

 The vegetation developing on this loose soil is invariably characteristic, 

 and owes its distinctive features to the substratum and to other physical 

 conditions, but particularly to those concerning the temperature and 

 the degree of moisture prevaihng in the substratum. Of paramount 

 significance in determining the nature of the vegetation on sand is the 

 depth of the layer of sand above the water-table ; when this layer is so 

 deep that the peculiar properties of the sand are brought into play, 

 then sand is physically dry and hears a definitely xerophytic type of vegeta- 

 tion. Here we deal solely with cases of this kind, and not with sandy 

 soil that is saturated with moisture because it fringes open water. 



The majority of sandy soils owe their origin to water, and especially 

 to the disintegrating and comminuting action of breakers, but in a less 

 degree to other agencies, such as the sun's heat which sphts stones into 

 pieces. Consequently one meets with sand along many coasts, often in 

 the form of dunes ; but one also encounters it inland (in tropical deserts) 

 occasionally in the form either of dunes or of sand-fields. 



The chemical nature of the soiP shows differences not only in com- 

 position of the grains, but also in the amount of salts present ; and in 

 the latter respect sand on the sea-coast contrasts with that lying inland. 

 Vegetation on sand bounding seas or salt -lakes is influenced by the salts 

 contained in the adjacent water, and may largely owe its xerophytic 

 character to these salts .^ 



In the north of Europe, near the sea and on sandy shores one may 

 encounter the following formations, which to some extent show a zonal 

 succession : 



1. Sand- Algae : in the aestuarium. 



2. Iron-sulphur-Bacteria. 



3. Psammophilous halophytes. 



4. Shifting, or white, sand-dunes. 



5. Stationary, or grey, dunes, 



6. Dune-heath and dry Sand-field. 



7. Dune-bushland. 



8. Dune-forest. 



* See p. 59. * See Section VII. 



