The whiteand mobile dunes are nearest the sea, where 

 the wind brings new sand and often breaks down the already existing 

 dunes. These change, when the vegetation grows more close to gray 

 dunes, with at first a thin, low, often unstable cover, which however 

 is able at last to suppress the high dune grasses. 



Several of the herbs, mosses, lichens and dwarf shrubs mentioned 

 may appear in the form of associations, and the dunes farther 

 from the sea especially will often show a Calluna association. 



II. Sandy salt-marsh. On the north end of the island we have 

 quite a different development. Here we find large sandy flats (sestuaria, 

 Vades, Watten), covered by water at flood-tide, dry at ebb-tide. 



1. Outermost is the sand -worm flat with thousands of 

 characteristic excrement-heaps formed by Arenicola marina. This worm 

 lives in an U-shaped tube with branches up to 40 cm. long. The en- 

 trance is marked by a funnel-like depression caused by the worm in- 

 gesting the sand and water; the excrements are expelled at the other 

 opening. 



2. In more shallow water live millions of a small crab, Corophium 

 grossipes, which also dwells in U-shaped but much smaller tubes; at 

 flood-tide it emerges from the tube and we can notice its tracks on the 

 sand. Many birds seek their food here on these Corophium-sestuaria. 



3. Belt of the sand algae. This begins in the Corophium- 

 aesluarium but it extends farther inland on bottom which is 

 not so often covered by the water. It is especially Myxophyceae 

 of many genera, which bind together the sand grains and form a layer 

 ca. 2 5 mm. thick about 1 mm. below the surface. Many diatoms and 

 some few other algae also live here intermingled and the tunnel-digging 

 insects occur on the drier of these flats. 



4. An association of iron sulphide bacteria forms black 

 masses of sand under the surface. 



Nearer the land the flowering plants begin. 



5. Farthest out is a Salicornia association (Salicornietum S. 

 herbaceae), very open with blue-green alga bottom. 



6. The Glyceria association (G 1 y c e r i e t u m Gl. maritimae). 

 These species collect the drifting sand and form low tufts. 



7. Successively many other plants immigrate, first especially h a- 

 lophytes and others, such as Cakile maritima, Cochlearia danica, 

 Agrostis alba f. stolonifera, Juncus Gerardi, Glaux maritima, Cochlearia 

 danica, Atriplex hastata, A. patula, Festuca rubra, Honckenya peploides, 

 Matricaria inodora, Odontites rubra, Plantago coronopus, P. maritima, 

 Potentilla anserina, Sagina maritima, Spergularia salina, Triglochin mari- 

 timiim, Trifolium fragiferum, T. repens etc. Gradually a low, dense 

 carpet is formed on the sand, a marshy meadow, a s an d marsh, 

 which affords nourishment to the cows of Nordby. 



Many other, less halophilous plants, herbs and mosses, occur on 



