278 



BRITISH PLANTS 



1. Flora of Mud-Flats and Salt-Marshes. — On the muddy 

 margins of estuaries, and in low-lying ground liable to 

 periodic floods from the sea, the soil is saturated with salt- 

 water, and the plants are typical halophytes, with fleshy 

 leaves or stems (p. 88). 



Usually submerged in the water of the estuary is a 

 zone of Zostera, and above this, on the flat reaches, wholly 



or partially submerged at 

 high tide, the only vegeta- 

 tion consists of Salicornia 

 herhacea (annual glass- 

 wort), or in some parts 

 S. radicans (perennial). 

 These may form a thick 

 sward if the mud is only 

 just covered at high tide, 

 but when the water is 

 deeper the plants stand 

 far apart. The Salicornias 

 have fleshy green stems 

 and minute, adpressed, 

 succulent leaves (Fig. 113) ; 

 the cell-sap is highly con- 

 centrated, and in every 

 way the plant is excellently 

 adapted for life in this 

 extreme xerophytic en- 

 vironment. Salicornia is 

 the first inhabitant of the 

 mud - flat, and sediment 

 brought down by the river 

 is caught at the base of 

 the plants. As the mud 

 accumulates, the flat be- 

 comes higher and drier, and 

 other plants now begin to 

 colonize the ground. The earliest of these new-comers 

 are Glyceria maritima and Triglochin maritimum, which 

 sometimes form a distinct zone above the Salicornia. 



Out of reach of the highest tides the general salt-marsh 

 flora develops. The vegetation is frequently of two 

 types. In the wetter parts a salt reed-swamp develops, 

 characterized by monocotyledons with erect, narrow leaves, 



Fig. 113. — Salicornia heriacea 

 (Annual Glasswoet). (Aftee 



SOWEKBY.) 



