PLANT COMMUNITIES. 



91 



silvery leaves and large golden-yellow blossoms ; the lovely Sea-bindweed, 

 Sea-purslane — both with fleshy succulent leaves, and many others. (See 

 fig. 16.) 



Nearer the sea, on the sand about high-water mark, are found the 

 Sea-rocket (Cakilc maritima), Saltwort (Salsola Kali), and various species 

 of A triplex. 



On muddy shores, especially in estuaries, the Grass-wrack (Zostera) 

 and Glass-wort (Salicomia) are often extremely abundant, and are com- 

 pletely submerged at high water ; while higher up, and out of reach of 

 all but the highest tides, occur such species as Triglochin maritimum, 

 Suceda maritima, Glaux, Aster TripoUum, Sea-plantain, Scurvy-grass, 

 and Sea-lavender. 



The loose shifting sands of sand-dunes are occupied chiefly by Grasses 

 such as Ammophila, Elymus, and Agropyron, all of which have glaucous 

 leaves and long creeping rhizomes, which help to bind the dunes together. 

 The Sand- sedge {Car ex arenaria) is also an efficient sand-binder. 



Few woody or shrubby plants occur, but along our east and south- 

 east coast the Sea-buckthorn {Hippophae rhamnoicles) is abundant in 

 places. 



Besides the plant associations thus far considered — associations which 

 depend on the varying adaptability of plants to different external con- 

 ditions — there are others in which the determining conditions are the 

 adaptability of plants to one another for purposes of support or nutri- 

 tion. To the former set belong the climbing plants and epiphytes ; to 

 the latter those which are associated in a symbiotic union, or in a state 

 of parasitism. 



Climbing plants have slender straggling stems with long internodes, 

 and in the absence of a suitable mechanical support would trail along 

 the ground and so become covered up and smothered by other plants. 

 According to the nature of their climbing organs we may distinguish : — 



(1) Twiners, those whose main axis twists spirally round the up- 

 right supporting stem of another plant. Such are the Hop, Convolvulus, 

 Tamus. 



(2) Climbing plants proper, which develop lateral prehensile organs 

 or tendrils, which are very sensitive to contact and coil round a suitable 

 support (Pea, Vine, Bryonia). 



(3) Boot-climbers, which develop adventitious aerial roots on their 

 stems which insinuate themselves into the crevices and roughnesses of 

 the bark of trees (Ivy). 



(4) Glamberers or Scramblers. — These develop lateral organs not 

 sensitive to contact, such as recurved spines, prickles, or hooked hairs, 

 which enable the plant possessing them to clamber over and support 

 itself upon other plants (Roses, Brambles, Goosegrass). 



Epiphytes differ from the above in that they are not rooted in the 

 ground, but merely "perch " upon other plants. They hold on to the 

 support by means of rhizoids, or by adventitious roots, and usually 

 exhibit contrivances for collecting and storing water. Their mineral 

 food is obtained from the dust which settles upon them and — together 

 with some organic matter— from the decaying vegetable debris which 

 collects among their roots. 



