Clare Island Survey — Marine Algae. 15 65 



the banks of rivers and where the water is brackish a zone of V. coronata 

 is the characteristic v egetation near high-water line, but in Achill Sound that 

 species is absent, and Microcoleus cMhonoplastes is the dominant alga. 



Vegetation of Vertical Banks. 



Vertical peat-banks, abutting on the shores of Clew Bay and Achill, 

 exhibit a similar vegetation to that described later for the salt-marsh formation 



D 



(seep. 82). 



Bhodochorton floridulum Association. 



This association, though not hitherto described, is very distinct and of 

 common occurrence in many parts of the British Isles, including the Clare 

 Island district. It is characteristic of sandy shores where rocks are present, 

 and is found on moderately open as well as sheltered coasts. Bhodochorton 

 fioridv.lv. m is the most important of a group of finely branched, upright-growing 

 algae, which retain quantities of sand, if they do not actually bind it together, 

 Other members of the group are : — 



Cladophora arcta, Ceramium strictum, 



Polysiphonia fibrata, C. ciliatum. 



In suitable localities the association extends from the Fucus spiralis belt 

 down to low-water line, but it also descends into the sub-littoral region, being 

 found abundantly between the islands in Clew Bay. It prefers flat or slightly 

 sloping shores ; on steep shores the sand is washed away, and the plant is 

 practically absent. Being of small size, Bhodochorton cannot fix sand on the 

 same scale as phanerogamic plants, nor does it occur on sand apart from rocks 5 

 except in very sheltered areas, or where it is submerged and beyond the effect 

 of rough water. It is, however, capable of covering flat or sloping rocks with 

 a dense mossy growth ; and of carpeting the intervening sand-spaces so that 

 they completely resist normal wave-action. In its young state the plant 

 is probably attached to rock, but as growth proceeds, sand collects, and the 

 abundant production of rhizoidal filaments helps to retain it. The growth 

 forms either pads which may measure a foot or so across, or vast sheets more 

 or less interrupted, but extending at times over a very wide area. When well- 

 grown, the Ehodochorton sward is about 3 cm. high ; but in favoured spots 

 pads up to 6 cm. have been met with. Though sometimes exposed to the full 

 light, especially on the lower part of the shore, Bhodochorton grows best with 

 partial shade. But even when exposed to light it is the ends of the branches 

 only that feel the full effect of sun and air, as the base of the plant is 

 covered with sand. 



P.I.A. PEOC, VOL. XXXI. I 15 



