LICHEN COMMUNITIES^ 385 



grass that lichens begin to develop, though in the cool damp weather of 

 autumn and winter, as observed by Wheldon and Wilson^ certain species 

 associated with Myxophyceae, such as Collemaceae, may make their appear- 

 ance, among others Leptogium scotinum, Collemodiuni turgidum and Collema 

 ceranoides. Watson^ makes the same observation in his study of sand-dunes. 



When the loose sand on th)£ dunes of South Lancashire becomes cemented 

 by algae and mosses several rare Lecideae are to be found on the decaying 

 vegetation, and with further accumulation of humus Cladoniae appear and 

 spread rapidly along with several species oi Peltigera and the ubiquitous Par- 

 melia physodes. The latter starts on dead twigs of Salix repens and spreads 

 on to the surrounding soil where it forms patches some inches in diameter. 

 The association also includes Lecidea uliginosa and Bilimbia sphaeroides. 



On the more inland portions of the dunes numerous rather poorly de- 

 veloped Cladoniae and Cetraria aculeata were associated, while on the sides 

 of "slacks" or "dune-pans" Collema pulposum, Cladonia sylvatica and several 

 crustaceous lichens covered the soil. The wetter parts of the dunes were 

 not found to be favourable to lichen growth. 



Sandstede" found on the sandy shores of Riigen, from the shore upwards : 

 first a stretch of bare sand, then a few dune grasses with scattered scraps 

 of Cladoniae, Peltigerae and Cetraria aculeata. Next in order sandbanks 

 with Parmelia physodes, Cladonia sylvatica, CI. alcicornis and Stereocaulon 

 paschale. All these are species that occur on similar shores in the British 

 Islands. Sandstede adds an extensive list of maritime species observed by 

 him in Riigen. 



A very careful tabulation of lichens at Blakeney Point in Norfolk was 

 made by McLean* and the table on p. 386 is reproduced from his paper. 

 Sand, he writes, is present in all the associations and the presence or 

 absence of stones marks the great difference between the two formations 

 determined by dune and shingle. 



(i) "Bare sand, which is the first association listed, is an area practically 

 without phanerogams ; the few lichen plants, Cladonia furcata and Cetraria 

 aculeata f. acantkella, are attached by slight embedding in the soil. 



(2) Grey dune. The sand-loving lichens of the association grow in 

 company with Hypnuni cupressiforme and attain their greatest development. 

 Other species which also occur there are Parmelia physodes and Evernia 

 prunastri var. stictocera. 



(3) Derelict dune. This part of the dune formation occurs here and 

 there on the seaward margin where the grey dune has been worn down by 

 the wind. It is more shingly, hence the presence of stone lichens ; dune 

 phanerogams are interspersed and with them a few fruticose lichens, such as 

 Cladonia furcata. 



'Wheldon and Wilson 1915. ^ Watson 1918^ ^ Sandstede 1904. ^ McLean 1915. 



S. L. 25 



