16 54 Proceedings of the Royal Irish Academy. 



A few notes are appended on the species composing the association. 



(a) Pelvctia canalicv.lata. — This is less variable than any other species. In 

 the sheltered parts of Clew Bay and in Achill Sound, it usually occurs on the 

 irregularly scattered stones, and consequently the zone is interrupted. The 

 growth is entirely flooded at spring-tides, but at neap-tides the upper part 

 of the belt is unmoistened, the plants remaining quite dry, and brittle for 

 several days in succession. Though Pelvetia may form a zone several yards 

 wide, its vertical distribution is not more than li-2 feet. It fruits freely 

 during summer and autumn ; but the growth being more interrupted 

 and not so luxuriant as on open shores, the orange-yellow band is not so 

 conspicuous. 



(b) F. spiralis. — Not only is F. spiralis found in all the sheltered parts of 

 Clew Bay, but it is capable of growing on peat and loam, and ranges from 

 such situations to the roughest and most exposed parts of the open coast 

 (cf. Borgesen, '05, p. 746). Leaving aside its growth on peat (see p. 124), we 

 find that on flat shores the plants are usually stone- attached, and do not form 

 so dense a vegetation as when growing on rock. For this reason they become 

 drier during the long exposure to the air, and are shorter and less branched. 

 When the shore is composed of rocks and large stones, the growth is more 

 vigorous ; and under these conditions the plants attain their maximum size, 

 the fronds being wide, much branched, and bearing large fleshy conceptacles. 

 This form remains as long as the conditions are sheltered, but when exposure 

 is experienced, or where a current strikes the shore, the plants become shorter, 

 though they usually retain their bushy habit. 



The belt formed is much wider than on steep open coasts, and its width is 

 proportional to the degree of slope. In Achill Sound it varies from a few 

 feet to 10 yards, and usually forms a pure growth, though F. vesiculosus and 

 Ascophyllum occasionally intrude into the lower part. The fruiting season 

 on quiet shores appears to extend over a slightly longer period than on those 

 which are more open. In Clew Bay it commences in May, and does not cease 

 till autumn. On sheltered shores also, a difference in colour is noticeable 

 during the autumn, the plants turning a bright rusty brown. The same 

 peculiarity has been observed on the south coast of England ; and it serves to 

 distinguish at a glance F. spiralis from F. vesiculosus, the latter being usually 

 yellow or only slightly brown. In some places two sharp zones of colour are 

 produced. Another feature, which on flat shores is very characteristic of 

 F. spiralis, is the presence of long blister-like swellings on the fronds. These 

 are invariably present in the inner part of Clew Bay, and are entirely absent 

 from the short forms found on Clare Island. Sauvageau has lately examined 

 F. spiralis (= F. platycarpue, Thur.) with great thoroughness ('08). 



