134 H. JONSSON 



in many places at about low-water mark, and often covers rather 

 large stretches of rock with a dense but, at times, low vegetation. 

 As previously mentioned, it grows luxuriantly on flat rocks, without 

 any protection whatever during the period of exposure, which is cer- 

 tainly short, and indeed exceedingly short during a heavy sea. Here, 

 as in the upper littoral zone, the association occurs also on the face 

 of the rocks, and the species seems as a rule to be larger than 

 when on flat rocks. The association appears to have an extended 

 vertical distribution since, as previously mentioned, it occurs as 

 shade- vegetation or undergrowth in the upper littoral zone; further- 

 more it must be assumed that it has a larger sublittoral distribu- 

 tion than is shown by the dredgings, as it has been found outside 

 the 10-metre contour (see below). From what I have seen, the 

 stretch of shore just above and somewhat below low-water mark 

 is essentially the home of this association. It usually grows on a 

 rocky substratum and on Laminaria stems at no great depth. 



Cystoclonium purpurascens grows socially in many places 

 both on a rocky substratum and on Gigartina. Sometimes it occurs 

 in such abundance as an epiphytic vegetation in the Gigartina- 

 Corallina-belt, that it is the alga which characterizes the vegetation, 

 and not until the plants are moved aside is it seen that they are 

 attached to a living substratum. On Cystoclonium various epiphytes 

 occur, for example, Chantransia, Monostroma Grevillei and others. 



Rhodomela lycopodioides very frequently grows scattered, 

 and then plays only an insignificant role in the vegetation ; but 

 Rhodomeleta of limited extent also occur, often in contact with the 

 Polysiphonia zz/reo/afa-associations. 



Ceramium rubrum, C. acanthonotum and Callitham- 

 nion Arbuscula realty belong to this association. In many places 

 these species grow fairly socially and Ceramieta of C. rubrum and 

 C. acanthonotum and Callithamnioneta of limited extent occur both 

 on rocky substrata and on one of Gigartina; however, I think that 

 these species occur too sparsely to be termed association-formers. 



Sphacelaria radicans also belongs to this association. Often, 

 even at low-water mark, it covers flat stones with a dense vegeta- 

 tion, but though these small Sphacelarieta are considerably distri- 

 buted, yet they cannot be called associations. 



In the Faeroes a similar Polysiphonia zzrceo/ota-vegetation occurs 

 (Borgesen, 12, p. 731). 



