■!^ 



104 H. JONSSON 



mass. Rosenvinge (63, p. 201) has pointed out that this mass 

 must be of importance as a reservoir for water during low-tide. 

 Ulothrix flacca also has very thick cell- walls, especially in the fruit- 

 bearing filaments (cf. Rosenvinge 1. c). The remaining species also 

 are rather thick- walled. 



Although the {//ozTirzx-association is distributed to an extra- 

 ordinary extent, yet it cannot be expected to be found everywhere. 

 It prefers that part of the littoral zone which is rocky and stony, 

 and grows luxuriantly on a somewhat exposed coast, and even, 

 indeed, on one which is very exposed. It also extends right into 

 the fjords, if the nature of the shore is favourable to it. 



The succession of the associations of filiform algae is seen most 

 distinctly on vertical rocks on somewhat exposed coasts, where the 

 /7/o?7in;r-association appears uppermost; but where the substratum 

 is uneven — a talus of debris or irregularly heaped-up fragments 

 of rock — the zonal division of the community becomes less ap- 

 parent, and it may then well happen that the Bcmgz'a-association 

 extends above the t//oz7irz':r-association. 



The Bangia-association prefers the rocky part of the litto- 

 ral zone, and vertical rock- walls in particular, and is only rarely 

 found in the stony part of that zone; it occurs normally below the 

 f/ZozTzrz'x-association , but where the latter is absent the Bangia- 

 association is not infrequently the algal vegetation which reachest 

 highest. In many places it has an extremely wide horizontal dis- 

 tribution, and is often perceived at a considerable distance as a 

 reddish-brown band in, and at the edge of, the water. This Bangia- 

 belt may attain a considerable breadth, as much as three feet, and 

 it often displays several shades of colour. Thus, in a less exposed 

 spot facing the south, I have seen the uppermost part coloured 

 green, the middle part brownish and the lowest part a fresh reddish- 

 brown. This is certainly connected in some way with the long 

 period of drought which, when the weather is calm or the wind 

 blowing off the land, may well continue from neap-tide till about 

 spring-tide. During the period of drought Bangia behaves somewhat 

 similarly to Ulothrix flacca. The filaments are rather long, as long 

 as 10 cm., and are very closely packed together; on vertical rocks 

 they hang straight down during low-tide, and are, on the whole, 

 protected by their structure and manner of growth from too 

 severe desiccation, in the same way as has been mentioned in 

 connection with the t//of7zrza>association. The Bangia-belt is, as a 



