122 H. JONSSON 



axis dies away from below and in this way branches, or systems 

 of branches, with well developed rhizoids are detached from the 

 parent-plant and become independent individuals (cf. Jonsson, 32). 

 The rhizoids of one individual become entangled with those of 

 others, and in this way the basal parts of the entire vegetation be- 

 come matted together. In addition to this, the lower branches are 

 hook-shaped, or bent downwards and outwards; thus it easily hap- 

 pens that they are entangled between the branches of other indi- 

 viduals, which further increases the matted condition. The upper 

 hranches are directed obliquely upwards, without being entangled 

 with the branches of other individuals. 



An A. a/frescens-association occurs everywhere along the coasts, 

 but on very exposed coasts the species generally grows dispersed. 

 Here it occurs in separate tufts which are, no doubt, composed of 

 several individuals, densely matted together below, but branching 

 upwards in various clusters, which are themselves densely matted 

 together by their hook-shaped branches; the branch-systems above 

 are, as usual, free. These characteristically matted branch- clusters 

 are probably an effect of the heavy beat of the waves; the matted 

 Acrosiphonia "tufts" will not be able to resist the force of the waves 

 and, even at an early stage, will become divided into very densely 

 matted branch-clusters. 



Acrosiphonia albescens occurs both in the upper and the lower 

 littoral zone, while A. incnrva belongs to the lower littoral zone, 

 but yet also extends upwards into the pools of the upper littoral 

 zone, and below low-water mark. 



Spon go morphetum. Spongomorpha vernalis grows very so- 

 cially, but never forms such a dense vegetation as does Acrosiphonia, 

 although it is sufficiently dense to characterize the spot. The species 

 has been found in only a few places; it occurs in abundance at 

 Grotta, where it grows both on a stony substratum and also as 

 epiphytic vegetation in the Corallina-Gigartina-he\l and in the Polu- 

 siphonia u/reo/a/a-assoeiation of the lower littoral zone. 



A Cladophoretum formed of Cladophora sericea f. (see under 

 Tide-Pools) also belongs to this association. It forms a densely 

 matted belt in pools high up in the upper littoral zone. As a rule 

 it follows the edge of the water round the entire pool. 



Cladophora rupestris ought also to be considered to belong to 

 the Acros/p/jo/u'a-association. 



An association similar to the Atrosz/j/iom'q-association is, no 



