SEA WEEDS 359 



attached by little discs to some larger weed, and from which the 

 tufts of branched fronds stand out erect. On the west and south- 

 west coasts of Britain we may often meet with the allied Crossed 

 Callithamnion (G. cruciatum), which grows on rocks, close to low- 

 water mark, that are covered with a muddy deposit. It grows in 

 tufts, somewhat resembling those of C. plumula, but its plumules 

 are arranged two, three, or four at a level, and are very crowded at 

 the tips of the branches. 



Still another section of this large genus contains weeds of a more 

 shrubby growth, with veined stem and branches jointed obscurely. 

 Of these the Rosy Callithamnion (C. roseum) is not uncommonly 

 found on mud^ shores, and especially in and near the estuaries of 

 rivers. It grows in dense dark-coloured tufts, two or three inches 

 long, with alternate branches much divided. The tetraspores occur 



FIG. 245. Callithamnion Fio. 246. Callithamnion 



roseum tetricum 



singly, one at the top of each of the lower joints of the pinnules of 

 the plumes. C. byssoideum grows on larger weeds in the rock pools, 

 and especially on Codium tomentosum (p. 353), in dense tufts of 

 exceedingly fine filaments, jointed, and branched irregularly. The 

 upper branches are plumed, and their tips bear very fine colourless 

 filaments. The spore-clusters are arranged in pairs, and the tetra- 

 spores are thinly scattered on the pinnules of the plumes. This 

 species is so very delicate in structure that a lens is absolutely 

 necessary to make out its structure. It is, in fact, impossible to 

 distinguish between the various species of Callithamnion without 

 such aid ; and many of them, particularly the species last described, 

 require the low power of a compound microscope. 



Among the other common species, belonging to the same section, 

 we may mention C. corymbosum, distinguished by its very slender, 



