246 DICTYOTEJE. PART n. 



Sea 15 to 20 miles long, and more than 600 feet 

 wide ; there is a submarine forest of it in Skapta Bay, 

 Orkney; and in passing through the sounds of the 

 western islands, as between Kerrera and the mainland, 

 there are others. The long cords always lean in the 

 direction of the tide, and must oscillate between two 

 zones of rest, one at the turn of the flood, and another 

 at the turn of the ebb. When dried the people use 

 them for fishing lines. In the Chordaria divaricata 

 both kinds of spore cysts are external, and give rise to 

 zoospores. 



In the preceding divisions of the Melanosperms the 

 fronds consist of articulated threads ; in the succeeding 

 divisions the fronds are inarticulate. The latter comprise 

 four very remarkable groups, of which the Dictyotese 

 are distinguished by a leathery or membranous frond, 

 sometimes cylindrical, but mostly flat, the surface of 

 which is reticulated and sprinkled with groups or little 

 patches of naked spores or cysts. The endochrome in 

 the cysts is sometimes quadripartite, or even divided 

 into eight parts. In one of the genera only, anything 

 like antheridia have been found. The zoospores pro- 

 duced from the quadripartite endochrome are large, of 

 a dark colour, and have two lateral cilia, while the 

 bodies in the filiform much divided antheridia seated 

 variously in the tufted threads are far more minute and 

 pale, but with similar cilia. This order obtains its 

 maximum of development in the tropical and subtropical 

 regions ; several species are found in the Mediterranean, 

 while a few occur on our coasts, and on those of North 

 America. 6 



The genus Dictyota begins the zonarioid group, whose 

 structure is very curious. Every band (lacinia) of 

 the frond terminates in a single cell, by the constant 



6 Berkeley's ' Cryptogamic Botany.' 



