110 SEAWEEDS 



by the division of their apical cells, and, at the 

 margin of a sorus, where they may be seen in an 

 early stage of development, push off and cause to 

 exfoliate the original cuticuloid gelatinous layer of the 

 epidermis. In Lithoderma sporangia of both kinds 

 occur in sori, but on different plants. The plurilo- 

 cular bodies, which may be presumed to give rise to 

 gametes, occur as lateral off-shoots from special 

 branches arising in turn from superficial cells. 

 These gametangia are either a single row of cells, 

 or more frequently several rows, and are more or 

 less cylindrical in shape. The unilocular sporangia 

 are terminal bodies in fact transformed superficial 

 cells, and are mostly obovate. The actual life-history 

 has not been followed, but the relationship with 

 Ealfsia cannot be doubted. The general relation- 

 ships of the order are vague, but probably Chor- 

 dariacece of which Ralfsiacece may be degenerate 

 allies, and Ectocarpacem (especially Ascocychui) exhibit 

 the most distinct claims. 



The Geographical Distribution extends from the 

 polar seas to the tropics, and though the species 

 are always few, the maximum is attained in the 

 North Atlantic. The distribution of the six or 

 seven species of Lithoderma is of limited range so 

 far as is at present known. They occur, however, 

 both in fresh-water and in the sea. The marine 

 forms are found in the Arctic Sea, North Atlantic, 

 and Mediterranean, while the two fresh- water forms 

 occur in the south of France, Germany, and Sweden. 

 L. fatisccns has recently been discovered on the 

 British coasts. 



