96 LAURENCIACEjE. 



&c.) the branches are usually hollow, and constricted at 

 regular intervals into joint-like portions, furnished at each 

 constriction with a diaphragm, which divides the cavity into 

 separate chambers : these chambers are filled with mucous 

 matter, through which a few vertical filaments, connecting 

 the diaphragms, are dispersed. In other genera (as Chrysy- 

 menid) the frond is also hollow and full of mucus, with a 

 few filaments dispersed through it ; but in these there are 

 neither constrictions nor diaphragm, but each branch consti- 

 tutes a chamber. Such genera evidently connect the two 

 extreme forms of the order. 



The ceramidia are sometimes imperfectly organized, and 

 reduced nearly to the structure of coccidia, the pore being 

 indistinct. The contents, however, appear to be constantly 

 pyriform spores, tufted as is normal in this kind of concepta- 

 cle. The tetraspores are sometimes collected near the tips 

 of the ramuli, but very commonly they are dispersed through 

 the branches in a very indefinite manner. On this dispersion 

 is founded the chief technical character by which the order 

 is distinguished from Rhodomelacete, but in habit there is 

 between the two groups that difference which marks a natural 

 family. 



The colours of the Laurenciacea are often fugacious. In 

 Laurencia itself, the prevalent colours are shades of purple, 

 but in most other plants of the order they are pink or lake-red. 

 When exposed to sunshine all become yellowish, and most 

 lose their colour in fresh water. None turn black in drying. 



The genera of this order, and many of the species, are re- 

 markable for their wide dispersion, the same forms inhabiting 

 the most distant countries. Thus, all of our Laurencia are 

 natives also of the Southern Ocean ; and L. pinnatifida is 

 equally common in the Pacific and Atlantic basins, in both 

 temperate and tropical climates. 



SYNOPSIS OF THE BRITISH GENERA. 



I. BONNEMAISONIA. Frond solid, filiform, (rose-red), much 



branched ; the branches margined with subulate, dis- 

 tichous cilia. [Plate 12, B.] 



II. LAURENCIA. Frond solid, cylindrical or compressed, 



(purplish or yellowish), pinnatifid; the ramuli blunt. 

 [Plate 12, C.] 



