52 ULVACE^. 



cells are closer, with narrower liyaline interspaces, and the gelatine has a firmer consis- 

 tence more like that of ordinary cellulose ; and in TJlva there is perfect cohesion 

 between thin-walled cells, and the membrane formed by them is firm, and often rigid 

 and tough. Perhaps in all cases the cells multiply by a fissiparous division into four, 

 the old cell dividing longitudinally and transversely. This is very obvious wherever the 

 cells stand sufficiently apart, as in Tetraspora and Frasiola, and in the more trans- 

 parent EnteromorpJm ; but is less evident in the ordinary marine UIvcb. Most of the 

 Ulvacece have the brilliant, grass-green common to the Chlorosperms ; but in the genera 

 Porphyra and Bangia the frond assumes a more or less pure dark-purple hue, and 

 hence some authors have removed these genera to the Rhodosperms. But I cannot 

 think such removal natural or desirable ; for there is really no difference between 

 IJlva and Porphyra in structure or fructification, and the occurrence of a purple colour, 

 or even of a purer red, is by no means limited among Chlorospervis to these plants. 

 We frequently find purple colours in Batrachospermeae, especially in Thorea ; they 

 occur also in Oscillatoriacete and in Palmellacese ; and in the latter, and also in the 

 spores of (Edogonia a pure carmine or scarlet is often seen. 



The fructification of the Ulvacete consists in zoospores, which are formed indifferently 

 in all or in any of the cells of the frond, and are furnished with two or four cilia. Their 

 development and germination are beautifully figured by Thuret in his valuable memoir 

 on the zoospores of Alga% in An. Sc. Nat. Ser. 3, vol. 14. 



Ulvaceffi are universally dispersed either in salt or fresh waters throughout the world, 

 and several are found on damp soil, or in half inundated places. All the genera and 

 most of the species are cosmopolitan. Their specific characters are difficult to fix, and 

 authors differ very much in their opinions respecting them. Kiitzing describes a mul- 

 titude of species, which other writers find it difficult to separate, even as varieties. The 

 form of the frond, in the foliaceous species, is assuredly a most uncertain character; and 

 the comparative size and branching of the tube, in the tubular, equally variable. 



SYNOPSIS OF THE NORTH AMERICAN GENERA. 



* Porphyreoj : frond purple. 



I. Porphyra. Frond leaf-like, purple. 



II. Bangia. Frond filiform, purple. 



** Ulvea3 : frond green. 



III. En'TEROMORPHA. Frond membranous, tubular, simple or branched. 



IV. Ulva. Frond meml)ranaceous, leaf-like. 



V. Tetraspora. Frond gelatinous, expanded. 



