52 - ULVACEiE. 



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

 tence, more like that of ordinary cellulose ; and in Viva 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 Enteromorphce ; but is less evident in the ordinary marine Ulvce. Most of the 

 UlvacecB 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 difierence between 

 JJlva 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 Chlorosperms to these plants. 

 We frequently find purple colours in Batrachospermea;, especially in Tliorea ; they 

 occur also in Oscillatoriacese and in Palmellacege ; and in the latter, and also in the 

 spores of CEdogonia a pure carmine or scarlet is often seen. 



The fructification of the Ulvaceee consists in zoospores, which are formed indifierently 

 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 Algte, in An. Sc. Nat. Ser. 3, vol. 14. 



Ulvaceae 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 difiicult 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. 



* Porphyrese : frond purple. 



I. Porphyra. Frond leaf-like, purple. 



II. Bangia. Frond filiform, purple. 



** Ulvea; : frond green. 



III. Enteromorpha. Frond membranous, tubular, simple or branched. 



IV. Ulva. Frond membranaceous, leaf-like. 



V. Tetraspora. Frond gelatinous, expanded. 



