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 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 Prasiola, and in the more trans- 
parent Enter omorphos ; but is less evident in the ordinary marine Ulvce. Most of the 
TJlvacece have the brilliant, grass-green common to the Chlorosperms ; but in the genera 
Porpliyra and Bangia the frond assumes a more or less pure dark-purple hue, and 
hence some authors have removed these genera to the Khodosperms. But I cannot 
think such removal natural or desirable ; for there is really no difference between 
TJlva 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 Batrachospermese, especially in Thorea ; they 
occur also in Oscillator! aceae and in Palmellaceae ; and in the latter, and also in the 
spores of (Edogonia a pure carmine or scarlet is often seen. 
The fructification of the Ulvacese 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 Algae, in An. Sc. Nat. Ser. 3, vol. 14. 
Ulvaceee 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. 
*' Porphyreae : frond purple. 
I. Porphyra. Frond leaf-like, purple. 
II. Bangia. Frond filiform, purple. 
** Ulveoe : frond green. 
III. Enteromorpha. Frond membranous, tubular, simple or branched. 
IV. Ulva. Frond membranaceous, leaf-like. 
V. Tetraspora. Frond gelatinous, expanded. 
