The L'Iraccer include plants which are found in the sea, in fresh water, or on the damp ground : they are generally of an herbaceous green or fine purple colour, and have a thin, tender, inembranactious, reticulated structure, rarely gelatinous ; they are generally furnished with a very minute scutate root, which is either expanded or tubular and continuous; the reproductive organs consist of roundish, mostly quaternate granules, or minute opercular grains, which are imbedded in the delicate membrane of the plant. This order contains about ten genera, of which the fivo following are British :—Perphyra, taro, Tetra:pora, Entcromorpha, and Pavia,.
Porphyra (from ropcinSpeos), the Purple Layer, has a plain frond, excemlingly thin, and of a purple colour. The reproductive organ!' are of two kinds :—1, Roundish granules arranged in fours, and imbedded in the whole substance of the frond ; 2, Masses of smaller ovate granules, which are scattered without order, chiefly towards the margin of the frond. Four species of the Purple Lave• are enume rated. The most common is the Porphyra laciniata, which has its fronds aggregated and deeply cleft, the segments dilated, and variously cut and waved. This plant is common in the sea from spring to autumn, and grown on rocks and stones, whence it is often torn by the violence of the waves, and thrown on the shore. The frond of this plant abounds in a viscid gelatinous matter, which is said to be very nutritious. On this account this plant, under the name of Layer, is much eaten in many places, especially the south of England. 'When collected, it is kept in jars with salt, and when brought to the table is served up with loinon-juice. Dr. Lightfoot states that in the Western Isles it is gathered in the month of March. and that, when pounded and macerated with a little water, the inhabitants eat it with pepper, vinegar, and butter. It is sometimes stewed with leeks and onions. But although this plant is abundant enough, it is only very partially used as an article of diet L'Ira the Green Laver, has a membrana.coous frond of a green colour with its reproductive granules arranged in fours. There are seven British species of Ulra : three growing in the sea, one in fresh water, and three in damp places on the land.
U. latissima, the Broad Green Laver, has a plain, widely-oblong or roundish frond, waved, and of a green colour and tender substance.
It is an abundant plant on the rocks and stones of the sea in summer and autumn. In common with U. Lactuca, the Lettuce Green Layer, it is gathered and eaten in the same way as the Purple Laver. It is also known under the name of Oyster Green. This plant is popularly supposed to be good for scrofulous habits. It is sometimes applied to the forehead to relieve headache in fevers, and also to procure sleep. U. bullosa, the Blistered Green Laver, is the fresh-water species. It has an obovate, saccate frond, which is gelatinous, and at length becomes irregularly expanded, waved, and bullate. It is a very fre quent plant in stagnant pools and ditches of fresh water, often covering the whole surface of the water, and giving it the appearance of being in a state of fermentation. Microscopically examined, this is an object of no common beauty ; it seems as if composed of little green balls, about as big as the blood-cells in the human blood, having no sort of adhesion with each other, but holding together by a transparent thin jelly. It is by these little green balls, or by the matter they contain, that the Ulva is propagated. The common Laver of the shops very nearly resembles it but is a marine species. The terrestrial species of Ulva are found growing on walls, rocks, the reefs of thatched houses, and especially in places exposed to much moisture. U. t herrnalis grows in hot-springs at a temperature of 117° Fahrenheit.
The genus Tetraspora, named from the quaternary arrangement of its granules, inhabits fresh water, and includes two British 'specie& The fronds are tubular or inflated, and gelatinous.
The Enteromorpha, Water-Gut, has a tubular, hollow, membra naceous frond, of a green colour and reticulated structure; the reproductive granules are arranged in threea or fours in the reticu lations. Seven or eight species of this genus have been described as British. They are all inhabitanta of the sea, or of pools and ditches of salt-water, with the exception of E. intestinalis, which is also found in fresh-water pools. All the species are long, varying from two ar three inches to three feet in length, and when floating in the water very much resemble the intestines of an animal—hence their name.