C. lætevirens is also very common on rocks between the tide-marks. Its fronds are tufted and freely branched, of a pale-green colour and soft flexible texture, and about six inches long. The branchlets are usually slightly curved.
C. gracilis is a beautiful plant that grows on large weeds, especially the Sea Grass (Zostera) in deep water; and although not very common, it is sometimes found on the beach after storms. It is characterised by its slender silky fronds, from a few inches to a foot in length, of a yellowish-green colour. It may always be known by the comb-like branchlets growing only on one side of each branch.
C. refracta grows in dense tufts, two or three inches long, in rock pools near low-water mark. Its fronds consist of rigid stems in rope-like bundles that are very freely branched, the whole tuft being of a yellow-green colour and silky texture. C. albida closely resembles it in structure and habit, but may be distinguished by its paler colour, which disappears when the weed is dried, and by its longer and more delicate branches.
In another order of the green-spored algæ (the Siphoneæ or Siphonaceæ) the frond is formed of single branching cells, and many of these are often interwoven into a spongy mass, and sometimes coated with a deposit of calcareous matter.
In the genus Codium the fronds are of a sponge-like texture, composed of interwoven branching fibres, and are of a globular, cylindrical, or flattened form. The commonest species is C. tomentosum ([Plate VII.]), which consists of sponge-like, dark-green cylindrical fronds, which are forked and covered with short hairs that give it a woolly appearance when in the water. Each frond is composed of slender interwoven fibres with club-shaped filaments passing vertically to the surface. It grows on rocks in the pools between the tide-marks, and is abundant on nearly all our coasts.
The Purse Codium (C. bursa) has spongy hollow fronds of a globular form, varying from a quarter of an inch to five or six inches in diameter. It is a rare species, being found only at a few places on the south coast. Another species (C. adhærens) adheres to rocks, over which the fronds spread in irregular soft patches, the club-shaped vertical filaments of its interwoven fibres giving it the appearance of rich green velvet.
An allied weed (Bryopsis), named from its moss-like appearance, grows in erect tufts, each frond consisting of a branched one-celled filament. There are two species of the genus, one (B. plumosa) characterised by the light feathery nature of its fronds, the stems of which are branched only near the top. It is found in rock pools on most of our coasts. The other (B. hypnoides) is more freely branched, and the branches are long, and issue from all sides of the stem. Like the last species, it has branches on the outer part of the stem only, but it is of a softer texture.
The best known of the green-spored weeds are certainly those belonging to the Ulvaceæ, characterised by their flat or tubular fronds, sometimes of a purplish colour, the cells of which multiply both horizontally and vertically as the plants grow. In the typical genus, Ulva, the frond is sometimes in two distinct layers, and becomes more or less inflated by the accumulation of either water or oxygen between them. The commonest species are U. lactuca and U. latissima, both of which are eaten by the dwellers on some of our coasts. The former, commonly known as the Lettuce Ulva, has a frond of a single layer of cells, and grows on rocks and weeds between the tide-marks. It is common on many oyster beds, and is employed by the fishermen to cover the oysters when sent to market; they call it ‘oyster green.’ This species is shown on [Plate VIII.] U. latissima or the Broad Ulva sometimes reaches a length of two feet, and a breadth of nearly a foot. The fronds are composed of two layers of cells, are of an irregular shape, with a very wavy, broken margin, and of a bluish-green colour, It is known as the Green Laver, and is used as food in districts where the true laver (Porphyra) is not to be obtained. A third species—the Narrow Ulva (U. Linza)—has smaller and narrower fronds, of a more regular shape and of a bright-green colour. The fronds are composed of two layers of cells.
Plate VII.