Fig. 258.—Polysiphonia Brodiæi
P. byssoides, so called on account of the pink filaments that fringe the fronds, has also seven siphons. It is a large and beautiful weed, moderately common on our coasts, of a bright-red colour, with conspicuous fructification. The branches are alternate, and the branchlets are clothed with the byssoid filaments above referred to.
P. violacea is of a reddish-brown colour, with long silky alternate branches, and four siphons. It receives its specific name from the fact that it turns to a violet colour when dried.
P. nigrescens has, as the specific name implies, blackish fronds, and these are freely branched. The tubes, about twenty in number, are flat, and are arranged round a large central space.
Fig. 259.—Polysiphonia nigrescens
Our last example—P. atro-rubescens—is of a dark reddish-brown colour, with rigid and densely-tufted fronds. It has twelve tubes, arranged spirally round a central cavity. It is common in the lower rock pools of some coasts.
In the same order we have the genus Chondria, so called on account of the cartilaginous nature of its thread-like fronds. These are pinnately branched, and the club-shaped branchlets taper below. The main stem is jointed and contains many siphons. The genus includes a common species (C. dasyphylla), with thick fronds, that is found in shallow sandy pools, where it grows on pebbles, shells, or on other weeds, the colour varying from pink to a dark purple. C. tenuissima is a very similar weed, but may be distinguished by its more slender growth, and by its long, rod-like simple branches, clothed with slender, bristle-like branchlets that taper from the middle towards both ends.