matter of these seaweeds is so easily affected by external influences, that only in such plants can we expect to find it unchanged, and the change which it commonly undergoes is to green or brown, the colours which respectively characterise the two other great divisions of the seaweeds. Now, the red seaweeds are, for the most part, deep water plants, and their rich and brilliant colours are only developed to perfection in situations where they are never exposed to the injurious influence of air or light. Thus the well-known Chondrus crispus, or Irish moss, when growing in deep water, or in shady pools, is of a dark purple colour, but in shallow rock pools, exposed to the rays of the sun, it becomes bright green, pale yellow, or sometimes nearly white. Another of the red seaweeds, Laurencia pinnatifida, known in some parts of Scotland as the pepper dulse, is still more variable, and specimens of this plant, gathered from different localities, show well the influence of light in altering and destroying the red colouring-matter. Thus plants growing near low-water mark, are of a deep purple, or red, where they are a little more exposed the red fades and becomes tinged with brown, while near high water, the red entirely vanish, and is replaced by yellow or light green.
Still more remarkable changes are produced in the colouring matter of these plants by long exposure to the air, or by the action of heat, or of fresh water. The effect of exposure to the air varies strangely, not only in the case of different species, but even in that of individuals of the same species, gathered in different localities. There is one British seaweed, naturally of a bright crimson, whose colour, in specimens gathered on the west coast of Ireland, becomes brighter by exposure, while in specimens taken from the east coast of Ireland, or the south coast of England, it almost entirely fades. The common Plocamium coccineum, which every reader knows is originally dull red, but after a short exposure to the air, it assumes the bright scarlet tints which so often attracts the eye in specimens thrown up by the tide. Dasya coccinea, another not uncommon plant, is an instance of a similar change. There is an African species, common at the Cape of Good Hope, which after a short exposure to the air, presents the most splendid varieties of colour. The original colour of this plant is dull pink, but the dying frond passes gradually through bright red, orange, yellow, and green, to white. Our English Chysemeniæ derive their name (golden membrane) from the property which they possess of assuming a golden tint, on being placed in fresh water. On many others of the red seaweeds fresh water produces a marked effect. Some are changed almost instantaneously from rich pink or crimson to bright orange. Several instances of this peculiarity occur in the beautiful and well-known order of the Delesseriaceæ, one species deriving its name, versicolor, from its liability to be so changed. The pretty little Griffithsia setacea, whose colour is a fine transparent crimson, on being placed in fresh water instantly discharges its colouring matter, the discharge being accompanied by a crackling sound produced by the bursting of the membrane of the cells. The colouring matter of this seaweed, and of some others of the same family, stains paper a fine crimson, which remains for a long time unchanged. It has been suggested that a valuable pigment might be prepared from these plants, if they could be procured in sufficient quantity. Perhaps the most singular change produced by fresh water occurs in the case of a South African seaweed which, when growing, is of a dull brown colour, very slightly tinged with red, but which on being placed in fresh water instantly discharges a considerable quantity of brilliant purple powder, and almost immediately becomes putrid. Some of our English Polysiphoniæ discharge in fresh water an offensive black juice, and these may with advantage be steeped for some time before being dried, as the natural colour is much better preserved after the plant has been freed from this dark pigment. Almost all the red seaweeds may have their colour changed to green, by placing them for a few minutes in boiling water.
There are certain of the Rhodosperms with whose names and properties everyone is familiar. These are the plants which in legal phraseology are parties to the petition which so often meets the eye in the shop-windows of our seaside towns. They owe their popularity to the possession of two qualities, a brilliant colour which does not fade in drying, and the power of adhering firmly to paper. The seaweeds most in request for the construction of the cards to which the aforesaid petition is appended, are those which belong to the family of the Delesseriacieæ, particularly Delesseria sanguinea, Delesseria alata and Plocamium coccineum. Perhaps the most generally known of all the red seaweeds is the beautiful delesseria sanguinea, whose bright crimson leaf-like fronds cannot fail to attract the least observant eye. This is its summer form: in winter few of its admirers would recognise it. For with the approach of cold weather the membrane of the leaves withers, and only the midrib and nerves remain. But in this ragged and forlorn-looking state it is more interesting to the student than when clothed in its summer dress. For now the midrib and the stem are fringed with small tubercles placed on short stalks, each containing a tuft of filaments bearing the spores. In other plants, instead of these tubercles, we find small leaf-like projections, also attached to the midrib, and containing the tetraspores. The two kinds of fruit are never found growing on the same plant. When the leaf-like membrane has once decayed, it never grows again; but in the spring new leaves shoot out from the old stem, so that the midrib of this year’s frond becomes the stem on which next year’s fronds are borne.
Another very beautiful plant of the same family is the Nitophyllum punctatum, the spotted Nitophyllum, so called from the frond being covered with minute dots, containing the tetraspores. It is distinguished from the Delesseriæ by not possessing a midrib, and from some other seaweeds which slightly resemble it by the extreme delicacy and transparency of the frond. This plant is worthy of notice, as being probably the largest of our English seaweeds, occasionally attaining a size far surpassing that of the largest oarweed.
