Page:Once a Week Volume V.djvu/507

 500 Some fronds were gathered at Cushendall Bay, in the north of Ireland, which measured five feet in length and three in breadth. Such gigantic specimens are, however, of very rare occurrence, the frond seldom exceeding a foot in length.

Many other of our English Rhodosperms, which are little inferior to the Delesseriaceæ in beauty, are less generally known, either because they are of less common occurrence, or because the rapid fading of their colour on exposure to the air renders it impossible to see them to advantage anywhere except upon their native rocks. Very beautiful forms occur among the family of the Ceramiaceæ, seaweeds which are characterised by their frond being constructed like that of the confervæ of a single string of cylindrical cells. The simplicity and transparency of these plants renders them very favourable subjects for miscroscopicmicroscopic [sic] examination. Seen with a low power, each cell appears like a tube of the clearest glass, filled with a bright crimson liquid. The spores, too, and tetraspores, especially the latter, are often very beautiful objects. In one species, belonging to a closely allied family, the tetraspores are described as forming strings of bright red beads, brilliant as rubies, and each marked with a faint St. George’s cross. Many species are clothed with transparent or coloured hairs and prickles, invisible to the naked eye, but adding much to the beauty of the plant when magnified.

Beautiful as are these and many other of our native Rhodosperms, they are not to be compared in beauty with some exotic forms. There is one Australian species which bears a close resemblance to the skeleton of an oak leaf, except in its colour, which is a fine crimson. Another forms a beautiful lace like frond, each fibre of the network consisting of a minute leaflet, and the points of these leaflets growing together on a regular plan to form the net. Some tropical species have a similar lace-like frond elegantly coiled in a spiral around a central stem, while in others the lower half of the frond is plain, the upper beautifully reticulated.

Very curious, though not strikingly beautiful (at least in our English species), are the seaweeds belonging to the family of the Corallinaceæ, which are characterised by their property of absorbing lime from the water in which they grow, and depositing it within and around the cells of which they are composed, so that when the plant decays, a perfect cast of the frond remains. Every one knows the common coralline; but there are some other plants of the same family which, though equally remarkable, often escape notice. These are the nullipores,—solid, strong, shrublike masses of lime, fixed to rocks between tidemarks, or thin crusts spreading like lichens over stones or over other seaweeds. The most common of these plants is the Melobesia polymorpha, which is everywhere to be met with, forming thick shapeless lumps, upon the rocks, or sometimes rising into short thick branches. On some coasts, one species of nullipore forms extensive submarine fields, and is obtained in such abundance as often to be used as manure. It is said to be applied with great advantage to soils which are deficient in lime. Few people would imagine these strange organisms to be plants, and even naturalists were long in discovering their vegetable nature. Examination with the microscope, after removing the lime by the action of weak acid, shows them to be composed of cells, and leaves no doubt of their true nature. To the family of the Corallinaceæ belongs the curious little Lithocystis Allmanni, the smallest of the red seaweeds, which grows as a parasite upon other seaweeds, forming minute dots only to be recognised as plants by the aid of the microscope. The corallines differ from all other English seaweeds in having their tetraspores contained in ceramidia or pitcher-shaped cases. In the common coralline these ceramidia generally occur at the ends of the branches, the last joint being hollow, and containing a tuft of oblong tetraspores.

The Rhodosperms are pre-eminently the seaweeds of the temperate zones, the number of species diminishing rapidly as we approach the equator or the poles, and their place being supplied in the one case by brown, in the other by green seaweeds. On our own coasts the red seaweeds equal in number of species the green and brown seaweeds taken together. The distribution of the different species depends principally upon climate, but is also affected to a very considerable extent by other causes, with which we are at present imperfectly acquainted. The occurrence of corallines in large quantities is said to be injurious to the growth of other forms. Probably the causes which favour the growth of these plants—the presence, for instance, of much lime in the water of the sea—are unfavourable to the development of other families which do not possess the same property of depositing this mineral between their tissues. Many of our English seaweeds are found only on our southern coasts, while others are confined to the extreme north. On some coasts only common forms occur; while on others, plants elsewhere rare are comparatively plentiful. The coasts of Cornwall and Devonshire are peculiarly favoured localities, an advantage which they owe in a great measure to the influence of the Gulf stream. The mineralogical nature of the coast has no doubt its effect, different species preferring to attach themselves to different kinds of rock; but there are some peculiar cases of local distribution for which no cause can be assigned. Thus, Polysiphonia variegata occurs abundantly at Plymouth, while it is rarely, if ever, found elsewhere.

Many of the red seaweeds are employed, in places where they occur plentifully as articles of food. Perhaps the best known and most extensively used is Chondrus crispus, or Irish moss, which by long boiling is converted into a jelly-like substance, and may then be employed as a substitute for isinglass. Rhodymenia palmata, or dulse, is another edible seaweed, largely consumed by the poor in Scotland and Ireland. It is simply washed and dried, and is then eaten without farther preparation. This seaweed has a rather sweet taste, unlike the Laurencia pinnatifida, or pepper-dulse, which is hot and pungent. Iridrea edulis, another common seaweed, is eaten both raw and fried, and when thus cooked is said to resemble roasted oysters in flavour. An East Indian species,