The Sea Shore

by William S. Furneaux

None of the rhodosperms are of really microscopic dimensions, and they all grow by the repeated division of the cells of the apex, while the branches are derived by the similar division of new cells at the sides.

All plants are particularly interesting during the period of fruiting, and this is remarkably the case with many of our red-spored sea weeds, which are brighter and prettier while laden with their spore-producing cells; and the collector of marine algæ should always endeavour to obtain as many species as possible in fruit, not only on account of the brighter appearance that may characterise them at this time, but mainly because the opportunity of studying the mode of reproduction should not be missed.

In the rhodosperms the reproduction may be either asexual or sexual. In the former case fertile spores are produced without the necessity for any outside fertilising element, and four are usually produced in each one of the sporangia, hence they are generally known as tetraspores. Where the reproduction is of the sexual type, the male cells are produced singly in the terminal cells of the fronds, and since they are usually crowded together in considerable numbers, and contain none of the red colouring matter that exists in the other parts of the plant, their presence is easily observed.

The female cells (carpogonia) are also produced on the tips of the branches, and when the male elements escape from their cells, they are conveyed passively by the movements of the water, for they have no vibratile cilia by which they are propelled, and on coming into contact with the female cell they adhere closely. An opening is then formed in the latter, and the male element enters the carpogonium, which germinates, deriving its nourishment from the parent plant, and the spores are thus formed. Lastly, it is interesting to note that the asexual spores, the male cells, and the female cells are generally produced on different plants of the same species.

We will now proceed to examine some of the best known and most interesting of the rhodosperms, beginning with the order Ceramiaceæ, which contains a number of red or reddish-brown weeds with jointed, thread-like fronds that enclose a single tube, and which are generally surrounded by a cuticle of polygonal cells. The spores are contained in transparent berry-like sacs which are naked; and the four-parted spores (tetraspores) are formed in the cells of the cuticle or at the tips of the fronds.

Over twenty British species belong to the genus Callithamnion, and nearly all of them are pretty red or rose-coloured, feathery plants that are conspicuous for their beauty. Nearly all are of small size, the largest measuring only seven or eight inches, while some are so small that they would scarcely be noticed except by those who search diligently for them. The principal features of the genus are, in addition to those mentioned above as common to the order, that the spores are angular, and clustered within a transparent sac, and the tetraspores are naked and distributed on the branches.

In some species the fronds have no stem, and these are very small, generally only about a quarter of an inch in height or less, and they grow on rocks or weeds, sometimes clothing the surfaces with a velvet-like covering. C. floridulum forms a kind of reddish down on the rocks, sometimes in little rounded patches, but sometimes completely covering the surface. It occurs on several parts of the English coast, but is so abundant on the west coast of Ireland that the beach is strewn with it after stormy weather. Other allied species grow in minute tufts on rocks, or are parasitic on other weeds, and are so inconspicuous that they are but little known.

Another section of the genus is characterised by pinnate fronds with opposite segments, and the species are very pretty plants with fronds generally a few inches in length. One of the commonest of these is the Feathered Callithamnion (C. plumula), a great favourite with collectors of sea weeds, and a most interesting object for the microscope. Its soft and flexible fronds grow in tufts from two to five inches long. The branches are regularly arranged, and the comb-like branchlets bear the tetraspores on the tips of the plumules. This beautiful weed grows near low-water mark, and in deep water, and is often very abundant on the beach after storms. C. Turneri is another common species, easily known by its creeping fibres, attached by little discs to some larger weed, and from which the tufts of branched fronds stand out erect. On the west and south-west coasts of Britain we may often meet with the allied Crossed Callithamnion (C. cruciatum), which grows on rocks, close to low-water mark, that are covered with a muddy deposit. It grows in tufts, somewhat resembling those of C. plumula, but its plumules are arranged two, three, or four at a level, and are very crowded at the tips of the branches.