We have now to deal more particularly with those marine Algæ that are commonly known as Sea Weeds, and which add so much to the beauty of our rocky coasts. These exhibit such a variety of graceful forms, and such charming colours, that they are admired and treasured by thousands of sea-side ramblers, who are attracted by them merely on account of their pleasing general appearance; but the naturalist has all this and a great deal more to interest and instruct him, for the sea weeds possess quite a number of peculiar and characteristic features that render them well worthy of a detailed study, especially when they are compared and contrasted with the better-known flowering plants of our fields, woods, and hedgerows.

It has already been observed that sea weeds differ from the majority of flowering plants in that they have no true roots or leaves, though they are often attached to rocks and other substances by a root-like disc, and sometimes have leaf-like expansions that are supported by stem-like rods. The root-like structures, however, serve simply for the attachment of the plant, and are never concerned in the absorption of nourishment like the true roots of higher plants; and the leaf-like expansions, though they are sometimes symmetrical in form, never exhibit the spiral arrangement that obtains in the leaves of higher plants, from which they also differ in function.

The plant-body of a sea weed is called a thallus, and differs considerably in the various species. Sometimes it has no expanded portion whatever, but is more or less cylindrical in all parts, and may be either branched or simple; and in some species it forms a simple crust or a soft jelly-like covering on a rock.

All portions of a sea weed are made up of cells, and these are never modified into vessels such as we see in the stems, leaves, and roots of higher forms of vegetable life; and one who is commencing the study of the algæ will find much interesting work in the examination of their microscopic structure. Thin sections of various parts of the larger weeds, cut with a sharp knife or a razor, and examined in a drop of water under a cover-glass, will show the cellular structure perfectly; while minute fragments of the small and slender species are sufficiently thin and transparent to display the form and arrangement of their cells without any previous preparation.

One of the principal charms of the marine algæ lies in the great variety of colour that they display. They all contain chlorophyll—that remarkable green colouring matter which enables a plant, under the influence of light, to feed on the carbonic acid gas existing in the atmosphere, or held in solution in water; and with its aid the sea weeds can utilise this product of decay and animal respiration that would otherwise accumulate in the water of the sea. But, in addition to this bright green chlorophyll, many of the sea weeds contain a second colouring substance, and in these the great variety of tint is dependent on the nature of the latter and on the proportion in which it is present as compared with the chlorophyll itself.

The different means by which the algæ reproduce their kind forms a most engrossing subject, and to the botanist a most important one, for it has much to do with the classification of the species. The affinities of plants may be better determined by the nature of their reproductive processes than by any other features, but unfortunately this is not so well understood with regard to the algæ generally as compared with many other divisions of the vegetable kingdom; and, as a consequence, there is still a considerable difference of opinion, not only as to the extent of the whole group, but also as to its divisions and subdivisions. The reason for this is clear; for while it is quite an easy matter to trace a flowering plant through its complete cycle from seed to seed, it requires a much more careful examination, combined with much microscopic work, to trace a lowly organised plant from spore to spore.

Some of the algæ may be reproduced without the agency of any sexual elements; that is, without the aid of parts that correspond with the ovules and the fertilising pollen of a flowering plant. Some of these are reproduced by a repeated subdivision, or by the separation of a portion of the plant that is capable of independent growth; while others produce spores that do not result from the fusion of two different cells. In most, however, sexual differences are to be observed, some cells being modified into female organs, containing one or two more minute bodies that are capable of developing into new plants after they have been fertilised, and other cells produce the male elements by means of which the fertilisation is accomplished. The fertilised cells are spores, but are named differently according to the nature of their development. They all differ from true seeds in that they never contain an embryo plant, but germinate by the elongation of some particular part, which subsequently grows by a continuous process of cell-division; or the cell-division may originate directly in the spore without any previous elongation or expansion.

The sea weeds are usually classified according to the colour of their spores; but, since this colour generally corresponds with that of the plant itself, we may almost say that they are grouped according to their general tints. There are three main divisions:—

The Chlorospermeæ, or Green-spored;
The Rhodospermeæ, or Red-spored; and
The Melanospermeæ, or Brown-spored.

The Chlorospermeæ contain no colouring matter other than the chlorophyll. They are mostly small weeds, of a delicate green colour; and, although they are not particularly conspicuous on our shores, they contribute very considerably to the beauty of the rock pools, where their delicate green fronds contrast richly with the olive Melanosperms and the pink and white corallines. The thallus or plant-body is very varied in form, sometimes consisting of a broad membrane, but more commonly of tufts of slender green filaments or of narrow, flattened fronds.

These weeds are most beautiful objects for the microscope, and they are generally so thin and transparent that no section-cutting is necessary, nothing being required except to mount very small portions in a drop of water. In this simple manner we may study the beautiful arrangement and the various forms of the cells of which they are composed. The more delicate species will be found to consist of a single layer of cells only, while in the larger forms—the Ulvaceæ, for example—the thallus may be formed of two or three distinct layers, and some of the cells may be elongated into tubes.

A remarkable feature of the green-spored weeds is the large size of the chlorophyll granules as compared with those of the other groups, and also the great variety of forms which these granules assume. They may be easily seen under a low power, and the examination of the weeds will show that the thalli are not uniformly green, but that the colour of the plants is due entirely to the chlorophyll granules, the remainder of the plant substance being quite colourless.

If a green sea weed be placed in alcohol for a short time, it will be found that the liquid assumes a green colour, while the plant itself becomes colourless. The explanation is, of course, that chlorophyll is soluble in alcohol. The presence of starch also in the weed may be proved in a very simple manner, as follows:—Mount a small piece in water, and then put a drop of iodine solution by the edge of the cover-glass. The solution will gradually diffuse itself around the object, turning the starch-grains to a deep blue colour, and so rendering them very conspicuous under a moderately high power.

The manner in which the green weeds are reproduced is very interesting also. In some cases the fragments of a thallus that have been detached by storms or other mechanical means possess the power of independent growth, and develop into plants; and this mode of reproduction may often be watched in the indoor aquarium. Another method is by the agency of little spores (zoospores) that are produced at the edges or extremities of the thallus. Certain of the cells become modified into what are called zoosporangia, and the minute zoospores are formed within them. The walls of the cells either gradually degenerate, or are fractured, and the zoospores are thus set free. The latter are provided with little vibratile cilia, by which they move about freely in the water. Some eventually settle down and germinate without any further aid, but others are unable to develop until they have been fertilised by fusion with another cell. The former is therefore an asexual development, while the latter is sexual.

Some of the delicate, filamentous green algæ are reproduced by another process termed conjugation. In this case two adjacent threads that lie close together become lightly united by a covering of gelatinous substance, and a cell of each throws out a process. The processes are directed towards each other, and unite to form a tube in which the contents of the two cells become fused together, with the result that zoospores are produced.

Among the lowest of the green sea weeds are the plants known collectively as the Confervaceæ, which consist of delicate green filaments, usually attached to rocks in dense masses, but often found floating freely in the rock pools. The filaments are composed of cells joined together at their ends, and are always unbranched.

Confervæ are found principally in the tide pools, especially near high-water mark, and often abound in hollows in the rock even above high-water mark, where the spray of the waves is mingled with rain-water or the drainage from the land. They exist in both fresh and salt water, and some species seem capable of thriving in brackish water of any degree of salinity.

Allied to the confervæ is a group of marine algæ called Cladophora, very similar to the former in general appearance, and found in similar situations, but readily distinguished by the branching of their jointed filaments. The various species of this group are very beautiful weeds, their delicate filaments looking very pretty as they float and sway in the water of the pools. They are also exquisite objects for the microscope; but, unfortunately, often lose their natural colour when preserved dry. They vary in colour, some few being of a dull green tint, while others are bright green, sometimes with a beautiful silky gloss.

One species (C. pellucida) is more rigid than most of the others; its fronds stand out erect and firm, and are repeatedly forked near the tips. It is a moderately common weed, found in the lower rock pools, and may be readily recognised by the long one-celled joints, from the tops of which the branches proceed. Another species (C. diffusa) is also very firm in structure, so much so that its bristly tufts retain their form when removed from the water, instead of becoming matted together in a shapeless mass. Its branches are rather long, and bear a few simple branchlets towards their extremities. It is found in rock pools between the tide-marks. C. lanosa is a very pretty little weed, growing in dense globular woolly tufts, an inch or more in diameter, on the olive tangles between the tide-marks. It is of a pale yellowish-green colour, which becomes much paler, or is even altogether lost, when the plant is preserved in a dry state, and, at the same time its fine glossy appearance is lost. Its fronds have straight branches, all making very acute angles, and they have also small root-like filaments. It much resembles another species (C. arcta), which grows in dense tufts on rocks, but the latter is larger, not so slender, and more freely branched. The cells, too, of C. arcta are longer, being about ten times the length of the diameter, and the fronds are silvery at the tips.

Nearly thirty species of Cladophora have been described, but it is impossible to give here a detailed description of all. We add, however, a brief summary of the distinguishing features of a few other species that are common on our coasts.

C. rupestris is common everywhere, and easily recognised by its rigid, branching, tufted fronds, of a dark greyish-green colour; its branches, which are opposite, bear awl-shaped branchlets. It is found in rock pools from half-tide downwards, and in deep water beyond the tide-marks, the plants growing in the latter situations being generally of a fine dark-green colour.

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.

The Ulvæ retain their colour perfectly when dried, and, with the exception of U. latissima, are of a mucilaginous nature, and adhere well to paper, but, unfortunately, the graceful wavy outline of the fronds is lost in pressed specimens.

The ‘true laver’ mentioned above, which is also popularly known as Sloke, is closely allied to Ulva, but may be distinguished from it by the colour of its membranous fronds, which vary from a light rose to a deep purple or violet, occasionally inclining to olive, but never green. Its scientific name is Porphyra laciniata (Plate VII.), and it differs from the majority of the chlorospermeæ in having dark-purple spores, which are arranged in groups of four in all parts of the frond. The fronds are very variable in form and size, being sometimes ribbon-like, and sometimes spreading into an irregular sheet of deeply-divided segments; and the remarkable variety of form and colour has led to a division into several species. These, however, merge into one another so gradually that the separation seems to be hardly necessary.

The same remark concerning the multiplicity of species applies to another allied genus called Enteromorpha, in which the fronds are green and tubular, and often more or less branched. In these the colour varies from a pale to a dark green, and the cells are arranged in such a manner as to give a reticulated appearance. The commonest and best-defined species are E. intestinalis, the tubular fronds of which are constricted at intervals in such a manner as to resemble the intestines of an animal, and E. compressa, with branched fronds of variable form and size. The former is common on all our coasts, and may even be found in rivers and ditches some distance from the sea. It thrives equally well in fresh and salt water, and appears to grow most luxuriantly in the brackish waters of tidal rivers. The latter species also thrives best in similar situations.

Coming now to the red-spored sea weeds (Rhodospermeæ), we have to deal with some of the most charming of the marine algæ that invariably attract the sea-side rambler, and provide many of the most delightful objects in the album of the young collector. Their brilliant colours, varying from a light red to dark purple and violet, are sufficient in themselves to render them popular with the collector, but in addition to this striking feature they are characterised by extreme elegance of form and delicacy of texture. They are to be found in most rock pools, from near high-water mark downwards, the smaller and more delicate forms adding much to the beauty of these miniature seas; but the largest and many of the prettiest species exist only at or beyond the lowest ebb of the tide, and hence the algologist, in quest of these beautiful plants, will find it necessary to work at the very lowest spring tides, with the occasional aid of a small boat drifted into the narrow channels among outlying rocks, and a long hook with which to haul up submerged specimens; and it will also be advisable to search the line of débris at high-water mark after stormy weather for rare weeds that may have been detached and washed ashore by the angry waves.

While engaged in the former of these employments—the searching of outlying rocks with the boat—and also when examining the outer rock pools which are disturbed by the waves that wash over their banks, the simple instrument known as the water-telescope will prove invaluable. Everyone must have noticed how difficult it is to observe objects in water, the surface of which is disturbed by the wind or some other cause; but the simple appliance named, consisting only of a long tube of metal, a few inches in diameter, and painted a dead black inside, will enable the observer to see all submerged objects with the greatest of ease when the water is itself clear. The principle of the water-telescope is as simple as its construction; for the tube, protecting the surface of the water within it from the disturbances outside, prevents the light from being refracted successively in different directions, while the dead-black surface of the interior prevents those internal reflections that would otherwise cause the vision to be indistinct.

A few hours spent with the rhodosperms at the sea-side will be sufficient to show not only the great variety of their form and colouring, but also that the same species may vary according to the position in which it grows. Most of the smaller forms are delicate and filamentous, but others have expanded fronds which are very leaf-like. The brightest colours are usually to be found at or beyond low-water mark, where the weeds are covered with a considerable height of water for hours together, and also in shady situations at higher levels, while some of the species that grow in the upper rock pools are often of such a deep colour, with so much admixture of brown, that they may be easily mistaken for the olive melanospores to be presently described.

Most of the rhodosperms are attached directly to the rocks, and the larger species have often a root-like disc by which they are very firmly held; but some of the smaller species grow attached to larger weeds, into the substance of which they frequently penetrate; and it is possible that these derive some amount of nourishment from the sap of their supporters. Some are of a recumbent nature, being attached to the rock throughout their whole length, while others are so incrusted with carbonate of lime which has been extracted from the water that they resemble corals rather than forms of vegetable life. Nearly all of them contain a bright-red colouring matter in addition to the chlorophyll by which they are enabled to feed on carbonic acid gas.

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.

Still another section of this large genus contains weeds of a more shrubby growth, with veined stem and branches jointed obscurely. Of these the Rosy Callithamnion (C. roseum) is not uncommonly found on muddy shores, and especially in and near the estuaries of rivers. It grows in dense dark-coloured tufts, two or three inches long, with alternate branches much divided. The tetraspores occur singly, one at the top of each of the lower joints of the pinnules of the plumes. C. byssoideum grows on larger weeds in the rock pools, and especially on Codium tomentosum (p. 353), in dense tufts of exceedingly fine filaments, jointed, and branched irregularly. The upper branches are plumed, and their tips bear very fine colourless filaments. The spore-clusters are arranged in pairs, and the tetraspores are thinly scattered on the pinnules of the plumes. This species is so very delicate in structure that a lens is absolutely necessary to make out its structure. It is, in fact, impossible to distinguish between the various species of Callithamnion without such aid; and many of them, particularly the species last described, require the low power of a compound microscope.

Among the other common species, belonging to the same section, we may mention C. corymbosum, distinguished by its very slender, rosy, jointed fronds, with the ultimate divisions of the branches disposed in a level-topped (corymbose) manner, growing on rocks and weeds near low-water mark; C. polyspermum, growing in globular tufts on Fucus serratus and F. vesiculosus, with short awl-shaped pinnules, and closely-packed clusters of spores; C. Hookeri, with opaque stem and branches, and spreading branchlets that are themselves branched, and bear spreading plumules at their tips; and C. arbuscula, found on the west coasts, with a stout stem, naked below, and having a very bushy habit.

It is often by no means an easy matter to distinguish between the different species in such a large genus as Callithamnion, and we strongly recommend the beginner to first study the characteristics on which the classification of the Algæ is based, and to arrange his specimens according to the orders and genera to which they belong; and then, after mastering the principles of classification, he should refer to one of those larger works in which all known British species are described, and make himself acquainted with the features of each individual species in his collection.

Before leaving the present genus we ought also to mention the fact that many of the species lose their natural colour rapidly when placed in fresh water; hence when they are being cleansed for mounting salt water should be employed. Further, even after they have been satisfactorily mounted, they are liable to be spoiled if left exposed to moist air. The salt water used need not be the natural sea water; a solution of common table salt, made up to approximately the same strength as sea water, will answer the purpose just as well.

The genus Griffithsia includes some very beautiful weeds of delicate threadlike structure and of a fine rose colour. The frond contains a single tube, and is jointed and forked, the joints being usually transparent. The spore clusters are enclosed in a gelatinous sac surrounded by a whorl of little branchlets, the spores themselves being minute and angular. The tetraspores are attached to the inner side of whorled branchlets.

The commonest species is G. setacea, which is of a bright-red colour and slightly branched. It is also of a somewhat firm structure, but soon loses both firmness and colour when removed from salt water; and, like Callithamnion, rapidly fades if put into fresh water, which is readily absorbed through its membranes, causing them to burst and discharge their colouring matter. It receives its specific name from its bristle-like forked fronds. G. secundiflora is somewhat like Setacea, but is larger, and the tips of its branches are obtuse. Its fronds grow in fan-shaped tufts five or six inches long. It is not a common weed, but may often be met with on the coast of Devon and Cornwall.

G. barbata, or the Bearded Griffithsia, receives its name from its very delicate fibres, which bear spherical, pink tetraspores. It seems to occur only on the south and south-west coasts, where it grows on stones or attached to other weeds. Our last example of the genus is G. corallina, which is of a deep-crimson colour, and is so jointed as to have the appearance of a coralline. Its fronds are from three to eight inches long, regularly forked, and of a gelatinous nature. The joints are somewhat pear-shaped, and the spore clusters are attached to their upper ends. It soon fades, and even if its colour is satisfactorily preserved, the pressure of the drying press destroys the beautiful rounded form of its bead-like joints. It forms a lovely permanent specimen, however, when preserved in a bottle of salt water, with the addition of a single grain of corrosive sublimate.

Our next genus (Halurus) contains a common weed of the south coast which was once included in Griffithsia. It is the Equisetum-leaved Halurus (H. equisetifolius), so called because its branches are regularly whorled round the nodes of the jointed branches, thus resembling the semi-aquatic Mare’s Tail. Its frond is tubular, and the spore-clusters are situated on the tips of the branches, surrounded by a whorl of small branchlets.

The genus Pilota has a slightly flattened cartilaginous frond, divided pinnately, and the axis surrounded by a cuticle of two layers of cells. The spore-clusters, at the tips of the branches, are surrounded by a whorl of branchlets. It contains only two British species, one of which (P. plumosa) is a very feathery species, with comb-like branchlets, growing on the stems and fronds of other weeds found on our northern shores. The other (P. elegans), with narrower fronds, in long flaccid tufts, is found all round our coasts.

Our last genus of the Ceramiaceæ is the large and typical one Ceramium, which contains about a dozen British species in which the frond is threadlike, jointed, branched or forked repeatedly, with the tips of the branchlets usually curled. The spore-clusters are enclosed in transparent sessile sacs, surrounded by a whorl of very short branchlets; and the tetraspores are embedded in the cortex, but distinctly visible. As a rule the fronds are very symmetrical, and the branches radiate in a regular fan-like manner.

In one species of the genus the frond is completely covered with cortex cells, and at each node of the frond there is a single spine which, although so small as to be invisible without a lens, so effectually locks the threads together that they form an entangled mass that is not easily arranged to the satisfaction of the collector. The species referred to is C. flabelligerum—the Fan-bearing Ceramium—and is very rare except in the Channel Islands.

Other species are armed with one or more spines at the nodes, but the nodes only are covered with cortex cells, which render them opaque, while the internodes or joints are transparent. In this group we have C. ciliatum—the Hairy Ceramium, with reddish-purple segments, and a regular whorl of hairs, directed upwards, round each node; each hair or spine consists of three segments. This plant is common during the summer and autumn, and may be found in the tide pools at all levels, either attached to the rocks or parasitic on other weeds. The same section contains C. echinotum, with rigid, forked fronds, and irregularly-scattered one-jointed spines; it is common on the south coast, where it may be found on the rocks and weeds of the upper tide pools; and C. acanthonotum, also common in the rock pools, with a single strong three-jointed spine on the outer side of each filament. The last-named weed is found principally on the northern shores, especially on rocks covered with the fry of the common mussel.

Other species are characterised by transparent internodes as above described, but have no spines at the joints, and may thus be easily floated on to a sheet of paper without the troublesome matting of their fronds. These include the Straight Ceramium (C. strictum), with erect and straight branches growing in dense tufts, and conspicuous tetraspores arranged round the nodes of the upper branchlets, C. gracillimum, of the lower rock pools, with very slender gelatinous fronds, swollen nodes and small fan-shaped branchlets; C. tenuissimum, closely resembling C. strictum in general appearance, but distinguished by having its tetraspores only on the outer side of the nodes; and the Transparent Ceramium (C. diaphanum), which may be found throughout the year on rocks and weeds in the rock pools. The last species is the largest and most beautiful of the genus, and may be readily recognised by its light-coloured, transparent stem with swollen purple nodes, and its conspicuous spore-clusters near the tips of the filaments.

Our last example of the genus is the Common Red Ceramium (C. rubrum), which may be found in the rock pools at all levels. It is very variable in form, but may be known by its contracted nodes, in which the red tetraspores are lodged, and its spore-clusters surrounded by three or four short branchlets. It differs from most of the other species in having both nodes and internodes covered with cortex-cells, and hence the latter are not transparent.

The order Spyridiaceæ has a single British representative which may be found in various localities on the south coast. It is Spyridia filamentosa, a dull-red weed with thread-like, tubular, jointed fronds, from four inches to a foot in length. The main stem is forked, and densely clothed with short and slender branchlets. The frond is covered with a cortex of small cells. The spore-clusters are grouped together, several being enclosed in a membranous cell in conceptacles, or external sacs, at the ends of the branchlets; and the tetraspores are arranged singly along the jointed branchlets.

The next family (Cryptonemiaceæ) is an extensive one, containing nearly twenty British genera of red or purple weeds, with unjointed, cartilaginous, gelatinous, and sometimes membranous fronds. The spores are irregularly distributed, and are contained either in sunken cells or in conceptacles. The tetraspores are either in cells at the edges of the frond or collected together in compact groups.

Of the genus Dumontia we have only one species (D. filiformis), the frond of which is a simple or branched tube, from an inch to more than a foot in length, containing a loose network of filaments when young, and only a gelatinous fluid when the plant is mature. The spores exist in rounded clusters among the cells of the tube, and the tetraspores are similarly situated. A variety with wide wavy fronds is sometimes found in the brackish water near the mouths of rivers.

Gloiosiphonia capillaris is a very delicate and beautiful weed found in the lowest tide pools of the south coast. Its frond is a very slender branched tube, filled with a gelatinous fluid, and composed of delicate filaments embedded in transparent gelatine. It is a beautiful object for the microscope.

Schizymenia (Iridæa) edulis has flat, oval, dark-red fronds that grow in clusters; and, being eaten by various marine animals, is often found imperfect and full of holes. The fronds are sometimes a foot or more in length, and five or six inches wide. They are thick and leathery, and each is supported on a short, cylindrical stem.

In the lower tide pools we commonly meet with Furcellaria fastigiata, with brownish-red, cylindrical fronds, solid, forked, and densely tufted. The branches are all of the same height, with sharp tips; and the spore-clusters are contained in terminal lanceolate pods. This weed is very much like Polyides, of another order, but may be distinguished by its fibrous, creeping root, while that of Polyides is a disc.

The genus Chylocladia is characterised by a tubular rounded frond composed of two layers, the inner consisting of branching filaments, and the outer cellular. The spores are contained in external cones with a pore at the apex, and the tetraspores are among the superficial cells of the branches. There are two common British species of the genus, one of them—C. articulata—with long, tubular fronds, constricted at intervals, the lower branches forked and the upper whorled and tufted; and C. clavellosa, with freely branched fronds bearing short spindle-shaped branchlets.

One of the best-known algæ of the present family is the Irish Moss or Carrageen (Chondrus crispus), which will be at once recognised by its representation on Plate VIII. Its fronds are cartilaginous, forked and fan-shaped; and, when growing in deep, sheltered pools, its branches are often broad and much curled. This weed is an important article of commerce, being still used as a food for invalids. When boiled it yields a colourless gelatine.

In the genus Gigartina the frond is cartilaginous, flat, or threadlike, irregularly branched, and of a purplish-red colour. The spores are contained in external tubercles, and the tetraspores are arranged in masses beneath the surface. The only common species is G. mamillosa, which has a linear, furrowed stem, with fan-shaped, deeply-cleft fronds. The spores are contained in mamilliform tubercles scattered over the surface of the frond.

Callophyllis (Rhodymenia) laciniata is found on most rocky coasts. It has bright-red, fleshy fronds that are deeply cleft into wedge-shaped segments, the fertile specimens with waved edges and small marginal leaflets. It is found on rocks and Laminaria stems beyond the tide-marks, but is commonly washed up on the beach during storms. It is a beautiful weed, and retains its colour well when dried.

Cystoclonium (Hypnæa) purpurascens is a very common weed, growing on other algæ between the tide-marks, and sometimes reaching a length of two feet. Its cartilaginous, purple fronds are much branched, and become almost black when dried. The spores are embedded in the smallest branches, and the tetraspores are arranged among the superficial cells.

The genus Phyllophora contains a few British weeds with a stiff, membranous frond, bearing leaf-like appendages, and supported on a stalk. The tetraspores are contained in external wart-like swellings. The commonest species is P. membranifolia, the fronds of which are divided into wedge-shaped segments, and grow in tufts from an expanding root. The spores are contained in stalked sporangia, and the tetraspores are near the centres of the segments. Another species—P. rubens—has a shorter stem, and grows in deep and shady rock pools. Its fronds are densely tufted; and, as the plant grows, new series of segments are formed at the tips of the older ones. A third species (P. palmettoides) has a very bright-red frond and an expanded root.

The order Rhodymeniaceæ includes a number of red or purple sea weeds with flat or thread-like unjointed, cellular fronds, the surface cells forming a continuous coating. The spores are lodged in external conceptacles, and are at first arranged in beaded threads. The tetraspores are either distributed among the surface cells, collected in clusters, or situated in special leaflets.

The typical genus (Rhodymenia) contains two red, membranous weeds, the commoner of which is R. palmata (Plate VIII.), so common on the Scottish and Irish coasts, where it forms an important article of diet, and is known as the Dulse or Dillisk. It is also widely distributed over the English coasts. Its broad, fleshy fronds are divided into finger-like lobes, and are either sessile or supported on a stalk that proceeds from a small discoid root. The frond is very variable in form, being sometimes divided into very narrow segments, and sometimes quite undivided. One variety has a number of small stalked leaflets on its margin (see Plate VIII.); and another is very narrow, with wedge-shaped irregular lobes. R. palmetta is a smaller and less common species that grows on rocks and large weeds in deep water. The tetraspores form crimson patches on the tips of the lobes.

Maugeria (Delesseria) sanguinea (Plate VIII.) is a large and beautiful weed, of a blood-red colour, that grows in the lower rock pools or beyond low-water mark, under the shade of high rocks or hidden by the olive tangles. Its frond is thin and membranous, with a well-defined midrib. The spores are contained in globular stalked conceptacles, usually on one side of the midrib; and the tetraspores may be seen in pod-like leaflets attached to the bare midrib during the winter.

Passing over some of the rarer membranaceous Rhodymeniaceæ, we come to the beautiful Plocamium, distinguished by its linear compressed crimson fronds, which are pinnate, with comb-like teeth, the branchlets being alternately arranged on either side in threes and fours. The spores are on radiating threads, in globular conceptacles; and the tetraspores are in the outer divisions of the frond. We have only one species of this beautiful genus, and that is P. coccineum, which is of such a brilliant colour that it is always a favourite with collectors.