Chapter 15

Fig. 23.A, Polyides rotundus:—a, thin slice showing the wedge-shaped spores;b, tetraspores.B, Furcellaria fastigiata:—c, thin slice showing a nucleus with the dividing spores;d, one of the large cells;e, a tetraspore.

The simple spores are produced within colourless tubercules called nuclei, variously situated upon the plant, as atfig. 23a,c. These nuclei contain many microscopic spores. Sometimes the nuclei are enclosed in conceptacles or ovate sacs, which are either perforate or not at the apex. These contain many microscopic strings of cells like jointed threads, and the endochrome in each joint of these threads is converted into a spore successively from the summit downward. Sometimes the endochrome in one or two joints only, becomes a spore whether terminal or central, and when the spores break through the joint wall and fall off from the threads, they are collected without any definite order into a mass within the nuclei. Sometimes new joints or cells are produced on the threads when the old ones have yielded their fruit. Occasionally a globose nucleus contains several secondary nucleoli full of spores. Inevery instance, the perfect spore is a dense grumous mass surrounded by a hyaline sub-gelatinous coat consisting of at least two membranes. The situation, mode of growth, and structure of the nuclei vary almost infinitely, and together with the structure of the frond afford the distinctive marks by which the genera are separated from each other.

The spores and tetraspores are equally capable, like buds, of reproducing their species; but the spores are believed to be in some cases fertilized by spindle-shaped particles, and consequently are considered to be the true fruit. Antheridia, or sacs containing these particles, have been discovered in various genera of Rhodosperms. Although, as a rule, the red Algæ have two modes of vegetative reproduction, yet there are various species in which tetraspores only have hitherto been met with.

Fig. 24. Vertical sections of conceptacles:—a, Gracilaria armata;b, Grinnelia americana;c, Corallina officinalis, the membrane of which, more highly magnified, is shown atd.

A large proportion of the higher Rhodosperms is distinguished from those possessing the preceding mode of fructification by the internal structure of their reproductive nuclei. In some of these Algæ the nuclei are divided into two equal chambers by a fibro-cellular substance to which the spores are attached; in others, pear-shaped spores radiate from a fibro-cellular substance at the base of the nucleus. There are, moreover, Algæ which have nuclei containing conical spores whosebroad bases radiate from the centre, and other arrangements occur.

The Rhodosperms are comparatively small plants. Some which form velvety cushions on stones, or minute tufts on small Algæ, are only the fraction of an inch high, but the larger kinds range from one to four, six, ten, or twenty inches; probably none exceed two feet. In thickness, some fronds are fine, like jointed and branched hairs, while others are thick, like hog’s bristles or crow quills. Numerous as the forms are, the simple jointed filamentous frond is connected by a series of forms with the highest order of the class.

A great portion of the Rhodosperms on the British coasts is composed of the exquisitely beautiful order of the Ceramiaceæ. They abound in every rocky pool, on every piece of wood that has been long exposed to the waves, on rocks and stones, and, above all, they fringe the Zostera marina, or sea wrack, as well as the firmer Algæ, with every shade of red from bright crimson to purple. They are articulated filiform plants, approaching in simplicity of form to the Confervas. The genus Callithamnion, which has thirty species in the British seas, consists of cylindrical jointed threads more or less profusely branched, and distinguished by having the divisions between the joints opaque and of various shades of red and purple, while the joints themselves are transparent and colourless, so that the stem and branches appear to be striped across by alternately white and coloured bands which are often visible to the naked eye, notwithstanding the smallness of the plants and the delicacy of their filaments, as the C. sparsum,—which is a soft purple tuft of jointed threads scarcely one-tenth of an inch high.

The Callithamnion corymbosum has a soft jointed filamentous stem, hair-like below, fine as a cobweb above, and excessively branched, with dichotomous branches. Infig. 25arepresents a thread of this plant with tetraspores, much magnified;b, a portion of the same, more highly magnified;c, a thread with naked nuclei, gongylospermous, that is, filled with a mass of spores, magnified; andd, a spore, magnified more highly. The nuclei are naked in all the Ceramiaceæ.

Fig. 25. Callithamnion corymbosum.

The genus Ceramium, some species of which have spinulose branchlets, is characterized by the tips of the forks of its terminal branchlets being hooked inwards, and by the stems and branches being striped by alternate hyaline and coloured bands as in the preceding genus, though the arrangement of the colours is somewhat different. The Ceramium ciliatum, which is a dense tuft of capillary jointed filaments, from two to six inches long, repeatedly and regularly forked, has the tips of the last forks so much hooked inwards, that the extremities of the branchlets look as if they were heart-shaped. It has minute spores in globular nuclei, sessileon the branches with two or three branch-like hairs beneath them, and tetraspores set in the coloured parts of the joints with a thorn between each, for in this plant the centre of the joint is hyaline, the rest coloured.

The genus Griffithsia contains various species of bright rose-coloured plants, which become bleached when put into freshwater, and form a circle when spread out. Soft, tender, and gelatinous, they form dense tufts of jointed and branched filaments on rocks at low water mark. The filaments are slender below, capillary and forked above, and the joints contain one linear upright rose-coloured tube, which is seen throughout their transparent walls: a distinguished mark of the genus. Tetraspores are borne on the hair-like jointed ramuli, and spores are amassed in coated roundish sessile nuclei, surrounded by minute hair-like fibres. Several species once called Griffithsia differ so much from the others, that they are by some referred to Halurus, which has the stems and branches thickened by overlapping whorls of tiny forked jointed and curved ramuli. They are propagated by spores, enclosed in clusters of nuclei borne on the tips of short branches, with a mass of curved ramuli folding over them, and by tetraspores attached to the inside of another set of curved ramuli. Antheridia have been discovered in several genera of the group Ceramiaceæ, especially in Ceramium, Callithamnion, Griffithsia, and Halurus. They consist of little clusters of cells variously arranged, in which the active particles known as spermatozoids are generated.

The Polysiphonia are Algæ, seen in tufts from ten to twelve inches long, of usually much branched jointed filaments, on rocks, corallines, and the smaller Algæ at low water mark. The joints of the filaments contain upright tubes, full of purple or reddish brown matter, which is seen through their transparent walls. The number of these colour tubes vary from four to ten,eighteen, or even twenty, and form the characteristic of the genus. Thus there is a similarity of structure between the Polysiphonia, a genus of the highest order amongst Rhodosperms, and the Griffithsia, which is one of the lowest. The Polysiphonia elongata, which is from six to twelve inches high, has four primary and several secondary colour tubes in the transparent joints of its filaments. Like many of its congeners, this plant does not come to perfection or bear fruit till the second spring. In its youth, it resembles the full grown plant but is smaller, and the colour tubes are not formed in the capillary threads of the tufts, which with many of its branchlets are deciduous, leaving the plant in its naked winter state. With returning warmth, it assumes its perfect form, and in March and April bears fruit, which consists of nuclei in conceptacles, sessile on the branches, either clustered or scattered. The spores are at the top of jointed threads rising from a substance at the base of the nuclei. In some species of this genus, tetraspores only have been found.

The Cryptonemiaceæ are the most numerous and diversified of all the orders of the Rhodosperms. Thirty-five genera are widely dispersed throughout the world, chiefly in the northern hemisphere; twenty-four genera at least occur on the east coast of North America; and fifteen genera have representatives in the British seas. This multitude of generic forms is divided into two groups of gelatinous structure, the one having inarticulate fronds composed of articulate threads closely incorporated, the other membranaceous, formed of cells closely incorporated into a foliaceous expansion. Most of these plants have a stratum of cellular tissue, interposed between a spongy matter in the interior of the frond, and the epiderm or external skin, which for the most part consists of a simple layer of minute cells firmly united by their sides, generally forminga mere film; but it may be thin and flexible, thick, tough, or leathery, according to circumstances.

The Furcellaria fastigiata (fig. 23B) has an intermediate layer of cellular tissue between its skin, and a pulpy interior. The frond is cylindrical, smooth, strong, and opaque, repeatedly forked with long narrow forkings. The root is fibrous, and the stem short and tapering. Masses of spores nestle under the skin and swell out the upper forkings, and oblong tetraspores are deeply imbedded in the same.

In the Dumontia filiformis the simple undivided stem and branches are filled with a watery jelly.

The stem of the Chylocladia kaliformis is a cylindrical tube, from four to eighteen inches high, constricted at intervals of half an inch or more into long hollow joints; branches of the very same construction but smaller spring from each constriction either opposite to one another or in whorls; these again have lesser branches, all tapering more or less to each end. The plant, which is of a pink colour fading to greenish yellow, is propagated by tetraspores imbedded in the branches, and by transparent conceptacles sessile on the branchlets, enclosing nuclei containing pyramidal spores. We neither possess the Constantinea rosa marina, nor the C. sitchensis, some of the largest and finest plants of the group, both being inhabitants of high latitudes, but there are some very pretty species on the British coasts. They are supposed to be annuals.

The red dulses belong to the foliaceous and gelatinous part of this order. The Chondrus crispus, or Irish moss, sold as Carrigeen, is very common on rocky coasts in the northern seas. It is from three to eight inches high, and exceedingly varied in form. The frond is thickish, firm, and elastic, with a stratum of cellular tissue under the skin, which is probably much developed, as the plantbecomes horny when dried. It is reproduced by tetraspores, in large oval groups scattered all over the surface, often prominent on one side only, and, in some rare instances, spores in prominent oval conceptacles are immersed in the lesser frond divisions. Besides these are warts composed of radiating threads, possibly antheridia, but not made out.

Fig. 26.—A, Rhabdonia Coulteri, portion of nucleus.B, Sphærococcus coronopifolius; portion of nucleus and single spore.C, Wrangelia penicillata, spore threads.D, Cruoria pellita, tetraspores.

The Rhodymeniaceæ are sometimes filiform, but for the most part they are compressed flat cellular fronds, spreading widely from a short delicate stem. They are usually of a blood red, but Rhodymenia palmata, or common Scotch dulse, is of a dark purple. The tetraspores are variously disposed, and simple or compound globular conceptacles containing nuclei are either attached externally to the filiform fronds, or partly immersed in those that are foliaceous. The spores are produced in the joints of moniliform threads within the nuclei, which are sometimes divided into two chambers bythreads running from wall to wall. Rhabdonia (fig. 26A) belongs to this group.

The Wrangeliaceæ are filiform, many species consisting of a central thread coated more or less with smaller ones, sometimes so disposed as to form a most elegant lacework. Each joint of the stem, branches, and branchlets is beset with whorls of short slender forked and jointed ramuli. They have clusters of spores in stalked capsules. The spore threads of Wrangelia penicillata (fig. 26C) are surrounded by a whorl of ramuli composed of radiating pyriform spores arising from the endochrome of terminal cells.

The Squamariæ resemble lichens in spreading themselves in a red crust over stones and rocks. They have roots below, and warts, on their upper surface, in which there are tufts of moniliform spore-bearing threads. The tetraspores of Cruoria pellita are shown infig. 26D; its repeatedly forked filaments taper upwards, and the tetraspores are formed in the swollen centre cell of the filaments. The Peyssonnelia grows on shells and other marine objects, and extends from the Mediterranean to Ireland, and the east coast of North America.

The Polyides rotundus (fig. 23A), representing the only genus of the Spongiocarpeæ, has a dark purple solid gristly cylindrical stem, repeatedly and regularly forked, all being of the same thickness. The tips of the last forkings, which are small and equal, give the top of the plant a rounded form. The microscope shows that the stem and branches are composed of a central column of interlaced threads and radiating cells; it shows, moreover, that hyaline nuclei containing a cluster of conical spores whose broad bases radiate in all directions from a centre, as infig. 23a, are scattered among the articulated threads of oblong irregular spongy warts which clasp or embrace the stem and branches. The tetraspores are buried in the ends of the last forks.This plant is so like the Furcellaria fastigiata (fig. 23B), that it affords a remarkable instance of similarity of form and total diversity of fructification, not only in the spores and their arrangements, but in the form of the tetraspores; for in the Polyides they are formed by two sections, one vertical and the other horizontal, while in the Furcellaria the endochrome is divided by three annular sections, as infig. 23E.

The Gelidium corneum, common in Britain and almost everywhere, representing the group Gelidiaceæ, is opaque, firm, and of a dark purple. The axis with its alternate and repeated branches lying all in one plane, is composed of confervoid threads. This plant is distinguished by having its spore-cases or nuclei divided into two chambers by a fibro-cellular substance; the spores are either attached to this or to a network of threads; these bodies and the tetraspores are lodged in the tips of the branchlets. It is one of the most variable of all Algæ.

The Sphærococcoideæ comprise some of the most common and beautiful Algæ, remarkable for their brilliant rose and purple tints. This section consists of those red Algæ which have their nuclei lodged in an external subglobose conceptacle, the spores being formed at the tips of jointed threads rising from a substance at the base of the nucleus. A portion of a nucleus of Sphærococcus coronopifolius, and a single spore magnified, is shown atfig. 26b. The tetraspores are variously disposed. The fronds in this family are either gristly or membranaceous, and totally different from those which follow. They often assume a leafy aspect from the regularity of the nerves, which sometimes perform the functions of a stem when the membraneous border has decayed, and then they give rise in turn to new fronds. That happens in some species of Nitophyllum: a very short stem rises from a minutedisc, and spreads widely into a flat ribless expansion, more or less deeply slit into broad rounded divisions. Wavy nerves from the top of the stem spread through the fronds, which are left bare in winter, and give rise to new fronds in spring. The leaves of the Delesseria sanguinea, from two to eight inches long and from one to six inches broad, are of the richest colour and most delicate structure, with evenly curled edges, and a firm solid stem, with prominent midrib and nerves. In winter globose stalked spore conceptacles are borne on the skeleton midribs of the summer’s leaves from which the margin has decayed, which thus become the stems of the next year’s plant. In this plant tetraspores in small special stalked leaflets fringe the skeleton midribs; in the Nitophyllums they are either scattered in dots over the frond, confined to the centre, or in lines round the margin. As regards the internal structure of this order, nothing can be more various, but they never acquire a truly articulate form. The genera and species of this group are widely distributed; they have many representatives in the Mediterranean.[41]The genus Sphærococcus is confined to Europe, while numerous genera are exclusively tenants of the southern hemisphere. The Gracilaria lichenoides, the Ceylon moss, is celebrated for its gelatinous qualities; and the Gracilaria compressa on our own shores is excellent as a pickle or preserve, and very ornamental. One of the most beautiful Algæ known is the Grinnelia americana, which abounds on the eastern coast of North America;fig. 24b, is a vertical section of its conceptacle, showing the rudimentary placenta and spore threads. It differs singularly from the Delesseria sanguinea, of which it is an exact analogue, in the capsules being scattered over the surface of the frond instead of being situated onthe midrib.[42]The Delesseria sanguinea is now known as the Wormskioldia sanguinea.

The Corallines are florid Algæ, which absorb such a quantity of carbonate of lime from the surrounding water, that they become rigid, hard, and often stony. They are purple or pink when fresh, white and sometimes brittle when dry, and are propagated by strings of spore threads rising from the base of the nuclei which are enclosed in conceptacles or spore cases, open at the top. Some are articulate, composed of closely compacted threads, as the Corallina officinalis, a pretty little branched and bushy plant, most luxuriant in deep water, and particularly abundant in the rocky pools. Its urn-shaped spore sacs are attached to the tips or sides of the branches;fig. 24cis a vertical section of one of them magnified, anddis a membrane of the same, more highly magnified, with impressions of the external cells. The joints of the articulate corallines, which are flexible and vary much in length, are either free from carbonate of lime, or ornamented with calcareous plates; it is through these open spaces that the plant is believed to obtain nourishment. The forms of the corallines are varied beyond description; many are mere amorphous crusts on stones and sea weeds, increasing from the centre outwards as in the lichens, others are lobed and branched like real corals. Corallines ascend to very high latitudes, but abound most in warm and tropical seas: either free, or coating pebbles at vast depths, they form the last zone of vegetable life.

The Laurenciaceæ have fronds which are soft and thread-like, or solid, fleshy, and inarticulate; both are repeatedly branched. The colour of these plants is purple or a dullish red, but they are extremely sensitive to the influence of light and air, changing through every shade of orange, yellow, or green, according to theexposure, and like many other florid Algæ they lose their colour in fresh water. They are amongst our commonest sea weeds. The Laurencia pinnatifida is the pepper dulse of Scotland, and is also native on the eastern and western coasts of North America. Species have been found at the Cape of Good Hope, Australia, and New Zealand. The fructification in this section is quite peculiar. They have tetraspores lodged in the branchlets; and egg-shaped conceptacles with a terminal pore, enclosing nuclei with pear-shaped spores radiating from a fibro-cellular mass at their base. The antheridia, which differ in the different species, attain a greater degree of complication than in other tribes. In Laurencia tenuissima they form curious lateral twisted plates of a greyish tint, bordered with large cells. The plate is occupied by the productive cells of a much smaller size, evidently springing from a cellular branched axis. In Laurencia pinnatifida instead of a plate there is a somewhat hollow cup-shaped disc, formed of dart-like vertical groups of pale cells surmounted by two or three larger oily-looking sacs filled with yellow pigment. These bodies are sometimes forked, and appear to shoot out from the mass. L. dasyphylla presents a third modification, the antheridium being a sac, and the dart-like groups of cells being ejected from the minute terminal orifice. The moving particles produced in the cells of these three forms, differ a little in shape, and as they do not germinate they are believed to be spermatozoids, though no cilia have been found on them.

The Rhodomelaceæ, the last and highest family of florid Algæ, are, as the name implies, of a rich red brown colour. None of the other Rhodosperms can vie with them in peculiarity or variety of structure. The fronds may be areolate or reticulate, filiform or variously leafy, articulate or inarticulate.

Fig. 27. Dictyurus purpurascens.

Some genera, as for example Dasya, have slender,often elegantly branched threads, while such genera as Amansia and Odonthalia have instead a flat and pinnatifid frond. The latter, which has a very conspicuous cellular reticulation, is a genus of high latitudes, but is common on some parts of the Scotch and North American coasts. The British seas are rich in many genera of this order, and analogous forms occur in the southern hemisphere, where there are at least twenty-three genera. Many are remarkable for their singularity of structure: the Claudea for example, which is one of the most elegant of the Algæ, has a cancellated frond and is the ornament of warm seas; the Amansia and Leveillea which are distinguished by the beautiful reticulation of their fronds caused by large hexagonal cells; and the Dictyurus, in which the net forms a spiral web round the principal stem.Fig. 27shows a portion of the network of Dictyurus purpurascens magnified. All the genera of this order possess free areolate hollow conceptacles perforated above, and containing nuclei, from the base of which short tufts of threads arise, each bearing a large obovate spore at its apex. The tetraspores are arranged in series either within the frond, or in distinct pod-like receptacles called stichidia.Fig. 28shows the Polyzonia cuneifolia with its tetraspores arranged in rows in their pod-like stichidia, together with the areolated conceptacle and spores, all highly magnified. The antheridia differ in form in the different genera. In the Dasya they assume that of pods full of cells, in which the motile particles are generated; in the Rytiphlæa tinctoria the antheridia resemble those of the Dasya exceptin being elliptical, and in the Rytiphlæa pinastroides they are cellular bodies, without any investing membranes, clothed with delicate hairs.

The form of the Rhodosperms, as well as the limits of the species, like those of other Algæ, are affected by many circumstances known and unknown, such as the depth, temperature, saltness, and currents in the water. The Gelidium corneum varies to such an extent that its forms may not only be considered as distinct species, but even as belonging to different genera. The Delesseria alata is sometimes destitute of its margin, and then its midribs alone being left, it has the form of the Delesseria angustissima. Several species of the florid Algæ, which in their natural state have the tips of their fronds even and straight, occasionally produce hooked and clasping tips.

Fig. 28. Polyzonia cuneifolia.

Brackish water is often a cause of change. The Irish moss, Chondrus crispus, when exposed to the fresh water of an estuary acquires great breadth and thickness, while at low water mark it is thin and has narrow forked branches, and there are many intermediate forms. The fruit rarely varies with these changes; its disposition and intimate structure, as well as that of the frond, are the points of prime importance for the determination of genera and species in the Algæ.

TheMelanospermeæ, or Melanosperms, are olive-green Algæ, sometimes inclining to brown. They have fewer species than the Rhodosperms, but the individuals exceed in abundance and in magnitude all the other Algæ.

These large Melanospermous Algæ, which form marine forests in both hemispheres, are excessively strong and tough on the exterior but of a looser texture within, so that the cells of their tissue are of different sizes and forms, according to the degree of pressure. The stems and branches are more dense than the leaves. This highest order, however, has small and delicate Algæ united to the largest by many intermediate forms. The Melanosperms are either monœcious or diœcious, and bear their olive-green spores in cases, that is cysts, variously disposed on the plants. Many have two kinds of zoospores differing in nothing but size; they are produced in different organs; in some species both are fertile, in others only one, and, in these cases, the other is therefore supposed to be a fertilizing body, but however that may be, there are certainly antherozoids in this group of Algæ, especially in the order Fucaceæ.

The Ectocarpeæ have many representatives on our coasts, all of which are tufts of articulated threads from one to eighteen inches long, branched or simple. They are generally soft, some so flaccid that they cling together, but sometimes they are firm and stiff. The cysts which are attached to these threads have various forms; they are spherical, siliquose (that is, like long pods), or of other shapes, according to the species; but whatever form they may assume, they are filled with a dense endochrome. Besides these they have active granules contained in other distinct organs. M. Thuret has decided beyond a doubt that the latter are small zoospores, and it is presumed that the endochrome in the cysts is resolved into zoospores, but of a differentorder, as in the Ulvas. These two organs are for the most part situated on different individuals; in Ectocarpus pusillus (fig. 29b) they are on the same. The different forms of fruit carpels are represented magnified infig. 29.

Fig. 29. Fruit of Ectocarpus:—a, E. sphærosporus;b, E. pusillus;c, E. fenestratus;d, E. fasciculatus.

The Ectocarpeæ contain little or no gelatine, whereas the genera of the group Chordariæ have soft gelatinous fronds of many forms, either incrustations, convex lumps, or tubers, like the Leathesia so common on our coasts; small plants as the Mesogloias, which have soft slippery filiform stems beset with myriads of moniliferous worm-like branches; or lastly the Chorda filum, a simple unbranched slimy cylindrical cord, varying from a quarter of an inch to the thickness of a pencil, and from one to twenty or even forty feet in length in deep water. The cord is tubular, divided into chambers by transverse partitions, formed of interlaced vertical and horizontal articulated threads. It tapers at each extremity, and the exterior, which is brown, is clothed with pellucid hairs. Vertical spores are immersed throughout the whole surface of the cord, and Dr. Harvey says that, mixed with these, there are numerous narrow, elliptical, transversely striated cells, which according to M. Thuret produce zoospores. Each plant rises solitary from its own little disc, but as the Chorda filum is a social plant, vast assemblies of it cover extensive areas of sand and mud, and form dense thickets in our northern seas. There are bands of it in the NorthSea 15 to 20 miles long, and more than 600 feet wide; there is a submarine forest of it in Skapta Bay, Orkney; and in passing through the sounds of the western islands, as between Kerrera and the mainland, there are others. The long cords always lean in the direction of the tide, and must oscillate between two zones of rest, one at the turn of the flood, and another at the turn of the ebb. When dried the people use them for fishing lines. In the Chordaria divaricata both kinds of spore cysts are external, and give rise to zoospores.

In the preceding divisions of the Melanosperms the fronds consist of articulated threads; in the succeeding divisions the fronds are inarticulate. The latter comprise four very remarkable groups, of which the Dictyoteæ are distinguished by a leathery or membranous frond, sometimes cylindrical, but mostly flat, the surface of which is reticulated and sprinkled with groups or little patches of naked spores or cysts. The endochrome in the cysts is sometimes quadripartite, or even divided into eight parts. In one of the genera only, anything like antheridia have been found. The zoospores produced from the quadripartite endochrome are large, of a dark colour, and have two lateral cilia, while the bodies in the filiform much divided antheridia seated variously in the tufted threads are far more minute and pale, but with similar cilia. This order obtains its maximum of development in the tropical and subtropical regions; several species are found in the Mediterranean, while a few occur on our coasts, and on those of North America.[43]

The genus Dictyota begins the zonarioid group, whose structure is very curious. Every band (lacinia) of the frond terminates in a single cell, by the constantdivision of which at the lower side, the other cells of the frond are formed, the terminal cell of the frond being thus continually pushed onwards. Hence it results that the longitudinal lines of superficial cells converge, thus affording a ready method of ascertaining the genus in default of fructification. When a new centre of growth is to be made, that is, when the frond is to become forked, the terminal cell divides longitudinally and then each half-cell grows according to its own law.Fig. 30shows the tip of the frond of the Dictyota dichotoma magnified; the cells on its surface are square, and the interior of each has a spiral structure.

Fig. 30. Dictyota dichotoma:—a, tip of young frond;b, cell showing spiral structure;c, threads from marginal cells;d, sorus of spores;e,f, terminal cells dividing into new centres of growth.

The Padina Pavonia, or Peacock’s-tail laver of our southern coast, and those of North America and the Mediterranean, is sometimes included in the genus Zonaria. The species is remarkable for its wedge-shaped fronds, which are olive green shaded with rust colour, and, when in fruit, they are striped across with dark concentric zones, which are merely lines of spores immersed in the frond and seen through its transparent superficial membrane. Each zone is ornamented with a fringe of orange-coloured hairs. Parallel to, or rather concentric with, the spores, is a row of articulated threads, which bear so strong a resemblance to the antheridia of the Cutleria that a similarity of function is suspected by Mr. Berkeley. Species of Zonaria, Padina, and Haliseris, which is themost highly developed of the Dictyoteæ, are most abundant in tropical and low latitudes.

The Cutleria multifida is a small plant not exceeding eight inches in length, of an olive green varied with rusty tints. The frond is a flat ribless expansion many times variously slit in the upper part. It is beautifully marked by prominent dot-like tufts of fructification scattered over both sides of the frond, and grows on rocks and shells in from four to fifteen fathoms water.[44]

The great Laminariæ form the principal part of those vast submarine forests which encircle the globe in the arctic and antarctic oceans. None of these gigantic Algæ are to be met with in low latitudes, but there are several smaller species. The Laminaria debilis of the Mediterranean is not more than five inches high, and we have some ribbon-shaped species also of small size. Besides, many small individuals of the large species grow on our coasts at low water mark or below it; but the largest individuals are only found at depths suited to their size, so that the great Laminaria, or tangle forests, extend from low water mark to a depth of fifteen fathoms.

The fronds of these Algæ are for the most part leathery and of a fibro-cellular consistence. The Laminaria bulbosa is the largest of our sea weeds. Mr. Berkeley says that individuals are sometimes found which are a sufficient load for a man to carry. A flat stem, often more than a foot long, rises with a twist from a round hollow bulb a foot in diameter, throwing out numerous stout fibrous roots below; the stem is bordered by a thin wavy membrane, whence these plants are commonly called sea furbelows. At the top of the stalk there is a broad leafy expansion cut into straps or segments, twelve or more feet long, and from one to two feet wide.

The Laminaria digitata, commonly called the great tangle, oar weed, or sea girdle, has a fibrous root, a stem six or more feet long, with a wide expansion at its top cut into very long narrow segments. The fronds of some Laminariæ are deciduous; the stem increases in size year by year, a new frond springing from the apex and replacing the old one, which at last separates from the point of junction with the new frond, to which it is attached till the latter has attained its natural form and dimensions.

The Laminaria saccharina, called the devil’s apron on our northern coasts, is of a greenish olive when young, brownish when old. It has a fibrous root, a stem several feet long, ending in a flat ribless ribbon-like expansion, always very much longer than the stem, and terminating in a point. The margin of the frond is even, but wavy or puckered.

‘The fruit of these three great Laminariæ is imbedded here and there in the surface of the frond, thickening it and forming cloudy patches.’[45]It consists of thick club-shaped perpendicular cells in which the endochrome is ultimately divided into four parts. This is certainly the case in the Laminaria bulbosa, and also in the Alaria Pylaii, a species of which latter genus, the Alaria esculenta of our own coasts, is a much esteemed British dulse.

Abundance of colossal Algæ are found in the North Pacific, about the Kurile and Aleutian Islands, and along the deeply indented and channel-furrowed northwestern coast of America. The Nereocystis Lutkeana forms dense forests in Norfolk Bay, and all about Sitka. Its stem resembles whipcord, and is sometimes 300 feet long. It is exceedingly slender at the top, where it terminates in an enormous air-bladder six or seven feet long, and about four feet and a half in diameter at its widestpart, the lower extremity passing into the stem. This huge air-vessel, which is the usual seat of the sea otter, is crowned with a tuft of twin leaves mostly rising on five stalks. These leaves, which are membranous and lanceolate when young, and from one to two feet long and two inches broad at the centre, are only marked with a few faint nerves, but they ultimately split lengthwise, cover a large space, and attain a length of twenty-seven or thirty feet, or even more. The growth of the Nereocystis must be enormously rapid, since it is an annual, and must therefore develop its whole gigantic proportions in one summer.[46]Boats cannot pass through the floating masses of this plant, whose stem is used for fishing lines, and whose cylindrical air-vessel serves as a siphon for pumping water out of boats.

The Thalassiophyllum Clathrus is also an inhabitant of the Russian coast of North America. It is about six feet high, very bushy and branched, each branch bearing a broad leaf at its extremity which unfolds spirally, and by this gradual development produces the stem with its branches and lateral divisions. A spiral border wound round the stem indicates the growth of the frond, which presents a large convex bent lamina without nerves, or a leaf of which one-half is wanting. Numerous long narrow perforations, arranged in a radiating form, give it the appearance of a cut fan.

The Macrocystis pyrifera and the Laminaria radiata are the most remarkable of marine plants, for their gigantic size and the extent of their range. They are met with on the antarctic coasts two degrees nearer the pole than any other vegetable, except the Diatomaceæ. The stem of the Macrocystis is slender, smooth, round, and slimy, rising from a fibrous root, like other Laminariæ, and bearing at its tip a lanceolateor oblong lanceolate frond. This frond divides at the base; the fissures extend upwards so as to form two petioles, each of which swells into an oblong or pyriform air-vessel. Another fissure is formed in a similar way a little above, and so on, till a single frond may at the same time have eight or ten fissures, each of which will ultimately gain the common apex. The margins of the fissures are at first perfectly smooth, but they soon become ciliated like the outer edge. The continuity with the fibrous base is at last broken, and the divisions of the leaves going on indefinitely, the whole reaches the length of some hundred feet, forming enormous floating masses which are wafted by the waves hundreds of miles from their origin. Fructification only takes place in young plants; consequently in such as are still attached to their native rocks. Even in that youthful state, Mr. Darwin mentions that such is the buoyancy of this powerful weed, that there is scarcely a loose block of stone on the coasts of Cape Horn that is not buoyed up by it.[47]The Macrocystis is native on the shores of the Atlantic, from Cape Horn to 43° S. latitude; but on the Pacific coast, according to Dr. Hooker, it extends to the river San Francisco in California, and perhaps to Kamschatka. The plantis reproduced by pyriform cells, full of endochrome, in nearly parallel rows imbedded in the fronds.

The rocky coasts of the Falkland Islands are covered with a vast growth of the gigantic Macrocystis mixed with forests of the arborescent Lessonia, which forms large dichotomous trees with a stem from eight to ten feet high and a foot in diameter. The leaves are two or three feet long, drooping from the forked branches like weeping willows. In the Lessonia nigrescens the quadripartite endochrome, ultimately resolved intospores, is contained in thickened club-shaped cells springing vertically between the surfaces of the frond.

A transverse section of the stem of many of the larger sea weeds presents zones, formed period by period, corresponding with the development of the laminæ, roots, and branches. The stem of the Lessonia bears a strong analogy to that of dicotyledons in having rings of growth, though there is a great difference. As increase in Lessonia takes place by the constant division of a flat leaf, the basilar portion of which becomes the petiole and ultimately swells into a branch, the stems have always a more or less elliptical form, and their section exhibits an elliptical core. This form of the core is not however peculiar, but exists in other Algæ. It is probable that the Lessoniæ, although attaining so large a size, are really of rapid growth.[48]

The Ecklonia is essentially a southern genus, though one species ascends to Spain and the Canaries. The frond is pinnatifid, the segments arising from the evolution of marginal teeth. The stem of the Ecklonia buccinalis, which is three or four inches thick and strongly inflated above, exhibits rings of growth with an orbicular central pith.

The group of the Fucaceæ exhibits the highest structure of all the olive-green Algæ, and forms a large portion of the sea weeds on our coasts, but they abound more in individuals than in the number of genera and species. A few have cylindrical stems and branches swelling out at intervals into large oblong inflated air-vessels, which gives them buoyancy in the water. The rest have a flat, ribbon-like stem, and for the most part dichotomous branches with a decided midrib, but no air-vessels, because they chiefly grow at half-tide level, and are exposed twice every twenty-four hours. The most common of our fuci, the Fucus vesiculosus, or bladder-wrack, hasa midrib with air-vessels, generally in pairs on each side of it, formed by the inflation of the frond; these vessels, however, are frequently wanting, for it is the most variable in form and most widely spread of the Fuci. The fructification of this group is contained in large clavate receptacles or expansions of an orange or greenish yellow colour situated at the extremities or borders of the branches.

MM. Thuret and Decaisne discovered, by microscopic investigation, that the fuci have a truly sexual fructification, consisting of male and female cells inclosed in these receptacles. In the common Fucus vesiculosus it was found that the male and female cells are either in different individuals, or in different conceptacles on the same individual; whilst in the Fucus platycarpus, both the male and female cells were found to be contained in a globular cavity enclosed in the flattened receptacles which grow at the extremities of the branches. The cavity is lined with jointed hair-like filaments formed of cells, some of which are so long as to project through a pore on the surface of the receptacle in a spreading brush (seefig. 31, where the whole is highly magnified). Towards maturity, the cells of some of these filaments assume an ovoid form; the white viscous, granular matter in their interior acquires an orange hue, and is divided into a multitude of hyaline particles, each having an orange spot and two cilia of unequal lengths, which enable these spermatozoids to swim with great vivacity in the water as soon as they are set free by the rupture of the cell in which they are inclosed. Besides these, dark olive-green female cells, of a large pyriform shape, are fixed to the walls of the same cavity by very short stems; their contents spontaneously divide into eight spore cells, never more; each contains a colourless viscous liquid, which is mixed with protein and yellow-green matter, and is inclosed in a double coat. ‘Thecoats are united at the base, and when the spores are ready for dispersion, the inner coat bursts through the apex of the outer one, dragging with it a portion of the latter in the form of a little peduncle. The immediate covering of the spores at length bursts, and they are set free.’[49]In Fucus serratus, vesiculatus, and nodosus, swarms of spermatozoids are produced, but M. Thuret has proved by experiment that they never come to anything of themselves, and the unfertilized spores perish.

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