Once a Week (magazine)/Series 1/Volume 8/Brown seaweeds
BROWN SEAWEEDS.
Some of my readers may perhaps remember to have been amused, as children, by an ingenious toy contrived for the exhibition of the curious changes of form, termed Anamorphoses. It consisted of a small cylindrical mirror, and a flat disc of cardboard, the surface of which presented to the eye a patch-work of divers colours, so disposed that the closest observation could detect in their arrangement neither regularity nor design. Here was a patch of brown, there a streak of red, there again a shapeless blotch of green, and in the centre of the disc a small circle left uncoloured. But on placing the polished metal cylinder upon this bare central space and looking, not at the pasteboard disc, but at its reflection in the mirror, a marvellous change was apparent. The patches of brown became transformed into horses, the pink dots into red-coated huntsmen, the green blotches into trees and fields; instead of a confused mass of colour, we had an orderly picture—a hunting field, a landscape, a Dutch interior, or some other scene more or less comic according to the taste or skill of the designer.
Just such a change as this is produced by a few pints of salt water, on the dirty brown masses of seaweed which are thrown by the waves upon our shores, or which cling to and disfigure the rocks. Just so strange is it to mark how quickly and suddenly, twisted and knotted fibres start apart as the water touches them, each assuming its own proper place and falling into lines and curves which no artist save Nature herself can hope successfully to imitate. Difficult indeed it is to believe that the lovely little tree-like structure waving its branches so gracefully in the rock-pool just filled by the advancing tide, is the same with the mass of inextricable confusion making the rocks hideous, from which a moment ago we turned our eyes with a feeling near akin to disgust.
And not over form only does water exercise an influence so powerful, calling order out of chaos, symmetry out of deformity. Over colour, too, its spell is equally potent, giving to the forms which it has created a new and brilliant lustre and collours not their own.
And in no case is the change thus produced more striking than in that of the plants belonging to the order of the Melanospores or Olive-spored seaweeds. For these, unlike most other seaweeds, have when left dry by the receding tide, no bright colours to attract the eyes; they have lost, not a part only, but the whole of their beauty, and have become for the most part utterly hideous and repulsive, and few people therefore know how lovely they really are. For indeed the sea, which has so many beautiful sights to offer us, can show us few more beautiful than a bed of the large brown seaweeds seen on a bright summer day a few feet below the surface of clear water. Looking down through the shadow of our boat we see huge fronds of Oarweed waving majestic in the gentle current, while here and there attached to the broad brown leaves shines a glorious living flower—the lovely green Anthea—one of our commonest English anemones. Long strings of chorda filum (Dead Men’s Ropes), clothed with delicate glistening fibres as of spun glass, shoot straight up to the surface ten, twenty, or even forty feet. Fuci, brown, yellow, and orange, nod and bow to one another in so grotesque a fashion, that we could fancy that like the knight in Fouque’s tale, we were looking down at the goblins in the centre of the earth rolling and tumbling over one another in their sport.
And not the most brilliant of the red seaweeds can show a more beautiful colour than the rich brown of the Oarweed or the lovely phosphorescent green which plays about the fronds of the heath-like Cystoseira, like the lambent light which flickers on a summer night along the edge of each retiring wave. Beautiful indeed is the colour of this last plant, and as evanescent as beautiful, vanishing at once when it is removed from the water, and reappearing as soon as it is replaced.
The head-quarters of the Olive seaweeds are the Equatorial seas. There they flourish with a luxuriance unknown in colder climes, and attain a size which dwarfs by comparison our largest English forms. Our own Oarweed is sometimes found with a stem five feet long, ending in a flat frond four feet long and three wide; and Chorda filum, as we said before, grows to a length of thirty or forty feet. But what say you to a seaweed a quarter of a mile long? The stem of the Microcystis, a tropical seaweed, is said to attain the length of 1500 feet, forming a simple unbranched cord until it approaches the surface of the sea, when it forks repeatedly, each division bearing a single small leaf, and the whole forming a floating mass of foliage some hundreds of square yards in extent. We shall be better able to realise the enormous length of this plant if we remember that the largest forest tree with which we are acquainted, the Californian Wellingtonia, attains a height of not more than 450 feet.
The Nereocystis again, which is found on the western shores of America, though less gigantic than the Microcystis, is still of a tolerable size. The stem of this plant, though no thicker than whipcord, is upwards of 300 feet in length, and bears at its top a huge vesicle or hollow bag six or seven feet long, shaped like a barrel, and crowned with a tuft of fifty or sixty forked leaves, each from thirty to forty feet long. The stem of this plant is so strong that the natives often use if for a fishing line.
Less again than the Nereocystis, but not less quaintly shaped, is the Eklonia, the South African Trumpet-weed; so called because the native herdsmen use it as a trumpet for calling the cattle home in the evening. The stem of this plant is about twenty feet long, two inches in diameter at the base, and gradually widening upwards until it ends in a fan-shaped cluster of leaves each about twelve feet long.
And whilst we are speaking of tropical forms we must not forget to make mention of the “Praderias de Yerva,” the seaweed meadows of Columbus, vast fields of floating seaweed covering the surface of the sea for hundreds of square miles. One of these, Sargassos, as they are termed, is thus described by Lieutenant Maury in his “Physical Geography of the Sea:”—“Midway the Atlantic in the triangular space between the Azores, Canaries, and Cape de Verd Islands, is the great Sargasso sea. Covering an area equal in extent to the Mississippi valley, it is so thickly matted over with Gulf-weed that the speed of vessels passing through it is often much retarded. When the companions of Columbus saw it they thought it marked the limits of navigation, and became alarmed. To the eye at a little distance it seems substantial enough to walk upon. Patches of the weed are generally to be seen floating along the outer edge of the Gulf stream. The seaweed always tails to a steady or constant wind, and thus serves the mariner as a sort of marine anemometer, telling him whether the wind as he finds it has been blowing for some time, or whether it has just shifted, and which way. Columbus first found this weedy sea on his voyage of discovery. There it has remained until the present day, moving up and down and changing its position like the calms of Cancer, according to the season, the storms, and the winds. Exact observation as to its limits and their range, extending back for fifty years, assure us that its mean position has not altered since that time.”
The plants of which these vast floating meadows are composed belong for the most part to the species Sargassum bacciferum—the Berry-bearing Gulf-weed—which, unlike most other seaweeds, floats freely in the water, and is seldom or never found growing upon stones or rocks. This peculiarity is shared by at least one of our English seaweeds, Mackay’s Fucus, which inhabits salt marshes connected with the sea. The Gulf-weed is sometimes found upon our own shores, being conveyed there by the agency of the same currents by which West Indian fruits and seeds are occasionally thrown upon the western coasts of Ireland and Scotland. In appearance Gulf-weed closely resembles our own Bladder-wrack, differing chiefly in having the air-bladders from which the latter plant derives its common name, attached to short stalks instead of imbedded in the substance of the frond.
In all the great oceans, there exist two currents, one of warm water, running from the Equator to the Pole, and the other a return current of cold water, from the Pole to the Equator. In the quiet water between these two currents the Gulf stream has its home, and there we may expect to meet with Sargassos or seaweed seas. Thus, besides the Sargasso, sailed through by Columbus, which is bounded by the Gulf stream and the Equatorial current, there is another in the Pacific Ocean, whose banks are formed by the two currents, known respectively as Humboldt’s and the Black Current. There are now known and marked upon our charts at least five well-defined Sargassos, besides several smaller collections of seaweed. Of these latter the most remarkable instance is to be found in the Straits of Magellan, where the seaweed is often so thickly crowded as sensibly to impede the motion of the paddles of large steamers.
Of the three great classes into which the seaweeds are divided, the brown seaweeds are, with a few exceptions, the most highly organised. In them Nature gives the first hint of the division of the plant into root, stem, and leaf, of which she elsewhere makes so frequent use. The resemblance, however, of these seaweeds to more highly-organised plants, is merely external. The so-called root has neither the structure nor the functions of a true root, and the distinction between stem and leaf is one of external form only. Often, indeed, that which is now the stem has at an earlier period formed part of the leaf, just as in some of the red seaweeds the stem of this year’s plant was last year the midrib of the leaf-like frond.
The root of a seaweed serves no other purpose in the economy of the plant than that of attaching it to the stone or rock on which it grows. In many seaweeds it is merely an expansion of the stem into the form of a cone, the lower surface of which fits closely to the rock, and cleaves to it like the “sucker” with which schoolboys lift stones. In other plants, as in the common oarweed, the root is composed of a number of stout cords growing out of the frond, which twist round projections of the rock and insert themselves into its holes and crevices, grasping it so firmly as to defy the efforts of the waves to tear the plant from its station. In a few instances the distinction between root, stem and leaf, vanishes altogether, and the plant becomes a mere crust growing over the surface of the rock, or a shapeless mass, either hollow or filled with loose branching fibres. A few of the brown seaweeds are almost microscopic, forming a minute tuft of fibres, scarcely visible to the unaided eye.
The shape and structure of the frond is often very curious. In the common Chorda filum, the frond is a hollow thong, about the thickness of whipcord, divided at intervals into chambers by their partitions. In one common and very curious species—Himanthalia lorea—which we may translate Strapweed, the plant in the first year of its growth has much the same shape as a pegtop fixed to the rock by the peg, the upper surface, however, of the top, being concave or flat. In the second year a long flat strap shoots from the centre of the top, which strap would, by the uninitiated, be certainly mistaken for the frond. The microscope, however, shows that it is not the frond, but the fruit or seed receptacle of the plant. Not the least attractive of our English seaweeds is the pretty little Padina Pavonia, so called from its fancied resemblance to a peacock’s tail. The grey powdery fronds of this plant, with their delicately fringed margins, have a peculiarly exotic appearance; and in fact, Padina Pavonia, though not uncommon on our southern shores, is more at home in warmer seas. With us it shows its love of warmth by choosing shallow rock-pools exposed to the full rays of the summer sun, and by only attaining its full size in our warmest summers. A few of the brown seaweeds are beautifully iridescent, displaying in the water brilliant prismatic colours. The Cystoseira, whose glowing green is only visible in the summer months, we have already mentioned, and Padina Pavonia occasionally affords an instance of the same peculiarity. The flat, fan-shaped, repeatedly-divided frond of Cutleria multifida, frequently glows with a brilliant series of prismatic colours.
But the chief point of interest in the brown seaweeds, as in most flowerless plants, lies in the study of the problems connected with their fructification, and with the various processes by which the species are reproduced. In many of them we find instances of that strange process of reproduction by means of zoospores, or moving spores, which so often occurs among their green-spored relations. The zoospores may be well seen in the plants which belong to the genus Ectocarpus, one or two species of which are very common in the summer, spreading in a matted mass over rocks and larger seaweeds, and looking when left by the tide like a layer of withered and rotting leaves. The moving spores are formed in the last cell of each of the fine threads of which the plant is composed. They are small, brown, pear-shaped bodies, each bearing a minute red spot, from which spring two cilia or vibrating threads, one much longer than the other. The longer of these is directed forwards, and seems to be the chief agent in producing the motion, while the shorter trails behind and acts as a kind of rudder.
But in addition to this multiplication by zoospores, there exists in many, perhaps in all these plants, a yet more marvellous mode of propagation. We will try to describe this process as it occurs in the common Fucus platycarpus, or Flat-fruited Fucus, choosing this plant partly because it is one of the commonest and best known forms, partly because in it the various organs and structures concerned in the process always exist together in the same individual.
If we examine a number of these plants—especially during the winter months—we shall find that in many of them the end of the frond is expanded into a flat swelling, roughened on either side with numerous small nipple-shaped warts. It is in this thickened part of the frond that we must look for the fruit. By examining a thin slice with the microscope, we find that each of these warts is hollowed into a spherical cavity, communicating with the air by means of a small pore. The interior of the cavity is thickly lined with a great number of fine fibres, some of which are very long and project through the pore, forming a small tuft on the exterior. In some of these fibres, lying within the cells of which they are composed, may be seen certain small oval bodies not unlike the zoospores just now described, each having a bright orange dot and two long thread-like cilia. These bodies, like the zoospores, when discharged by the rupture of the cells containing them, have the power of moving rapidly through the water by means of their vibrating cilia. But unlike the zoospores, if left to themselves they soon lose the power of moving and quickly decay. What their true nature is, and what part they play in the life history of the plant, we shall see directly; for the present we will only remark that they are called “antherozoids,” a term for which the English language has no equivalent.
Returning to the slice of the frond which we have been examining, we see nestling among the fibres with which the interior of the cavity is lined, a few rather large, dark, pear-shaped bodies, fastened by their smaller end or stalk to the wall of the cavity. These are the “sporangia,” or seed-vessels. When ripe they burst, discharging eight or in some cases four spherical brown spores. These spores have by themselves no more power of growing into a new plant than the antherozoids; like them, left to themselves, they soon decay. It is only by the union of the two that a new plant can be produced. In the species which we have been describing, this union takes place within the frond, and it is therefore very difficult to get a peep at the process. But in the equally common Bladder-wrack, the antherozoids and spores are always found on distinct plants, and the fructification of the spores takes place after they have been discharged from the frond. Plants containing spores may be recognised by the olive colour of the thickened extremity of the frond, those containing spores by the same portion being orange or yellow. By mixing in a drop of sea-water<!— see list of hyphenated words --> a small portion of the contents of the olive and yellow receptacles, the whole process may, with the aid of a good microscope, be easily seen.
After a short time the antherozoids will be seen to fasten themselves to the spores, communicating to them a rapid rotary motion. The field of the microscope becomes covered with these large brown spheres, bristling with antherozoids, and rolling quickly about among the crowd of spores which are still unattached. The motion continues for about half an hour, after which the spores will be seen to have become coated with a thin transparent membrane. The spore is now fertilised, and if allowed to remain in the water soon begins to grow, and finally becomes a plant in every respect similar to that from which it was produced.
Formerly the brown seaweeds were largely employed in the manufacture of soda, being first burned in pits dug in the sand, until they were reduced to hard cakes, in which state the product was termed kelp. A curious instance of popular prejudice is mentioned by Dr. Greville in connection with this manufacture. When the preparation of kelp was first introduced into the Orkneys, the islanders resisted its introduction by all means in their power, and it was gravely pleaded that “the suffocating smoke of the kelp would sicken or kill every species of fish on the coast, blast the corn and grass, introduce diseases of various kinds, and smite with barrenness their sheep, horses, and cattle.” Seaweeds are now little used in the manufacture of soda, but still form a valuable source of Iodine. Doubtless, however, the chief purpose which they serve is that of removing from the sea the excess of soluble salts washed into it by the rivers, and if we call to mind that an increase in the saltness of the sea would, by diminishing the amount of evaporation from its surface, cause a corresponding diminution in the total amount of rain which falls upon the surface of the earth, we shall see that it is scarcely possible to overrate the importance of the work performed by these apparently insignificant plants.