Fig. 31. Vertical section of receptacle of Fucus platycarpus.
Fig. 31. Vertical section of receptacle of Fucus platycarpus.
Fig. 31. Vertical section of receptacle of Fucus platycarpus.
When a fertilized spore begins to grow, it assumes apear shape, and sends out from its narrow end filaments or footstalks containing solid yellow grains at their extremities, where a hook or claw is formed by which it fixes itself to rocks or stones. The spore then divides itself into four equal cells of a brown colour, and by the continued subdivision of these into four, the plant increases in size, and assumes a form corresponding to the genus and species of the spore. Dr. Carpenter mentions that in the Fucaceæ there is also a multiplication by zoospores. These bodies are produced within certain of the cells that form the superficial layer of the frond, and swim about freely for a time in the water after their emission, until they fix themselves and begin to grow; but these are merely gemmæ.
All the Fucaceæ are tough leathery plants. This is even characteristic of the genus Cystoseira, various species of which may be seen on our coasts at low water mark, or in the tide pools. They are little shrub-like and somewhat thorny plants, not more than three feet high, with a cylindrical stem and many branches, near the extremities of which there are inflated air-vessels, sometimes two or three together; in some species they are lower down. Long spiny conceptacles are situated at the tips of the branches, but the endochrome does not divide in the germ cells as it does in the Fuci, so that each cell produces but one spore.
‘Throughout all latitudes the two divisions of Fucaceæ—Fucoideæ and Cystoseireæ, form the prevailing marine vegetation to which the name of sea-weed is commonly applied, and the different genera so arrange themselves as to present, with a few exceptions, a most harmonious assemblage.’ ‘None of these approach the tropics; the Fucoideæ abound towards the poles, and there attain their greatest bulk, diminishing rapidly towards the equator, and ceasing some degrees from the line itself; while the immense genus Sargassum finds its maximumin lower latitudes and under the equator itself. In the opposite cold and frigid zones the waters are inhabited by certain genera of Fucoideæ, which are in a great measure representatives of one another.’[50]The huge D’Urvillæa and the Sarcophycus in the Antarctic Ocean represent the Himanthalia and Fucus proper in the north, and the Cystoseireæ and Halidrys of the northern seas are represented by the Blossevillea and Scytothalia in the southern.
The frond of the Himanthalia lorea is a knob about an inch high, somewhat like a small mushroom; by degrees the top of the knob sinks in, and the frond becomes cup-shaped. In the second year of growth it throws out from its centre strap-shaped receptacles from two to three feet long and the sixth of an inch wide; they are slimy, forked, and entirely covered with fruit. The true frond sometimes becomes hollow and swells into a bladder. This singular plant, which grows on our coasts, extends from Norway to Spain. In the D’Urvillæa, its representative in the southern hemisphere, the frond and receptacle are united, for the plant, which is of large dimensions, has dichotomous fronds ten feet long, and an inch or more in breadth. Their surface is ornamented with large cavities like a honeycomb, and the fruit imbedded within them consists of antheridia and club-shaped germ cells with four spores in each. These plants form a large portion of the wrack and also of the living Algæ which surround the Falkland Islands and Cape Horn; and they extend to Western Chili, where the poorer class make a sweet mucilaginous soup of them. The Sarcophycus potatorum, the only species of its order, is nearly allied to the D’Urvillæa by the structure of its fruit, and is so named from pieces of its frond being used to carry water. Many other olive-green Algæ are peculiar to the southernhemisphere—among them the Hormoseira, in which the frond, at first even and filiform, becomes inflated so as to produce moniliform chains of vesicles, parts of which are at length rough with the apertures of the conceptacles; this plant has bladder-like air-vessels formed by swollen parts of the frond, like many of our Fucaceæ.
Those genera which have distinct organs containing air, as the Sargassum, of which there are numerous species, are either of low latitudes or tropical, but are sometimes drifted by currents to the extra-tropical shores. The Sargassum vulgare, however, grows on the rocks in the Mediterranean. The whole plant is of a translucent reddish brown; the stem has alternate branches, bearing lanceolate serrated leaves with a midrib, and generally dotted with dark pores. The air-vessels are small translucent round balls about the size of a currant, borne on flat stalks in the axils of the branches, and the spores are in conceptacles borne on the branchlets just above the air-vessel. In one variety of this most variable plant, the Uva di mare, the main stem ends in a loose bunch of these little air-balls.
The Sargassum bacciferum is often found in the Mediterranean, but only as a wanderer drifted in from the Atlantic, where masses of it, like floating meadows, occupy an area west of the Azores equal in extent to that of France, which has never changed its position since the time of Columbus, on account of the surrounding currents. Fields of it cover the seas near the Bahama Islands, and another permanent area of Sargassum of great extent occurs in the South Pacific. The Sargassum bacciferum is of a pale translucent olive colour, having branched stems, with lanceolate, midribbed, and serrated leaves, destitute of pores, and little stalked air-balls in the axils of the branches. The same individual continually produces new branches and leaves, and thus multiplies its species, but it never produces fruit; consequentlyits habits exactly resemble those of the Macrocystis, and as that plant becomes detached and floats after fructification, it is supposed that the Sargassum bacciferum may grow on rocks at the bottom of the Atlantic, between the parallels of forty degrees north and south of the equator, and when detached after fructification that it is uniformly drifted to particular spots which never vary. ‘Multiplication is so rapid in the floating beds of the Sargassum and Macrocystis, as to render fruit needless; and even the common Fucus vesiculosus occurs in the Mediterranean under a peculiar form consisting entirely of specimens derived from sea borne weed carried in by the current which sets in towards the Mediterranean from the Atlantic.’[51]
Kelp, the ashes of sea weeds, is the commercial source of iodine. Algæ growing in deep water contain most of that substance; consequently the kelp made at Guernsey, consisting chiefly of the ashes of Laminaria digitata, is richer in iodine than that made elsewhere.
Marine vegetation varies both horizontally and vertically with the depth, and it seems to be a general law throughout the ocean, that the light of the sun and vegetation cease together. It consequently depends upon the power of the sun, and the transparency of the water; so that different kinds of sea weeds affect different depths, where the weight of the water, and the quantity of light and heat, suit them best. One great marine zone lies between high and low water marks, and varies in species with the nature of the coasts, but exhibits similar phænomena throughout the northern hemisphere. In the British seas this zone does not extend deeper than thirty fathoms, but it is divided into two distinct provinces, one to the south and another to the north. The former includes the southern and eastern coasts of England, the southern and western coasts of Ireland, and boththe Channels; while the northern flora is confined to the Scottish seas, and the adjacent coasts of England and Ireland. The second British zone begins at low water mark, and extends below it to a depth of from seven to fifteen fathoms. It contains the great tangle sea weeds or marine forests mixed with fuci, and is the abode of a host of animals. A coral-like sea weed is the last plant of this zone and the lowest in these seas, where it does not extend below the depth of sixty fathoms; but in the Mediterranean it is found at seventy or eighty fathoms, and is the lowest plant in that sea. The same law prevails in the Bay of Biscay, where one set of sea weeds is never found lower than twenty feet below the surface, another only in the zone between five and thirty feet, another between fifteen and thirty-five feet. In these two last zones they are most numerous; at a greater depth the kinds continue to vary, but their numbers decrease. The distribution in the Ægean sea was found by Professor E. Forbes to be perfectly similar, only that the vegetation is different and extends to a greater depth in the Mediterranean than in more northern seas. He also observed that sea weeds growing near the surface are more limited in their distribution than those that grow lower down, and that with regard to vegetation, depth corresponds with latitude, as height does on land. Thus the flora at great depths in warm seas is represented by kindred forms in higher latitudes. There is every reason to believe that the same laws of distribution prevail not only throughout the ocean, but in every sea.