DISTRIBUTION OF MARINE VEGETABLE LIFE The distribution of plants on land is fixed by the con ditions of soil and climate; the greatest profusion being within the tropics, and gradually disappearing horizontally towards the poles, and vertically to the limit of perpetual snow. Marine vegetable forms are similarly distributed, both horizon tally and vertically. The prevalence of peculiar species within particular limits, permits of both the horizontal and vertical distribution to be broadly divided into more or less well-defined zones. Thus, horizontally, the Acne abounds in the North Atlantic, the sargassum in the tropical regions of the Atlantic and Pacific.
Vertically, the conditions of distribution are different, but scarcely less decided. On land the temperature decreases as the elevation increases ; but in the ocean there is a gradual fall of the temperature from the surface to the bottom. And as the height of the limit of perpetual snow decreases from the equator towards the poles, so also, but perhaps not so regularly, the limit of cold water, nearly ice-cold along the bottom, even under the equator, rises gradually until it reaches the surface at the Arctic and Antarctic Circles.
But vegetation would seem to suffer more from the exclusion of light than from any other cause, light being absolutely necessary to their continued existence. The sun's light pene trates but a comparatively small depth, there are therefore few plants at a depth of 50 fathoms, and none at 200 fathoms. The depths to about 100 fathoms (600 feet) have been broadly divided into zones, each zone having its peculiar plants. Thus the dulse and bladder-wrack abound in the littoral zone, as the space between high and low water mark is called ; while the tangle and scarlet sea-weeds characterize the laminarian zone, from low-water mark to a depth of 50 feet. The com mon sea-weeds decrease, and the corallines increase to a depth of about 300 feet. Lower still, to about 600 feet, is the coral zone. Except the microscopic vegetable forms called diatoms, the great depths of the sea are utterly devoid of vegetable life.