The Significance of Micro-plants

It has been shown that sea water possesses in solution all of the necessary inorganic elements, phosphorus, nitrogen, iron, and so on, for the manufacture of plant substance with the aid of light. In other words, the sea as a whole possesses the potentialities of sustaining such autotrophic organisms as are capable of capitalizing upon this tremendous supply of nutrient material held in solution. Although the sea offers these possibilities to plant growth, at the same time it also sets up certain serious physical difficulties and barriers by reason of the magnitude, depth, and fluidity of the oceans. We know that it is only the merest rim of the sea that has sufficient light and, at the same time, suitable substratum for attached and other bottom-living plants, and that in the open sea plant life is restricted by the factor of illumination to only the upper few meters represented by the euphotic zone. This relative restriction of living space makes it of vital importance to plants that they overcome or minimize materially the effect of gravity that might lead to their destruction below the euphotic zone. This serious challenge of the pelagic environment has been successfully met by the plants, and the method by which it has been conquered has determined largely also the kind of animal life that the sea can support.

In order that the vast stretches of the ocean may be populated and its nutrient resources used by plants beyond a depth of about 40 m, special types of plants adapted to a floating existence have evolved. To accomplish a pelagic habit, two methods appear to be open. (1) The development of buoyant forms with air bladders or similar features to keep them at the surface. This is the means evolved by the macroscopic alga Sargassum. This method, however, is subject to serious handicaps and can hardly be considered a successful one; moreover, although plants of this nature float in large masses and reproduce vegetatively in restricted places such as the Sargasso Sea, yet their natural home is along the shore attached to rocks from which they have been torn. Such buoyant plants can extract nutrients only from a shallow layer at the immediate surface and unless maintained in large eddies of relatively quiet water, they are subject to being rapidly driven by the wind and currents into coastal areas, there to be destroyed by stranding and by beating of the surf. The contribution of organic material by such plants is relatively small. (2) The really successful method by which plants have been able to

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conquer the barriers of the sea is by the evolution of single-celled microscopic plants. These plants, although of somewhat greater density than sea water, are able to remain in suspension in the euphotic zone long enough to assimilate nutrients and to reproduce (p. 764). Submerged microscopic plants escape the hazards of being blown ashore in mass and are able to extract the nutrients from a stratum of water extending from near the surface to a depth of about 20 to 40 m or more. The almost complete absence in the open sea and in many neritic waters of any but microscopic plants must in itself be evidence of the suitability of this type of adaptation for pelagic plants.