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18 MARINE FLOWERING PLANTS Most flowering plants become metabolically stressed if exposed to sea water, as they are incapable of dealing with the high salt content. This metabolic stress ultimately results in the death of the plant. A few species of land plants have, however, successfully invaded and adapted to sea-water environments, flourishing in a variety of marine habitats. Unlike algae, land plants can grow only where they can absorb nutrients through their roots. These higher plants also need nearly direct sunlight, so they can only tolerate relatively shallow waters (1-30 meters, 3-100 ft). Color the red mangrove, starting with the prop roots at the lower right. Notice the junction of the trunk and roots at the high-tide mark. Color the entire leaf mass green. Color the falling seed and new root protruding from it, and notice the four young leaves emerging from the seed. One of the largest flowering plants to flourish in the marine environment is the red mangrove. These plants occur in tropical and semitropical regions from Florida to South America and reach tree size (1-3 plus meters, 3-10 plus ft). The mangrove grows best in the mud bottoms of estuaries, coastal lagoons, and near the mouths of large rivers, where silt is deposited. The mangrove remains stationary in the unstable mud bottom by sinking a mass of large, arching prop roots deep into the mud. The prop roots divide off from the trunk at the high-tide level; the roots proliferate and trap more sediment, raising the substratum level. This, in turn, creates more space where the mangrove can grow, and, in this way, the mangrove colony expands toward the sea. Mangrove roots are the habitat for a myriad of attached algae and animals such as sea squirts, sponges, and sea anemones. The lattice-work of roots also offers protected nursery areas to young reef fishes and spiny lobsters. The mangrove leaves fall into the water and decompose, providing a detrital food source for many animals residing near the mangrove, and others further out to sea. Seeds of the red mangrove sprout and begin to grow before they fall from the tree. A long (36 cm, 14 in), slender root emerges from the seed, and several leaves may also | grow. If the seed falls at low tide, it may poke, dart-like, | into the soft mud near the parent plant, and continue to grow. Seeds that fall during high tide float upright in the water and may be carried far from the parent plant. When the tide recedes, if the seed has been left on the mudflat with the root facing downward, it will be “drilled” into the mud by the jostling of small waves, thus beginning a new colony of mangroves. Now color the turtle grass and the surf grass. Note that the grasses’ stems receive shades of the same color as the mangrove’s trunk, and the blades receive shades of the same color as the mangrove’s leaves. These structures are homologous (same origin) and have the same function in these higher plants. The flowering plants known as sea grasses can grow completely submerged. Various sea grass species occur from the intertidal zone to depths of 30 meters (100 ft). Turtle grass is found in eastern Florida, along the Gulf of Mexico, and in the Caribbean, where it thrives on soft bottoms of mud, shell, or sand. Once established, turtle grass proliferates by extending underground stems (rhizomes) from which leafy blades grow. Turtle grass forms extensive shallow-water meadows, which are frequented by numerous juvenile reef fishes. At night, parrotfish and sea turtles come into these meadows to feed. The stems and roots of turtle grass bind the substratum, which permits the colonization of burrowing animals, such as sipunculid worms and sea cucumbers (Plates 25, 41). Unlike most sea grasses, which require quiet waters, surf grass grows in the wave-swept rocky intertidal zone of temperate waters. Surf grass seeds possess two stiff, bristled, pointed projections. If the seed lodges on an alga, it can germinate and send out tenacious roots to grab hold of the substratum. The surf grass roots trap sediment, and a patch of long, thin, bright green blades may develop in a tide pool or at the low-tide line. This creates an environment that shelters many species of worms and other small creatures. . 
