General Features of Seagrasses

• Plants clone by creeping rhizomes, buried under sediments, and form adventitious roots at nodes along the lower surface of the rhizome.
• Leaves are narrow and strap-shaped (linear), a design that would tend to limit damage via wave action.
• As in typical monocotyledons, leaf growth occurs only from a basal (intercalary) meristem. This is beneficial in particular for seagrasses, which have leaves colonized by epiphyllous (epiphytic) algae, so that growth at the base may reduce shading on new tissues by epiphytes. Leaves are protected when young by sheaths, which may shield the immature leaf region from desiccation by the saline solution.
• No leaf of a seagrass has stomates, and yet the leaf is typically covered with a thin, waxy cuticle. At first glance, this would appear to restrict carbon dioxide from entering the leaf, because gases do not diffuse well through wax. However, studies have shown that this cuticle is relatively porous in texture, so that carbon dioxide enters at a relatively rapid rate through the leaf surface.
• Leaf photosynthesis takes place almost exclusively within the epidermis, the outermost layer of leaf cells, not the central mesophyll (as in typical land plants). Epidermal cells have well developed chloroplasts and many organelles associated with active metabolism, whereas mesophyll cells of seagrasses possess very few chloroplasts and appear to store starch grains.
• The roots and rhizomes occur in an anoxic (anaerobic) environment, so that oxygenation is required for proper and rapid growth of anchoring organs. In seagrasses, oxygen generated inside chloroplasts during the light reaction of photosynthesis diffuses most easily to the center of the leaf, where there are large intercellular air chambers (lacunae), in a tissue type called aerenchyma. Use of oxygen by the roots creates a sharp oxygen gradient, from high levels in leaf aerenchyma (greater than 30% in these lacunae) to near zero in growing roots; hence, via mass flow oxygen diffuses from leaf to stem to root and rhizomes through aerenchyma air spaces. At each node there occurs a diaphragm, a wall of cells that appears to block passage of air molecules into the next segment (internode), but this is merely an illusion, because there are air gaps large enough between cells of the diaphragm that no blockage likely occurs in the pathway (based on calculated resistances).
• Seagrass organs seem to have scanty development of xylem, water-conducting tissues, in comparison with related land plants with equal leaf size.
• Flowers on all seagrasses are unisexual, i.e., either male or female, and in some species there are separate male and female plants (dioecious). For sexual reproduction to be successful, a pollen grain from a male flower must be transported to the tip of the female flower, in the ocean without the aid of an animal vector. Female structures and some male structures appear to be waterproof, so that tissues to not desiccate in the saline solution.
• Pollen grains of numerous seagrasses are extremely long and thread-like (filament-like or filiform). In fact, the longest one of any seed plant is of a seagrass, three millimeters in length with forked ends (Amphibolis antarctica)! These thread-like pollen grains often have no germination apertures (sulci or colpae) and have considerably reduced wall layers, even lacking certain zones. Researchers have analyzed possible effects of these features on pollination in the submarine environment, and speculated, with some corroborative evidence, that the filiform grains, which may be bow-shaped and are lighter per volume by having less wall, have a slower sinking velocity that spherical pollen grains. This would allow the pollen grain to be suspended in the water column longer, therefore have longer time to come in contact with the female flower and to take a longer path and use a greater search area.
• Vivipary, germination of the seed and formation of a seedling while still attached to the mother plant, has been found in some seagrasses, and is well documented in Amphibolis and Thalassodendron. Vivipary in plants is most common among halophytes (mangrove swamp and salt marsh plants in particular), and is thought to be a way to minimize some of the harmful effects of chloride ions during seed germination.

Because seagrass communities are highly productive, exceeding that of most crop plants, plant biologists have attempted to study in detail how carbon dioxide is delivered to the leaf chloroplasts across the waxy cuticle. Investigators know that at a typical ocean water pH of 8.2, as much as 99% of the carbon dioxide is present instead in the form of bicarbonate, HCO3. Speculation has been that bicarbonate, because it is so abundant, could be the carbon-containing molecule used. The best models to date, however, have revealed that carbon dioxide, a much smaller molecule, diffuses much more rapidly across the unstirred layer next to the leaf and through cutinized cell walls than does bicarbonate, so carbon dioxide is now taken to be the chief gas used to enter the leaf, even if present at a very low concentration.

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