Which is the plant completely submerged in water and how do it's leaves look like
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Answer:
Completely submerged water plants like seagrasses and pondweed (Elodea canadensis) lack stomata. The leaves are mostly thin (e.g. Zostera marina, here left) or thread-like (round but with a small diameter; e.g. Syringodium isoetifolium). In general, a favorable ratio of surface (high) with respect to volume (small) is encountered. Fresh water plants have no water balance problems. They exchange gas optimally without risk of dessication through the thin cuticle which is air-permeable; the epidermis is also thin or event absent. Seagrasses by contrast have to struggle against the high salinity in their environment. The external cell wall is not so thin as in fresh water plants. The vascular system in water plants is reduced in general. Their buoyancy derives from large intercellular cavities that protrude into stems and roots and thus facilitate oxygen transport (Microscopical fluorescence and bright field views of aerenchym in a Zostera leaf). The intensity of light underwater is relatively low; a true palisade parenchyma layer is often lacking.
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