This article gives a brief description of the structural
similaries and differences between seagrasses and vascular plants
found on land.
Although seagrasses live in marine waters, they evolved millions
of years ago from land plants and have many of the same
morphological features, such as leaves, roots, flowers, seeds, and
conducting tissues. Similar to terrestrial vegetation, seagrasses
use the process of photosynthesis to manufacture their own food and
produce oxygen through structures called chloroplasts. Land plants
have chloroplasts in both the stems and leaves, but in seagrasses,
chloroplasts are found only in the leaves. Since seagrasses don't
need to overcome the force of gravity, they don't have the strong
supportive stems and trunks found in land plants; instead, they
depend on the natural buoyancy of the water to provide support.
This allows seagrass blades to remain flexible to bend and move
with the force of currents and waves. To protect leaves from this
high-energy environment, however, seagrasses have developed
tube-like structures, called sheaths, which extend down through the
vertical rhizome. This structure, not found in land plants,
protects the meristem and any newly formed leaves, which extend up
through the sheath of previously formed leaves.
Image source: Hemminga, M.A. and C.M. Duarte, 2000. Seagrass
Ecology. Cambridge University Press, Cambridge, UK. Modified with