An Eelgrass (Zostera marina) Shoot
An Eelgrass Shoot

An eelgrass plant consists of a horizontal stem or rhizome that grows on or just below the surface of the sediment. Leaves originate from a meristem which is protected by a sheath at the actively growing end of the rhizome. As the shoot grows, the rhizome elongates, moving across or within the sediment, forming roots as it progresses.

Root initiation is directly related to formation of leaves and during the growing season. A new leaf is initiated approximately every 10 to 14 days. When this occurs, a new set of roots emerges from a node on the rhizome at the base of what was an old leaf. Roots emerge from the node at a downward angle alternating from the left side to the right side of the rhizome, possibly adding to the stability.

Parts of an Eelgrass Shoot...

The leaves of eelgrass are the primary means of energy production and also serve to alter the movement of water over the bottom.  The length of the leaf will be determined by the time of year and environmental conditions.  Typically, leaves range from 6 inches to 5 feet during the growing season. Eelgrass leaves are much like those of a deciduous tree in that they produce energy via photosynthesis, but in this case each leaf only persists for approximately 1 month before it detaches from the parent plant.  Although the significance is not clear, eelgrass leaves can absorb nutrients directly from the water column to support growth.
The rhizome is the horizontal stem of the plant that serves a the backbone from which the leaves and roots emerge.  As a shoot grows, the rhizome extends along or just below the sediment surface.  Rather than piercing through the sediment like most other plants, eelgrass rhizomes crawl across the bottom similar to a hose being unrolled. During the growing season, the rhizomes branch to initiate new shoots.  Branching typically occurs in fall and spring.
A node is a growth break in the rhizome that represents the former connection point for a leaf.  There is meristematic tissue at the node that allows for the initiation of roots or new rhizome branches (lateral shoots).  The rate of node formation depends on the rate of leaf initiation.  Measuring node initiation, termed the plasticrone interval, is one means of measuring shoot growth rate.
Roots are the primary means of nutrient uptake for the shoots as well as the only means of anchorage in the bottom.  Roots initiate from the nodes and typically emerge at a slight angle downward away from vertical, alternating in direction from one side to the other. The tips of the roots often have root hairs that greatly increase the surface area allowing for additional uptake and anchorage.
The sheath is the bundle at the base of the leaves that holds the leaves together, protecting the meristem.
The meristem is the primary growth point of the shoot.  Protected at the base of the leaves in the sheath, the terminal meristem initiates a new leaf primordia approximately ever two weeks during the growing season.  If the meristem is damaged, the shoot often dies unless a lateral shoot, farther back on the rhizome, is already formed.  New lateral meristems are formed when the rhizomes branch.
lateral shoot
Lateral shoot
A lateral shoot is a secondary shoot that has formed on the rhizome during branching.  Lateral shoots eventually break away from the primary shoots and keep the clone alive after the primary meristem has flowered, died and degraded. The number of lateral shoots or branches can vary depending on time of year and conditions.
Epiphytes are organisms that grow on the surface of the eelgrass leaves.  In addition to the algae, the epiphytic community can include microbes, small invertebrates including colony-forming tunicates and tube-forming worms.  The level of epiphytic fouling can be an indicator of water quality and/or current regime.  The types of species (especially algae) found in the epiphytic community often vary with season.  High levels of epiphytic fouling are detrimental to shoot growth. (learn more...)

Next: Eelgrass: Growth Habit


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