The ecology of eelgrass meadows in the Pacific Northwest: A ...

CHAPTER 2
THE BIOLOGY OF EELGRASS
2.1 VEGETATIVE GlXWlTd STRATEGIES
Sauvageau (1889, 1890, 1891), Setchell
(19291, Phillips (1972), and Tomlinson
(1974, 1980) described the morphology and
anatomy of eelgrass. Eelgrass leaves
possess an anatomy typical of submerged
hydrophytes: namely, cutin on the leaf is
thin, there are no stomata, chloroplasts
are densely packed and lie principally in
the epidermis, the internal cellular
structure consists of large thin-walled
aerenchyma cells with numerous air canals
(lacunae), and the vascular tissue has
reduced xylem (Tomlinson 1980). As with
grass-like monocots whose leaves stand
erect, there are no dorsal or ventral
sides to the leaf blade. The lacunae are
continuous from the leaf blade through the
rhizome to the root tips and carry o2 and
C02 throughout the plant. Because these
lacunae may also store and recycle 02, the
standard method of measuring prlmary
productivity by measuring O2 changes in
the water around eelgrass plants is not
accurate (Hartmanand Brown1967; Jacobs
1979; Zieman and Wetzel 1980). The leaftips
are rounded, while the blades are
straplike. Leaf width depends on the
severity of the climate in a region, the
season, and tidal zone (Setchell 1920,
1929; Phillips 1972; Kentula 1983). The
leaves are produced by a meristem terminal
on a short shoot (erect branch from the
horizontal rhizome). The oldest shoot
(defined as an erect branch with a bundle
of leaves) is terminal on the rhizome.
Occasionally the meristem on the terminal
shoot gives rise to a lateral rhizome
branch with a meristem that produces
leaves.
Depending on the length of the growing
season, the number of leaves produced in a
year, andthe number of lateral branches
produced, the individual eelgrass plant
has a variable growth and expansion rate
over the bottom. There is one rhizome
ncde prcduced for each new leaf initiated;
the same meristem which produces a leaf
also produces a new internode for the
elongating branch. Setchell (1929)
theorized that eelgrass plants produced
two lateral branches during a growing
season and that the terminal shoot always
flowered during the second year following
development from a seed. In this scheme
eelgrass is a biennial plant.
Eelgrass rhizome are buried from 3-4 cm
(1.2-1.6 inches) up to 20 cm (8.0 inches)
deep in sediment, depending on the
sediment consistency. In firmer
substrates, rhizomes may be only half as
deep as in soft muddy substrates. Two
bundles of roots are produced at each
rhizome node.
New leaves grow on alternating sides from
the meristem on the shoot. Subtidal
shoots in Puget Sound typically carry five
leaves each during summer and four in
winter e hilli ips 1972), while intertidal
shoots in Oreyon averaged four leaves each
in June and as few as 2.5 each in August
(Kentula 1983). Keller and Harris (1966)
reported that the number of leaves per
shoot in Humboldt Bay ranged from two to
thirteen but averaged three to four.
Kentula (1983) analyzed the growth of a
leaf in relation to its age-position on
the shoot. The youngest leaf was
designated No. 1. The greatest proprtion
of growth occurred in position No. 2. In
April-May these leaves accounted for 48%-
65% of the total growth of the shoot and
from 75%-95% of the growth from June-
October.