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PHYCOLOGICAL SOCIETY OF AMERICA

Much of Cystoseira osmundacea's morphology is visible to the naked eye, the complex thallus construction being differentiated into floats, blades, a stipe, and a holdfast. However, the formation of these structures is not readily visible, as what sets Cystoseira apart from other algae is its anatomy, or cellular construction, and modes of growth.

Cystoseira osmundacea is one of few fucalean species anywhere that grow large enough to form a surface canopy. Cystoseira can grow to be eight meters in height and thirty meters in length (as part of a surface canopy), though during its dormant season, a considerable part of the branching thallus is shed, leaving the plant with a much smaller and strikingly different appearance.

Thallus
Cystoseira osmundacea's thallus, or body, construction is parenchymatous, meaning that the cells divide such that they form a fleshy sheet of tissue. Parenchymatous structure grows from cell division occuring in many planes, which allows for tissue differentiation similar to higher plants. For Cystoseira, that parenchymatous tissue is differentiated into the holdfast, stipe, blades, and floats, or pneumatocysts. The basal region of the alga includes the lower blades and branches, while the morphologically distinct apical region includes the pneumatocysts and associated branches.

Holdfast
Cystoseira has a discoid holdfast, triangular in shape. The holdfast is made of fine, hairlike outgrowths of algal tissue called hyphae. These hyphae grow into the crevices of the rocky substrata, creating an impenetrable mat that holds the alga firmly in place, often weather remarkably violent winter storms.

Stipe
C. osmundacea has a tough stipe with a triangular shape in cross section. The stipe often stands erect, giving the alga its upright stature (to match its dignified elegance!). During the periods of new growth and elongation of the main axis, the stipe will often display a marked increase in girth. The stipe tends to have a thicker-walled medulla than other parts of the alga.

Blades
Blades in the basal region have a flattened shape with a slight midrib (a result of meristoderm growth). These blades show pinnate structure: the lobes of the blades extend bilaterally at roughly equal angles from the midrib. Blades are arranged radially around the stipe, so that they form a spiral, or radial, organization. Blades tend to have relatively more peripheral and medulla tissue, and they tend to have higher concentrations of chloroplasts and mitochondria, indicating relatively high levels of photosynthesis and respiration.

Pneumatocysts are formed by growth of the intercalary meristem, and increases in their circumference are effected by growth initiated in the meristoderm. Thus, the pneumatocyst is an area of hollowed-out tissue, bordered on all sides by medulla and cortex tissue and often traversed by residual hair-like cells of the medulla (See right.) The gas within the hollow chamber is mostly oxygen and nitrogen, occurring in similar proportions as in the atmosphere, with some variable amounts of carbon dioxide. Oxygen and carbon dioxide arise partly from the metabolic activity of the bordering cortex cells, but the major source of O2 and CO2 is equilibration between gases in the pneumatocyst and the surrounding water.

SEASONAL MORPHOLOGY

A striking feature of Cystoseira osmundacea is the seasonal fluctuations in morphology. At the onset of winter, the apical region of the alga is shed, leaving only the basal region behind for the duration of the winter.

BASAL REGION

APICAL REGION
The apical region comprises the upper branching fronds characterized by the presence of pneumatocysts and, during fertile growth, reproductive structures. These fronds are produced and shed annually, with new growth commencing as early as spring and continuing into late summer. These fronds compose the main bulk of the alga, often accounting for over 80% of the organism's biomass. The apical region is responsible for the formation of the surface canopy, providing a distinctive structural habitat for other organism's in its community.

ANATOMY

The anatomy or cellular construction of an alga can give many insights as to its origin and functions. Cystoseira osmundacea's cells each contain a single nucleus, a feature that can taxonomically characterize some families of algae. The surface, or peripheral cells usually exhibit a palisade shape and are densely packed with photosynthetic pigments. This is easy to see in the picture of a blade cross section: the outer cells have the greenish, golden-brown color characteristic of Cystoseira, resulting from those pigments.

The colorless cells interior to the colored, peripheral cells are called the cortex cells. Often the active growth in surface cells, especially in the case of blades, will stretch these inner cortex cells, giving them an irregular arrangement.

At the very center of the blade cross section, one can just make out a group of cells distinct from the cortex. These cells form the medulla, which can function in the transport of photosynthetic products from the light-enfused fronds at the surface down to the shaded basal region of the alga.

The cell walls of C. osmundacea are compsed of an inner layer of cellulose fibers cemented together by a slimy, gummy later of alginic acid. Alginic acid (or alginate) is a polymer of five-carbon acids and gives the plant considerable flexible strength, in addition to functioning in ion-exchange. Alginates are also economically valuable and are used commercially for their stabilizing and emulsifying properties.