Brown algae exist in a wide range of sizes and forms. The smallest members of the group grow as tiny, feathery tufts of threadlike
cells no more than a few centimeters (a few inches) long. Some species have a stage in their life cycle that consists of only a few cells, making the entire alga microscopic. Other groups of brown algae grow to much larger sizes. The
rockweeds and leathery
kelps are often the most conspicuous algae in their habitats. Kelps can range in size from the sea palm
Postelsia to the giant kelp
Macrocystis pyrifera, which grows to over long and is the largest of all the algae. In form, the brown algae range from small crusts or cushions to leafy free-floating mats formed by species of
Sargassum. They may consist of delicate felt-like strands of cells, as in
Ectocarpus, or of flattened branches resembling a fan, as in
Padina. Regardless of size or form, two visible features set the Phaeophyceae apart from all other algae. First, members of the group possess a characteristic color that ranges from an
olive green to various shades of
brown. The particular shade depends upon the amount of
fucoxanthin present in the alga. Although not all brown algae are structurally complex, those that are typically possess one or more characteristic parts. A
holdfast is a rootlike structure present at the base of the algae. Like a root system in plants, a holdfast serves to anchor the alga in place on the
substrate where it grows, and thus prevents the alga from being carried away by the current. Unlike a root system, the holdfast does not serve as the primary organ for water uptake, nor does it take in nutrients from the substrate. The overall physical appearance of the holdfast differs among various brown algae and among various substrates. It may be heavily branched, or it may be cup-like in appearance. A single alga typically has just one holdfast, although some species have more than one stipe growing from their holdfast. A
stipe is a stalk or stemlike structure present in an alga. It may grow as a short structure near the base of the alga (as in
Laminaria), or it may develop into a large, complex structure running throughout the algal body (as in
Sargassum or
Macrocystis). In the most structurally differentiated brown algae (such as
Fucus), the tissues within the stipe are divided into three distinct layers or regions. These regions include a central pith, a surrounding cortex, and an outer epidermis, each of which has an analog in the stem of a vascular plant. In some brown algae, the pith region includes a core of elongated cells that resemble the
phloem of vascular plants both in structure and function. In others (such as
Nereocystis), the center of the stipe is hollow and filled with gas that serves to keep that part of the alga buoyant. The stipe may be relatively flexible and elastic in species like
Macrocystis pyrifera that grow in strong currents, or may be more rigid in species like
Postelsia palmaeformis that are exposed to the atmosphere at low tide. Many algae have a flattened portion that may resemble a leaf, and this is termed a
blade,
lamina, or
frond. The name
blade is most often applied to a single undivided structure, while
frond may be applied to all or most of an algal body that is flattened, but this distinction is not universally applied. The name
lamina refers to that portion of a structurally differentiated alga that is flattened. It may be a single or a divided structure, and may be spread over a substantial portion of the alga. In
rockweeds, for example, the lamina is a broad wing of tissue that runs continuously along both sides of a branched
midrib. The midrib and lamina together constitute almost all of a rockweed, so that the lamina is spread throughout the alga rather than existing as a localized portion of it. '' produces numerous gas-filled
pneumatocysts (air bladders) to increase buoyancy. In some brown algae, there is a single lamina or blade, while in others there may be many separate blades. Even in those species that initially produce a single blade, the structure may tear with rough currents or as part of maturation to form additional blades. These blades may be attached directly to the stipe, to a holdfast with no stipe present, or there may be an air bladder between the stipe and blade. The surface of the lamina or blade may be smooth or wrinkled; its tissues may be thin and flexible or thick and leathery. In species like
Egregia menziesii, this characteristic may change depending upon the turbulence of the waters in which it grows. Growth in most brown algae occurs at the tips of structures as a result of divisions in a single
apical cell or in a row of such cells. These filaments may be haplostichous or polystichous, multiaxial or monoaxial forming or not a
pseudoparenchyma. Besides fronds, there are the large in size
parenchymatic kelps with three-dimensional development and growth and different tissues (
meristoderm,
cortex and
medulla) which could be consider the trees of the sea. There are also the
Fucales and
Dictyotales smaller than kelps but still parenchymatic with the same kind of distinct tissues. The
cell wall consists of two layers; the inner layer bears the strength, and consists of
cellulose; the outer wall layer is mainly
algin, and is gummy when wet but becomes hard and brittle when it dries out. Cellulose, a major component from most plant cell walls, is present in a very small percentage, up to 8%. Cellulose and alginate biosynthesis pathways seem to have been acquired from other organisms through endosymbiotic and horizontal gene transfer respectively, while the sulphated polysaccharides are of ancestral origin. Specifically, the cellulose synthases seem to come from the red alga endosymbiont of the photosynthetic stramenopiles ancestor, and the ancestor of brown algae acquired the key enzymes for alginates biosynthesis from an
actinobacterium. The presence and fine control of alginate structure in combination with the cellulose which existed before it, gave potentially the brown algae the ability to develop complex structurally multicellular organisms like the kelps. ==Evolutionary history==