Editing Brown algae (section)

===Visible structures===
[[File:Laminaria hyperborea.jpg|thumb|240px|Two specimens of ''[[Laminaria hyperborea]]'', each showing the rootlike '''holdfast''' at lower left, a divided '''blade''' at upper right, and a stemlike '''stipe''' connecting the blade to the holdfast.]]
{{further|Thallus|Holdfast (biology)|Stipe (botany)|Lamina (algae)|Pneumatocyst}}

Whatever their form, the body of all brown algae is termed a ''[[thallus]]'', indicating that it lacks the complex [[xylem]] and [[phloem]] of [[vascular plant]]s. This does not mean that brown algae completely lack specialized structures. But, because some botanists define "true" stems, leaves, and roots by the presence of these tissues, their absence in the brown algae means that the stem-like and leaf-like structures found in some groups of brown algae must be described using different terminology.<ref name="Raven2005">
{{cite book
|last1=Raven |first1=P. H.
|last2=Evert |first2=R. F.
|last3=Eichhorn |first3=S. E.
|year=2005
|title=Biology of Plants
|url=https://archive.org/details/biologyofplants00rave_0 |url-access=registration |edition=7th |pages=[https://archive.org/details/biologyofplants00rave_0/page/316 316]–321, 347
|location=New York
|publisher=[[W. H. Freeman and Company]]
|isbn=978-0-7167-1007-3
}}</ref> Although not all brown algae are structurally complex, those that are typically possess one or more characteristic parts.

A ''[[holdfast (biology)|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 generally 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 (botany)|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 (algae)|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 [[Fucus|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.

[[File:Bladder Wrack (Fucus vesiculosus) - geograph.org.uk - 224125.jpg|thumb|left|240px|Species like ''[[Fucus vesiculosus]]'' produce 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.<ref name="Connor1989" /> In other species, the surface of the blade is coated with slime to discourage the attachment of [[epiphyte]]s or to deter [[herbivore]]s. Blades are also often the parts of the alga that bear the reproductive structures.

Gas-filled floats called ''[[pneumatocyst]]s'' provide [[buoyancy]] in many [[kelp]]s and members of the [[Fucales]]. These bladder-like structures occur in or near the ''lamina'', so that it is held nearer the water surface and thus receives more light for photosynthesis. Pneumatocysts are most often spherical or [[ellipsoid]]al, but can vary in shape among different species. Species such as ''[[Nereocystis|Nereocystis luetkeana]]'' and ''[[Pelagophycus|Pelagophycus porra]]'' bear a single large pneumatocyst between the top of the stipe and the base of the blades. In contrast, the giant kelp ''Macrocystis pyrifera'' bears many blades along its stipe, with a pneumatocyst at the base of each blade where it attaches to the main stipe. Species of ''[[Sargassum]]'' also bear many blades and pneumatocysts, but both kinds of structures are attached separately to the stipe by short stalks. In species of ''[[Fucus]]'', the pneumatocysts develop within the lamina itself, either as discrete spherical bladders or as elongated gas-filled regions that take the outline of the lamina in which they develop.