Editing Brown algae (section)

==Morphology==
Brown algae exist in a wide range of sizes and forms. The smallest members of the group grow as tiny, feathery tufts of threadlike [[cell (biology)|cell]]s no more than a few centimeters (a few inches) long.<ref name="Connor1989">
{{cite book
|last1=Connor |first1=J.
|last2=Baxter |first2=C.
|year=1989
|title=Kelp Forests
|publisher=[[Monterey Bay Aquarium]]
|isbn=978-1-878244-01-7
}}</ref> 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 [[Ascophyllum|rockweed]]s and leathery [[kelp]]s are often the most conspicuous algae in their habitats.<ref name="Dittmer1964">
{{cite book
|last=Dittmer |first=H. J.
|year=1964
|title=Phylogeny and Form in the Plant Kingdom
|url=https://archive.org/details/phylogenyforminp0000ditt |url-access=registration |pages=[https://archive.org/details/phylogenyforminp0000ditt/page/115 115–137]
|location=Princeton, NJ
|publisher=[[D. Van Nostrand Company]]
|isbn=978-0-88275-167-2
}}</ref> Kelps can range in size from the {{convert|2|ft|cm|-1|order=flip|adj=mid|-tall|sp=us}} sea palm ''[[Postelsia]]'' to the giant kelp ''[[Macrocystis pyrifera]]'', which grows to over {{cvt|150|ft|m|-1|order=flip}} long<ref name="Abbott 76">
{{cite book
|last1=Abbott |first1=I. A.
|last2=Hollenberg |first2=G. J.
|year=1976
|title=Marine Algae of California
|location=California
|publisher=[[Stanford University Press]]
|isbn=978-0-8047-0867-8
}}</ref><ref name="Cribb1953">
{{cite journal
|last=Cribb |first=A. B.
|year=1953
|title=''Macrocystis pyrifera'' (L.) Ag. in Tasmanian waters
|journal=[[Australian Journal of Marine and Freshwater Research]]
|volume=5 |issue=1 |pages=1–34
|doi=10.1071/MF9540001
}}</ref> and is the largest of all the algae. In form, the brown algae range from small crusts or cushions<ref name="Jones62">{{cite journal
 |last    = Jones
 |first   = W. E.
 |year    = 1962
 |title   = A key to the genera of the British seaweeds
 |url     = https://cdn.fieldstudiescouncil.net/fsj/vol1.4_23.pdf
 |journal = [[Field Studies (journal)|Field Studies]]
 |volume  = 1
 |issue   = 4
 |pages   = 1–32
}}</ref> 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 {{convert|1|ft|cm|order=flip|sp=us|adj=mid|-long}} flattened branches resembling a fan, as in ''[[Padina (alga)|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 drabs|olive green]] to various shades of [[brown]]. The particular shade depends upon the amount of [[fucoxanthin]] present in the alga.<ref name="Bold1987"/> Second, all brown algae are [[Multicellular organism|multicellular]]. There are no known species that exist as single cells or as colonies of cells,<ref name="Bold1987"/> and the brown algae are the only major group of [[seaweed]]s that does not include such forms. However, this may be the result of classification rather than a consequence of evolution, as all the groups hypothesized to be the closest relatives of the browns include single-celled or colonial forms.{{citation needed|date=January 2024}} They can change color depending on salinity, ranging from reddish to brown.

===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.

===Growth===
[[File:Capo Gallo Dicotoma.jpg|thumb|240px|Growth in ''[[Dictyota dichotoma]]'' occurs at each frond tip, where new cells are produced.]]

The brown algae include the largest and fastest growing of seaweeds.<ref name="Connor1989" /> Fronds of ''[[Macrocystis]]'' may grow as much as {{convert|50|cm|in|sigfig=1|abbr=on}} per day, and the stipes can grow {{convert|6|cm|in|sigfig=2|abbr=on}} in a single day.<ref name="Round1981">
{{cite book
|last=Round |first=F. E.
|year=1981
|title=The Ecology of Algae
|page=103
|location=Cambridge
|publisher=[[Cambridge University Press]]
|isbn=978-0-521-26906-3
}}</ref>

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. They are single cellular organisms.<ref name="Dittmer1964" /> As this apical cell divides, the new cells that it produces develop into all the tissues of the alga. Branchings and other lateral structures appear when the apical cell divides to produce two new apical cells. However, a few groups (such as ''[[Ectocarpus]]'') grow by a diffuse, unlocalized production of new cells that can occur anywhere on the thallus.<ref name="Bold1987" />

===Tissue organization===

The simplest brown algae are filamentous—that is, their cells are elongate and have septa cutting across their width. They branch by getting wider at their tip, and then dividing the widening.<ref>{{cite book
|last=Wynne
|first=M. J.
|year=1981
|chapter=The Biology of seaweeds
|editor1-last=Lobban
|editor1-first=C. S.
|editor2-last=Wynne
|editor2-first=M. J.
|title=Phaeophyta: Morphology and Classification
|chapter-url=https://books.google.com/books?id=QG4tqjFPWJ0C&pg=PA52
|page=52
|series=Botanical Monographs
|volume=17
|publisher=[[University of California Press]]
|isbn=978-0-520-04585-9
|access-date=11 May 2020
|archive-date=16 October 2023
|archive-url=https://web.archive.org/web/20231016193130/https://books.google.com/books?id=QG4tqjFPWJ0C&pg=PA52#v=onepage&q&f=false
|url-status=live
}}</ref>

These filaments may be haplostichous or polystichous, multiaxial or monoaxial forming or not a [[pseudoparenchyma]].<ref name="Textbook of Algae">
{{cite book
|last1=Sharma |first1=O. P
|year=1986
|title=Textbook of Algae
|url=https://books.google.com/books?id=hOa74Hm4zDIC&pg=PA298
|page=298
|publisher=[[Tata McGraw-Hill]]
|isbn=978-0-07-451928-8
}}</ref><ref>{{Cite book|last=Graham; Wilcox; Graham|title=Algae, 2nd Edition|publisher=Pearson|year=2009|isbn=9780321603128}}</ref>   Besides fronds, there are the large in size [[parenchyma]]<nowiki/>tic kelps with three-dimensional development and growth and different tissues ([[wiktionary:meristoderm|meristoderm]], [[Cortex (botany)|cortex]] and [[wiktionary:medulla|medulla]]) which could be consider the trees of the sea.<ref>Fritsch, F. E. 1945. The Structure And Reproduction Of The Algae. Cambridge University Press, Cambridge.</ref><ref>{{Cite journal|last1=Charrier|first1=Bénédicte|last2=Le Bail|first2=Aude|last3=de Reviers|first3=Bruno|date=August 2012|title=Plant Proteus: brown algal morphological plasticity and underlying developmental mechanisms|url=http://dx.doi.org/10.1016/j.tplants.2012.03.003|journal=Trends in Plant Science|volume=17|issue=8|pages=468–477|doi=10.1016/j.tplants.2012.03.003|pmid=22513108|issn=1360-1385|access-date=13 November 2020|archive-date=16 October 2023|archive-url=https://web.archive.org/web/20231016193229/https://www.cell.com/trends/plant-science/fulltext/S1360-1385(12)00055-6?_returnURL=https%3A%2F%2Flinkinghub.elsevier.com%2Fretrieve%2Fpii%2FS1360138512000556%3Fshowall%3Dtrue|url-status=live|url-access=subscription}}</ref> 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.<ref name="Textbook of Algae"/> Specifically, the brown algal cell wall consists of several components with alginates and [[Sulfation|sulphated]] [[fucose|fucan]] being its main ingredients, up to 40% each of them.<ref>{{Cite journal |doi=10.1016/j.tplants.2018.10.013 |title=Gazing at Cell Wall Expansion under a Golden Light |year=2019 |last1=Charrier |first1=Bénédicte |last2=Rabillé |first2=Hervé |last3=Billoud |first3=Bernard |journal=Trends in Plant Science |volume=24 |issue=2 |pages=130–141 |pmid=30472067 |s2cid=53725259 |url=https://hal.archives-ouvertes.fr/hal-02183603/file/PDF%20article%20accept%C3%A9.pdf |access-date=17 September 2022 |archive-date=8 January 2022 |archive-url=https://web.archive.org/web/20220108014851/https://hal.archives-ouvertes.fr/hal-02183603/file/PDF%20article%20accept%C3%A9.pdf |url-status=live }}</ref> 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.<ref>{{Cite journal |doi=10.1111/j.1469-8137.2010.03374.x |doi-access=free|title=The cell wall polysaccharide metabolism of the brown alga Ectocarpus siliculosus. Insights into the evolution of extracellular matrix polysaccharides in Eukaryotes |year=2010 |last1=Michel |first1=Gurvan |last2=Tonon |first2=Thierry |last3=Scornet |first3=Delphine |last4=Cock |first4=J. Mark |last5=Kloareg |first5=Bernard |journal=New Phytologist |volume=188 |issue=1 |pages=82–97 |pmid=20618907 }}</ref> 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.<ref>{{Cite journal |doi=10.1093/aob/mcu096 |doi-access=free|title=Chemical and enzymatic fractionation of cell walls from Fucales: Insights into the structure of the extracellular matrix of brown algae |year=2014 |last1=Deniaud-Bouët |first1=Estelle |last2=Kervarec |first2=Nelly |last3=Michel |first3=Gurvan |last4=Tonon |first4=Thierry |last5=Kloareg |first5=Bernard |last6=Hervé |first6=Cécile |journal=Annals of Botany |volume=114 |issue=6 |pages=1203–1216 |pmid=24875633 |pmc=4195554 }}</ref>