2020/2/13Top3Theories of Shoot Apical Meristem|PlantsNavigationTop 3 Theories of ShootApical Meristem / PlantsArticle Sharedby Prateeksha LADVERTISEMENTS:Label-freecellanalysisin3DHoloMonitor? uses nothing but lighttobuild,measure andanalyseyoursinglecellsin3D.The following points highlight the topthree theories ofshoot apical meristem.The theories are: 1. Apical CellTheory 2. Histogen Theory 3. Tunica-Corpus Theory.1. Apical Cell Theory:Nageli in 1944 advocated this theory.According to thistheory the apical meristem consists of a single apical cell(also called apical initial) and this cell is interpreted as the‘structural andfunctionalunitofapicalmeristem'.Thecellis very large and is shaped like an inverted pyramid.The apical cell is tetrahedral in shape and has three or fourcuttingfacesamongwhichsinglefaceisdirectedupwardandtherestpointsdownward.Thesideof apical cellthat isdirected upward istriangularor square in shape and forms apart of the outer surface of the shoot apex (Fig.7.7).1/17www.biologydiscussion.com/shoot-system/top-3-theories-of-shoot-apical-meristem-plants/69015
2020/2/13 Top 3 Theories of Shoot Apical Meristem | Plants www.biologydiscussion.com/shoot-system/top-3-theories-of-shoot-apical-meristem-plants/69015 1/17 Navigation Top 3 Theories of Shoot Apical Meristem | Plants Article Shared by ADVERTISEMENTS: The following points highlight the top three theories of shoot apical meristem. The theories are: 1. Apical Cell Theory 2. Histogen Theory 3. Tunica-Corpus Theory. 1. Apical Cell Theory: Nageli in 1944 advocated this theory. According to this theory the apical meristem consists of a single apical cell (also called apical initial) and this cell is interpreted as the ‘structural and functional unit of apical meristem’. The cell is very large and is shaped like an inverted pyramid. The apical cell is tetrahedral in shape and has three or four cutting faces among which single face is directed upward and the rest points downward. The side of apical cell that is directed upward is triangular or square in shape and forms a part of the outer surface of the shoot apex (Fig. 7.7). Label-free cell analysis in 3D HoloMonitor® uses nothing but light to build, measure and analyse your single cells in 3D
2020/2/13Top 3Theories of Shoot Apical Meristem|PlantsDicalcellFigure7.7A.Diagram showing an apical cell of leptosporangiatefern and its derivatives that are formed in helicalsuccessionThenew cells arenumberedas 1&2.B.Diagramshowing apacketofcells formedbyanapicalcellbydivisionandsubdivision.Thecuttingfacesofapicalcelldivideinanorderlyfashionthat is in helical succession. The cell divides by anasymmetricdivision;asaresultanarrowandflatcellisformed.Thenextdivision of theapical cell is alsoasymmetric. This type of asymmetric division is repeated inthe downwardly pointed faces of the apical cell.As a result all cutting faces have their daughter cells.Thedaughter cells also divide and form large packet of cells. Thepacket ofcells differentiates and forms different segments ofshoot. So the apical cell is regarded as a reserve of onegeneticallysoundcell'.A single apical cell composing an apical meristem is presentin vascular cryptogams.After the discovery of solitary apicalcell in vascular cryptogams, it was supposed that such apicalmeristem might exist in higher plants as well.Later extensiveinvestigationsrefutedtheuniversaloccurrence of solitary apical cell in a meristem. In higherplantstheapicalcelltheorywasreplacedbytheconceptthatthe different parts of a plant bodyhave independent origin.So the apical cell theory was later superseded by histogentheory.2.Histogen Theory:ADVERTISEMENTS:2/17www.biologydiscussion.com/shoot-system/top-3-theories-of-shoot-apical-meristem-plants/69015
2020/2/13 Top 3 Theories of Shoot Apical Meristem | Plants www.biologydiscussion.com/shoot-system/top-3-theories-of-shoot-apical-meristem-plants/69015 2/17 The cutting faces of apical cell divide in an orderly fashion that is in helical succession. The cell divides by an asymmetric division; as a result a narrow and flat cell is formed. The next division of the apical cell is also asymmetric. This type of asymmetric division is repeated in the downwardly pointed faces of the apical cell. As a result all cutting faces have their daughter cells. The daughter cells also divide and form large packet of cells. The packet of cells differentiates and forms different segments of shoot. So the apical cell is regarded as ‘a reserve of one genetically sound cell’. A single apical cell composing an apical meristem is present in vascular cryptogams. After the discovery of solitary apical cell in vascular cryptogams, it was supposed that such apical meristem might exist in higher plants as well. Later extensive investigations refuted the universal occurrence of solitary apical cell in a meristem. In higher plants the apical cell theory was replaced by the concept that the different parts of a plant body have independent origin. So the apical cell theory was later superseded by histogen theory. 2. Histogen Theory: ADVERTISEMENTS:
2020/2/13Top 3Theories of Shoot Apical Meristem| Plants广告XLabel-freecellanalysisin3DHoloMonitor? uses nothingbut lighttobuild,measure andanalyseyoursingle cells in 3D.Hanstein in 1868 put forward histogen theory (histogenmeanstissuebuilder)Accordingtothistheorythetissues ofaplantbodyoriginatefromamass ofmeristemwherethefollowing three (histogens)can be distinguished(Fig.7.8):Dermatogen-PeriblemlarnHistogenFigure7.8Schematic diagram ofshootapexaccordingtoHistogentheory(a)Dermatogen:(InGreekmeaningskin).Itistheoutermostlayerofthemeristem.Itgives risetoepidermisesofrootand stem.(b)Periblem:(In Greek meaning clothing). This region occurs internal todermatogen but peripheral to plerome.This histogen isdestinedtoformcortexofrootand shootandinnertissuesofleaves.It surroundsplerome.(c) Plerome:(In Greek meaning that this fills).This region gives rise tovascular cylinder of stem and root including pith. It is thecentral coreofstemandrootandthecellscomposingthiszone are very irregular. This region is enveloped by avariable number of mantle-like layers which are representedby dermatogen and periblem.ADVERTISEMENTS:3/17www.biologydiscussion.com/shoot-system/top-3-theories-of-shoot-apical-meristem-plants/69015
2020/2/13 Top 3 Theories of Shoot Apical Meristem | Plants www.biologydiscussion.com/shoot-system/top-3-theories-of-shoot-apical-meristem-plants/69015 3/17 Hanstein in 1868 put forward histogen theory (histogen means tissue builder). According to this theory the tissues of a plant body originate from a mass of meristem where the following three (histogens) can be distinguished (Fig. 7.8): (a) Dermatogen: (In Greek meaning skin). It is the outermost layer of the meristem. It gives rise to epidermises of root and stem. (b) Periblem: (In Greek meaning clothing). This region occurs internal to dermatogen but peripheral to plerome. This histogen is destined to form cortex of root and shoot and inner tissues of leaves. It surrounds plerome. (c) Plerome: (In Greek meaning that this fills). This region gives rise to vascular cylinder of stem and root including pith. It is the central core of stem and root and the cells composing this zone are very irregular. This region is enveloped by a variable number of mantle-like layers which are represented by dermatogen and periblem. ADVERTISEMENTS: Label-free cell analysis in 3D HoloMonitor® uses nothing but light to build, measure and analyse your single cells in 3D
2020/2/13Top 3Theories of Shoot Apical Meristem|Plants告XNewbrowserbasedAccordingtoHansteindermatogen,periblem andpleromearise from independent initials of the apical meristem.Laterinvestigationsrevealthatthesub-divisionsdermatogen,periblemandpleromehavenouniversal application due to the following tworeasons:(i)Ingymnospermandangiospermthereexistsnocleardistinctionbetweenperiblemandplerome(ii)TherespectiverolesofthethreehistogenscannotbedemonstratedADVERTISEMENTS:广告XRNASequencingServiceRNA-seqandDataAnalysisCompetitivepriceforRNA-Seg,ATAC-seq,WES,WGS,andChiPseg.fcontactus!quickbiology.comOPENThe main weakness of Hanstein's concept was to assignspecific destinies of histogens. The histogens -dermatogen,periblem and plerome are committal and respectfully giverisetoepidermis,cortexandstele.Laterthistheorywassuperseded bytunica-corpus theory because the zones arenoncommittal.Though histogen theory is rejected it is still regarded asclassic'duetothefactthat Hanstein regarded shootandrootapexasacompositeunitconsistingofdifferentgroupsofhistogens and thehistogens differedfundamentallyfromeach other producing different tissues.It is to note that4/17www.biologydiscussion.com/shoot-system/top-3-theories-of-shoot-apical-meristem-plants/69015
2020/2/13 Top 3 Theories of Shoot Apical Meristem | Plants www.biologydiscussion.com/shoot-system/top-3-theories-of-shoot-apical-meristem-plants/69015 4/17 According to Hanstein dermatogen, periblem and plerome arise from independent initials of the apical meristem. Later investigations reveal that the sub-divisions — dermatogen, periblem and plerome have no universal application due to the following two reasons: (i) In gymnosperm and angiosperm there exists no clear distinction between periblem and plerome. (ii) The respective roles of the three histogens cannot be demonstrated. ADVERTISEMENTS: The main weakness of Hanstein’s concept was to assign specific destinies of histogens. The histogens — dermatogen, periblem and plerome are committal and respectfully give rise to epidermis, cortex and stele. Later this theory was superseded by tunica- corpus theory because the zones are noncommittal. Though histogen theory is rejected it is still regarded as ‘classic’ due to the fact that Hanstein regarded shoot and root apex as a composite unit consisting of different groups of histogens and the histogens differed fundamentally from each other producing different tissues. It is to note that RNA Sequencing Service RNA-seq and Data Analysis Competitive price for RNA-Seq, ATAC-seq, WES, WGS, and ChIP seq. Please contact us! quickbiology.com OPEN
2020/2/13Top3Theories of Shoot Apical Meristem|PlantsHansteinvisualizedthislongbeforewhentheconceptofgeneandDNAwasestablished.The present-day concept is that the different zones in theshoot apex arefundamentally sameand capableofproducing all tissues.It is the position in the meristem thatdeterminesthedestinyofderivatives.Thisisduetothefactthat in zones some genes are activated and others arerepressed resulting in the production of different tissues.3.Tunica-Corpus Theory:Schmidt in 1924 postulated tunica-corpus theory on thebasis of studies of shoot apices of angiosperm.This theoryisconcerned with planes of cell division in the apex. Incontrasttoapical cell theoryandhistogentheorytunica-corpus theoryis applicable only to shoot apex and not toroot. Schmidt distinguishes two tissue zones in the shootapex and termed them as tunica and corpus.ADVERTISEMENTS:一告XNewbrowserbaMajority of angiosperm shoot apex exhibits tunicaconsisting of two layers of cells and corpus (Fig. 7.9A).Researchers designate the layers as Li, L2 and L3 to denoterespectivelyouterlayeroftunica,innerlayer oftunicaandcorpus.Plasmodesmata exist between the cells of tunica and corpus.It isthought that plasmodesma controlsthegene expressionthatleadstotheformation of protoderm,ground meristemandprovasculartissue.5/17www.biologydiscussion.com/shoot-system/top-3-theories-of-shoot-apical-meristem-plants/69015
2020/2/13 Top 3 Theories of Shoot Apical Meristem | Plants www.biologydiscussion.com/shoot-system/top-3-theories-of-shoot-apical-meristem-plants/69015 5/17 Hanstein visualized this long before when the concept of gene and DNA was established. The present-day concept is that the different zones in the shoot apex are fundamentally same and capable of producing all tissues. It is the position in the meristem that determines the destiny of derivatives. This is due to the fact that in zones some genes are activated and others are repressed resulting in the production of different tissues. 3. Tunica-Corpus Theory: Schmidt in 1924 postulated tunica- corpus theory on the basis of studies of shoot apices of angiosperm. This theory is concerned with planes of cell division in the apex. In contrast to apical cell theory and histogen theory tunicacorpus theory is applicable only to shoot apex and not to root. Schmidt distinguishes two tissue zones in the shoot apex and termed them as tunica and corpus. ADVERTISEMENTS: Majority of angiosperm shoot apex exhibits tunica consisting of two layers of cells and corpus (Fig. 7.9A). Researchers designate the layers as L1, L2 and L3 to denote respectively outer layer of tunica, inner layer of tunica and corpus. Plasmodesmata exist between the cells of tunica and corpus. It is thought that plasmodesma controls the gene expression that leads to the formation of protoderm, ground meristem and provascular tissue