Mechanical tissues

This presentation intends to explore the different mechanical tissues of plants in general and their properties in particular.

1. PLANTANATOMY
Welcome to
MECHANICAL TISSUES IN PLANTS
Properties & -Significance.
By
N. Sannigrahi, Associate Professor,
Department of Botany,
Nistarini College, Purulia, 723101(W.B) India

2. PLANTS UNDER STRESS

3. MECHANICAL TISSUES & PLANTS
Havingwiderangeofhabitats,plantshavetowithstand
differentenvironmentalforceslikeforceofgravity,wind,
weatheralongwiththeirbodystructure,weightandform.
Mechanicaltissuesaresuchgroupoftissuesas‘Stereome’
coinedbyHaberlandtand‘Sterides’ascellsofthetissues.
Plantshavetowithstand-
Longitudinalcompressionsduetoweightofthecanopy,
Longitudinalpullingduetobendingforceaswellasdueto
weightofsomeparts
Radialpressureandshearingstress,bendingonanydirection
duetowindcurrent.
Theplantsdevelopedmechanicaltissueswithmaximum
possiblerequirementsofrigidity,elasticityandtheeconomyof
thetissuedeposition.

4. DIFFERENT PLANT TISSUES

5. MECHANICAL TISSUES
Therearefourmajorbasicbuildingblocksofthecellwall-
cellulose,hemicelluloses,ligninandpectin.The
microstructureoftheplantcellwallsvariesindifferenttypeof
plantswithcellulosefibersreinforcingginmatrixof
hemicellulosesandpectinorlignininoneormorelayerswith
volumefractionandorientationofcellulosicfibersvaryingin
eachlayer.
Sometissuesarespeciallymeantforprovidingmechanical
strengthorsupporttotheplantmemberswhicharefrequently
subjectedtovariouskindsofstrainsandstresses.Themost
importantmechanicaltissuesaresclerenchymafiberswith
highlylignifiedwallsandpeculiarlyinterlockedends,
sclereids,withmassivelignifiedwalls,andcollenchymaswith

6. COLLENCHYMA -BEAUTY & STRENGTH

7. MECHANICAL TISSUES
unevenlythickenedcellulosewalls.Thesclerenchymafibers,
asalreadystated,maybepresentincortex,inpericycle,with
vascularelementsandeveninpith.Theyareundoubtedlythe
mosteffectivemechanicaltissues.Sclereidsmayoccurin
differentpartsoftheplantsforthesamepurpose.
Collenchymasformseithercontinuousstrandsorisolated
patchesinthesuperficialregionsoftheaerialorgansofthe
dicotyledons.Theyprovidesufficientstrengthtothegrowing
organs.Thesetissuesgivingmechanicalstrengthhadbeenput
underasystem,knownasstereome,bysomeworkersinthe
lastcentury.Thetrachearyelementsofxylem—thexylem
fibersandextra-xylaryfibers,bastfibersandwoodfibers
constituteamajorinputsinthisregard.Thus,plantsdeveloped
differenttypeoftissueswithspecializedstructuretoresistthe
variousforcesofnature.

8. SCLERENCHYMA -STRENGTH

9. PROPERTIES
tracheidsandtracheae,areprimarilymeantfortheconductionof
waterandsolutes.Butastheypossessthicklignifiedwallswith
differenttypesoflocalizedthickenings,theypossiblycangive
mechanicalsupportaswell.
Parenchymatissuesinstemhavethemicrofibrilsoriented
transverselyontheverticalwallssothatthecellsbendwithout
breakingandinroot,themicrofibrilshavesteeplypitchedand
helicalorientationtoresistextensionpressure.
Collenchymastissuesarecharacterizedbytheirtensilestrengthand
plasticity.Somecollenchymassclerifyandmodifytheirwallsin
suchwaytheyareabletowithstandtensileandcomprehensive
componentsofbendingstress.
Sclerenchymahavingmostplantligninpossessthehardnessofa
steel.Scleridesgivestrength,resistanceandinflexibleprotection
.Theyservemajormechanicalattributes.
Thus.Plantsdevelopdifferenttypeofspecializedtissuesto
withstandtheadverseenvironmentalconditionasemergefromtime
totime.

10. SCLERENCHYMA -ENGINEER

11. PROPERTIES OF MECHANICAL TISSUES
Ingeneral,plantsfacebending,pullingandcompression
pressure.Sotoresistthesestresses,plantshavedistributedthe
variousmechanicaltissuesintheinternalstructuresofthe
differentparts.Theprinciplesofdistributionwasin
conformitywiththeengineeringprinciplesinvolvedinvarious
constructionpracticedinengineeringsciences(Schwendener).
Theplantsachievedtheresistancetovariousforcesthrough
getmax.Strengthandstabilitywiththeminimumuseof
materialsbythetissueorientationandbiomechanicsof
differentorgans.
PROPERITIESOFMECHANICAL TISSUES
Therearethreeprinciplesbasedonwhichthemechanical
tissueshavebeendistributed-inflexibility,inextensibility,
incompressibility.Inadditiontothat,sheeringstressisanother
attributesinthisregard.
Inflexibility:Itreferstotheresistancetobending.Theorgans
whichexperiencethisaremainlyaerialtrunk,branches,stems,
petioles,peduncles,stalksoffruits,leafbladesetcastheseare
calledinflexibleorgans.

12. INFLEXIBILITY
Ifaloadisputatthemiddleportionofastraightgirder
supportedattheends,theresultwouldbeacurvaturewhenthe
uppersurfacewouldbeshortenedandthelowerlengthened.
Thatshowsthattheuppersurfaceissubjectedtocompression
andthelowertotension,whileatthemiddleportiontension
willcometozero.Sorequisitematerialsshouldbe
concentratedonthetwosurfaces,whichareregionsofgreatest
tension.Thetypicalgirdersareconstructedaccordingly,sothat
theyappearas‘I’incross-section.
Fromthisitisquiteclearthattopossessmaximum
inflexibilityintheI-girder,thestrengtheningmaterialbe
distributedontheperipheriesknownasflanges.Thestrength
ofthegirderdependsonthestrengthoftheflangesandit
increasesindistancebetweentheflanges.Thereisnoneedof

13. PROPERTIES
strengtheningmaterialsattheneutrallayersduetolackof
tensionorzerotension.
Thus,theI-girdercanresistthebendingforceswhetherit
comesfromonesideortheotherside.Theinflexibleorgansof
theplantshavebeenconstructedontheprinciplesofI–girder
withmultipleI-girdersinringtoresistthebendingdueto
radialpressure.
Inextensibility: It is the resistance to extension or pulling.The
roots and other organs are called inextensible organs. The
pulling of the water from well by a rope and the rope has a
tight winding of the fibers. The degree of the extensibility
depends on the cross sectional area of its resistant elements.

14. PROPERTIES
Theprinciplestatesthattheforceisevenlydistributedandthis
cannotbesatisfiedunlesstheresistingstructuresarecompact.
Themoreisthetightnessoftheresistingmaterials,thegreater
istheefficiencytoresisttheextensionpressure.Rootswhich
attachtheplantstothesoilorothersubstratumsufferfrom
longitudinalpullortension.Mechanicaltissuesare
advantageouslyputinthecentralregioninformofacompact
massintheseorgans.Thedegreeofresistanceofcourse
dependsonthecrosssectionalareaofthemechanical
elements.Thustherootshavemechanicaltissuesassociated
withthevascularelementsinsidethesteleandbuiltlike
structuretoperformthesamearope.
Thus,theplantscanwithstandtheadverseconditionand
remainintact.

15. PROPERTIES
INCOMPRESSIBILITY:Itistheresistancetocompressible
force.Theaxisofaspreadingtreewithitsarraysofbranches
andleaveshastobeartheweightoftheheavycrown,which
maybecomparedtoputtingaloadatthetopofacylindrical
axis.Heretheaxisissubjectedtolongitudinalcompression.
Themechanicaltissuesareeffectivelyaggregatedatthecentral
portionwhichservesasasolidcolumnforwithstanding
longitudinalcompression.Therefore,thetrunkisan
incompressibleorgan.Thetrunkoftheplantisbuiltonlikea
pillar.Inpillars,thestrengtheningmaterialarefoundstraight
andevenlydistributedwithintheconcretematerialsandalso
taggedtoeachother.Thisensurestheloadpassestothe
groundthroughthelongitudinalaxiswithoutanydamageto
thestructure.Trunkintheplantisnotonlyincompressible
part,itisalsoaninflexiblestructureasitisalsosubjectedto
radialforces.

16. PROPERTIES
Therefore,itisbuiltontheprincipleofI-girderaswellaslike
apillartoresistthebothforces
SHEARINGSTRESSES:
Theflatorgansliketheleavesareoftensubjectedtoviolent
shearingstressesduetomovementofsurroundingairorwater.
Thewindcurrentsworkatrightanglestothesurfaceofthe
leavesandcauseconsiderablelaceration.Tostandagainstthis
stresstheI-girderspresentforsecuringinflexibilityarefirmly
heldtogetherbyalargenumberofcrosstiesinformofveins
whichoftenformanetwork.
TheI-girderarrangementismorepronouncedin
monocotyledonousleaves,usuallyhavingparallelvenation.
Themarginsoftheleavesareparticularlyexposedtoshearing
stresses.Theyhavespecialarrangementforprotectionby
increasedthicknessoftheepidermis,andfrequentoccurrence
ofthick-walledcollenchymasinthesub-epidermalregion.

17. DISTRIBUTION OF MECHANICAL TISSUES
Compositeperipheralgirders-FoundinCyperaceae&
Juncaginaceae,
Sub-corticalfibro-vascularstrands-Fibrousbundleshavebeen
pushedfromthesurfaceintoamorecentralpositionlikein
Bamboo,
Simplehollowcylinderortubeofstereomewithembedded
mestomestrands-Cylindershollowbutvascularstrandsare
fused,asfoundinthemembersofLiliaceae,Cucurbitaceae,
Chenopodiaceaeetc,
Peripheralhollowcylinderortubeofsteromereinforcedby
isolatedsub-epidermalgirders-Theflangesprojectfromthe
outersurfaceofthetubesatmoreorlessregularintervals,
foundingrasses.

18. MECHANICAL TISSUES INFLEXIBLE ORGANS
Inflexibleorgansarethosecanresistbendingandradial
forces.Theseorgansincludestems,branches,petioles,
inflorescenceaxis,floweraxis,fruitstalks,seedsstalksetc.
ThedistributionofmechanicaltissuesarebasedonI-girder
principle.Themechanicaltissueslikecollenchymas,
sclerenchyma,xylemtissuesetcformtheflangesandthe
parenchyma,phloemtissuesinbetweentheflangesconstitute
thewebpositionofthegirder.Themultiplenumberofthe
girdersarrangedinaringtoformacompositegirdersystemin
cylindricalstructures.
Mainlyfivetypesoforganization(Haberlandt)ofmechanical
tissuesarefound---
Sub-epidermalgirders-FoundinLamiaceaeinsquareor
rectangularinoutline

19. TISSUES IN INEXTENSIBLE STRUCTURES
Inextensiblestructuresarethosesubjectedtolongitudinal
stressandtheseincludeallsuchpartswhichattachtheplants
tothesubstratum.Theseorgansareroots,rhizomes,stiltroots
,fruitstalksetc.Here,themechanicaltissuesareconnected
intoasinglecompactcablelikecentralstrand.Morethe
compactness,morethedistributionofforcetomakeitmore
resistant.Thisisachievedbytheformationofcentral
styereome3bundlesurroundedbyseveralhadromeand
leptomegroups.Intheroots,steleremainscompactbyringof
mechanicaltissues.Insomemonocotclimbers,longitudinal
tensionsinextensibilityisachievedbythesclerotic
developmentinthepith.Inpendulousfruitstalks,
inextensibilityisprovidedeitherbythecontractionoffibrous
cylinderandbythestrengtheningoftheirfibroussheaths
whichaccompanytheinternalmerestomestrands.

20. TISSUES IN INCOMPRESSIBLE STRUCTURES
Incompressibleorgansfacescompressionpressuresinplants
intrunk.Trunkbearstheloadofcanopyandresiststheradial
pressure.Thetrunkneedstwomechanicalneeds-Toresist
compression,Toresistbending.So,themechanicaltissues
followsboththeprincipleofincompressibilityand
inflexibility.TheVascularbundlesrunupwardtodownwardas
straightironrodsofaconcretepillarandtransmitloadtothe
grounds.Thesecondarywoodalsobearsthecompression
pressure.Theringarrangementofthevascularbundlesandthe
concentricringsofthesecondarywoodandphloemprovides
multipleI-girdersandhelpsthetrunktowithstandtheradial
tensions.,ThestiltrootsinPandanuscanwithstandthe
tensionsduetotheirinextensibleandinflexibleproperties.The
mechanicalsystemsofstiltrootsinmaizehavingouterbast
fibersperformsinflexibilityandtheinnerserves
inextensibility.

21. THANKS FOR YOUR PLEASURE OF ENGINEERING IN PLANTS

22. ACKNOWLEDGEMENT
Googleforthedifferentimages,
Differentwebsitesforcontent.
Anatomyofangiosperms-Mishra&Dash
Plantanatomy-B.P.Pandey,
Plantanatomy-PijushRoy
Others.
ThispresentationhasbeenpreparedfortheUGstudentsof
IndianUniversitiesandotherUniversitieswithoutany
financialinterest.