CYCAS-1.pdf

This is a part of the online class for the undergraduate Botany students

1. CYCAS
(Sago palm)
SYSTEMATIC POSITION
DIVISION-GYMNOSPERMAE
CLASS-CYCADOPSIDA
ORDER-CYCADALES
FAMILY-CYCADACEAE
GENUS-CYCAS
Classification
An old Cycasplant
Male plant
Female plant
2N =22
DrP B Mallikharjuna
GFGC Yelahanka
9
th
,16
th
, 23
rd
& 26
th
June 2021
1

2. GEOGRAPHICAL DISTRIBUTION
Cycas,withabout16species(Willis,1951)isthemost
widelydistributedgenusintheEasternHemisphere.
Itisanorientalgenus,foundaswildorcultivatedinthe
tropicalandsub-tropicalregionsoftheworld.
Thespeciesareusuallydistributedinthesouthernparts
ofJapan,India,China,theIslandsofIndianandPacific
oceansandthenorthernpartofAustralia.
AfewspeciesgrowasfarasMadagascar,Mauritiusand
EastcoastofAfrica.
Cycasrevoluta,anativeofChinaandJapan,isthe
commonestandthemostwidelycultivatedcycadsandis
grownthroughouttheworldforitsornamentalvalue.
2

3. C.medagascaransisisfoundinMadagascarandAfrica.
ThegenusCycasisrepresentedbysixspecies(Raizadaand
Sahni,1958)inIndiaandmainlydistributedinthenorth
easti.e.,EastBengal,Orissa,plainsofAssam,Nepaland
Sikkim;andinsouth-Mysore,MadrasandMalabar.
C.pectinata,C.siamensis,C.circinalis,C.rumphii,C.
revolutaandC.beddomei.Outofsix,thelastfourspecies
arequitecommonandarefoundinwildstateinthe
country.
C.revolutaisbyfarthemostcommonlycultivatedspecies
ofIndiangardens,C.siamensis,anativeofSiamandBurma,
isalsosometimescultivated.
InIndia,CycasbeddomigrowswidelyinTirumalaHillsand
C.rhumpiiinAndaman&NicobarIslandsandalsointhe
neighbouringcountrieslikeBurmaandSriLanka
RecentlyanewspeciesCycasswamyihasbeenreported
fromKarnataka(2008) 3

4. INDIAN SPECIES OF CYCAS
C. pectinata
C. siamensis
C. circinalis
C. beddomei
C. revoluta
C. rumphii
C. circinalis
C. revoluta
C. sowmyi
4

5. THE PLANT BODY :
EXTERNALMORPHOLOGY
Thesporophyticplant:slowgrowing,long-
lived,evergreenandwoodygenus.
•Ithasunbranchedcolumnarstemwithcrown
ofpinnatelycompoundleavesatthetopand
resemblestoapalmortreeferninits
externalappearance
•Fullygrownplantofmostofthespecies
normallyrangesfrom2to5metersinheight,
thoughsomespeciesmayattainaheightof
about13to20metersormore.
•C.media,anAustralianspecies,isperhaps
the oftenreachinga
heightof20meters.
•Cycasmorphologicallylookslikeapalm
(monocot)treeandanatomicallylikeadicot.
Cycasold plant
5

6. Root:Taprootsystem
•Primaryrootisashort-livedtaprootwhichlateronis
replacedbyadventitiousrootsarisingfromthestem
tissue.
•Welldevelopedprimaryrootislongandsometimes
almostasthickasthestem,andinseedlingstage
rootismuchlargerthanthestem.
•Mostoflateralbranchesofprimaryroot,thatare
commonlyknownasnormalroots,penetratethesoil,
andanchortheplant.
•Therootsareassociatedwithectomycorrhizae
6

7. CORALLOID ROOTS
•Someoftheroots,whichdevelopnear
thesurfaceorabovetheground,get
infectedwithcertainalgae,bacteria
andfungialreadypresentinthesoil
surroundingtherootsystemcalled
coralloidroots.
•Theserootsgrowatfirsthorizontally
andthenverticallyinthesoiland
becomeswollenattheirtipsdueto
entryabovementionedmicro-
organisms.
•Theybehavelikenegativelygeotropic
structurescomeoutintheairabove
thesoilsurfaceandbranch
dichotomouslytoformgreenishbrown
corallikemassand,therefore,are
namedascoralloidroots.
7

8. CYCAS -STEM
•Thestemisarborescent,erect,
stout,columnarandusually
unbranched.
•Treesmaygiveoutbranchesasa
resultofinjuryoraftertheyhave
reachedacertainage.
•Thestemwhenyoungistuberous,
shortandsubterranean.Mostpart
ofthestemiscoveredwitha
numberoflarge(foliageleaves)and
small(scale)leavesinmaleplants
and scaleleaves and
megasporophyllsinfemaleplants)
rhomboidalleafbases.
•Thishardarmorofwoodyleafbases
thatpersistsformanyyearsaround
thestemissaidtoprotectthestem
andcanbeusedasanindexofthe
ageoftheplant.
8

9. •Acrownofspirallyarrangedleaves
presentaroundthestemapex
makestheCycaslooklikeapalmor
atreefern.
•Itbearstheassimilatoryfoliage
leavesandbrownscaleleaves
arrangedinalternatingwhorls.
•Therearetwotypesofleaves:large
greenfoliageleaves.Foliageleaves
aredevelopedintheformofa
singlecrownunderthecoverof
armorofscaleleaveseachyear.
•Youngfoliageleavespushasidethe
scaleleavesandbegintocomeout
witharapidgrowthtilltheyreach
fullsizeinafewdays.
CYCAS -LEAF
Pinnately compound leaves
9

10. •Theyoungcrownwith
caneasilybedistinguishedfromthefully
expandedoldcrown.
•Thenumberofleavesupto100inold
plants.Foliageleavesareproducedinlarge
numbersandarepinnatelycompound,
showy,fairlylarge1to3meters,very
thickandleathery.
•Theyareattachedtothestemby
transverselyexpandedrhomboidalleaf
bases.Theypossessalongorshortpetiole
andalongstoutrachis.
•Petiolebearstworowsofshortandstiff
spinesnearitsbase.
•Thesegraduallybecometransformedinto
leaflets(pinnae)towardsthedistalendof
theleaf.
•Inveryyoungleavesrachismaybe
circinatewithcircinatelycoiledleafletslike
thoseofferns.
10

11. (II) SCALE LEAVES.
•Scaleleavesaresmall,rough,dry,and
triangularandthicklycoveredstructures
withbrownhairs.
•Youngstemapexissheathedwithsuch
scaleleaves.
•Theybearwelldevelopedbaseseachof
whichtapersintoashort,narrowand
broadlytriangularapicalportion.
•Apicesofthescaleleavesarespirally
twistedagainsteachother.
•Theylooklikethecordsofarope
enclosinginthemyoungstemapexand
otheryoungstructures.
•ThustheyServetoprotecttheother
organswhenyoung.
•Scaleleafbasesarealsopersistentand
formpartofthearmoroftheoldstems.
11

12. Cycas-Anatomy
•The root and the coralloid root anatomy.
•The Stem primary growth and the secondary growth
•The Leaf anatomy –The Rachis and the Leaflet structures
12

13. ANATOMY OF THE ROOT
Two types of roots are found in Cycas.
1. Normal root:
TheInternalstructureofnormalrootofCycas
resembleswiththatofadicotyledonousroot.
Epiblemaisthesinglelayeredoutermost
coveringlayerwiththinwalls,thatgivesout
roothairs
Cortexisthemiddlezoneinbetween
epiblemaandvascularcylinder.Itconsistsof
thin-walledparenchymatouscellsrichin
starch.
Thecortexcontainsalargenumberof
mucilagecanalswhichareboundedby
epitheliallayerorsecretarycells.
Sometimesabundantairspacesand
intersperseddarkbrowntannincellsarealso
seen.
Singlelayeredendodermisisprovidedwith
thethickeningofcasparianstrips..
13

14. Thethin-walledmultilayeredcellsof
periyclecontainabundantstarchgrains.
Thesteleexhibitsdiarchtotetrarchand
rarelypolyarchcondition
Eachvascularbundleshowsradial
arrangementofxylemandphloem,Xylem
andphloemalternatetoeachother.
Protoxylemconsistsoftrachciedswith
spiralthickeningsandmetaxylemwith
scalariformandpittedthickenings.
Thecentreoftherootisoccupied
completelybymetaxylemormetaxylem
withinterspersedparenchymatouscells.
Atanearlystagephloemconsistsofsieve
tubesandphloemparenchymabutin
olderpartsalsowithbastfibers.
Pithisreducedorcompletelyabsent.
Root Anatomy
14

15. SECONDARY GROWTH IN CYCAS ROOT
Followsthesamepatternasseenindicotrootsalthoughstarts
quiteearly.
NormalsecondarygrowthtakesplaceasaresultofCambium
formationofcambialarcsalongtheinneredgeofthephloem
strands.
Thesesooncutofmanoxylicsecondarywoodsimilartothe
stem.
Theprimaryphloem,asaresultofformationofxylemrays,gets
crushed.
Apairofcambialarcs,formingacontinuouscylinderof
secondaryvasculartissue,goesonaddingsecondarytissues.
Coincidentlyadistinctphellogen(corkcambium)layerappears
inthecortex.
Itcutsoffphelloderm(secondarycortex)onitsinnerandphellem
(cork)onitsoutersidewithlenticels.
15

16. 16

17. 2. CORALLOID ROOT ANATOMY
Internalstructureofthecoralloid
rootsissimilartothatofnormal
rootsbuttheydifferfromthe
normalrootsfornothaving
secondarygrowth,andthecortexis
differentiatedintothreedistinct
zones:
theoutercortexcomposedof
compactlyarrangedpolygonal
parenchymatouscells.
Theinnercortexisofthinwalled
parenchymatousand
Themiddlecortexformingthealgal
zone
17

18. Algalzone,whichisusually
onecellwide,consistsofloosely
connectedthinwalledandradially
elongatedcellsbothwith
abundantlargeintercellularand
intracellularspaces.
Theseareoccupiedbycertain
algae(Anabaenacycadeae,
Nostocpunctiforme,Oscillatoria
anddiatoms),somefungianda
fewbacteria(Pseudomonasand
Azotobacter).
Eachcellpresentinthealgal
zonecontainsasinglenucleus
andcytoplasm.
A portion is magnified
TS of Coralloid root
18

19. ANATOMY OF CYCAS STEM
ThestemanatomyofCycaspossessboththeprimarygrowthandthenormal
secondarygrowthwhichresemblesthedicotstem.Howeveritdiffersforthe
lackofsomecellsbothinthexylemandphloemtissues
Inatransversesection,stemassumesanirregularoutlineduetothepresence
ofnumerousleafbases.
Primarygrowth:
Epidermis:theoutermostlayer,onaccountofthepresenceoflargenumberof
persistentleafbasesappearsincompleteandruptured.
Cortex:amultilayeredmiddlezoneoccupiesgreaterpartinbetweenthe
epidermisandvascularcylinder.
Cortexconsistsofthinwalledparenchymatouscellswithabundant
starch.(HencecalledSagopalm)
Numerousmucilageducts,calciumoxalatecrystals,mucilagecanalsand
leaftracescanalsobeseeninthisregion.Twotypesofleaftracepresent
viz.,apairofGirdleleaftracesandtheradialleaftracesarepresent.
Thecortexcommunicateswiththepiththroughmoreorlessbroad
medullaryrays.
Endodermisandpericyclearenotdistinct.
19

20. LEAF BASES
20

21. VascularcylinderisEustelicoccupies
comparativelysmallerregionofthestem
appearslikethedicotstele.
Thevascularbundlesarenarrow,
radiallyelongated,collateral,conjoint,
openandendarch.
Theyaresmallinsizeandmorein
number,andarecloselyarrangedina
ring.
Primaryphloemelements,consisting
ofsievetubesandphloem
parenchymaandlackingcompanion
cells,butsievetubesareassociated
withalbuminouscells.
Short-livedprimarycambiumis
representedbyathinstripof
parenchymatouscellsbetweenthe
primaryxylemandprimaryphloem.
21

22. Primaryxylemconsistsofonlytrachciedswhichshowspiral
thickeningsinprotoxylemandscalariformthickeningsinthe
metaxylemoftenwithborderedpits.
Alargeparenchymatouscellsrichinstarchpresentinthemedullaor
pithregionanditcontainsalargenumberofmucilagecanals.
AstrikinganatomicalfeatureofCycasisthegirdlingofthestem.Itis
causedbythelargenumberofleaftraces.
Thesearesmallandendarchofthestem,pseudomesarchastheyenter
theleafduetotheformationofcentripetalxylemandexarchatthe
extremeterminalpartoftherachis.
Foreachleafthereareusuallyfourtracesofwhichtwoaremaintraces
girdletracebundleswhiletheothertwoaredirecttracesorradial
tracebundles.
22

23. Theypassoutoftheringthrougha
gapformedbythesplittingofthe
vascularcylinder.Maintracebundle
firstpassesobliquelythroughthe
cortex.
Thenitdividesintotwohalveseach
halfrunningroundinopposite
directionsintheoutercortexwhere
theyriseandmeetatthebaseofthe
leafandfinallybendsharplyintothe
petiole.
Becausethesetracegirdlesrunround
thecentralcylinderinthecortex,they
aregiventhenamegirdle-traces".
Theseinturnbifurcateandeachhalf
branchesfreelysoastoproducea
complexsystemofanatomizing
bundles.
LS of the Cycas
stem tip
23

24. SECONDARY GROWTH
ThesecondarygrowthoccursintheCycasstemduetotheformation
ofvascularcambiuminthestelarregionandthecorkcambiuminthe
corticalregionasinthedicotstem.
Secondarygrowthinthestembeginsquiteearly.
Atfirst,ittakesplacebytheactivityofinterfascicularandprimary
intra-fascicularcambiaresulting,theformationofvascularcambial
ring.
Thecambialringthatcutsofsecondaryxylemontheinnerand
secondaryphloemontheouterside.
Parenchymacellsandnarrowuniformlyparenchymatoussecondary
medularyrayscomposedofcellsrichinstarchfounddistributedin
thesecondaryxylemaresometimesalsocutoffbythecambiumon
theinnerside.Secondaryphloemcomprisesoffibresandsievecells.
Elongatedborderedpitsinthecellwallofmetaxylemtracheids.
Growthringsofanykindhavenotbeenobservedinthewood.
Occurrenceofabout8to12borderedpitsinthexylemtracheidscan
bemarkedbytheirpresenceintwoorthreeseries,sometimes
alternateorpressed.
24

25. Cycasstem TS with Secondary growth
Cycasstem is with the manoxylic,
nonporous, polyxylicwood anatomy
25

26. 26

27. Aftertheprimarycambiumceasestobeactive,secondarycambia
developinthecortexorpericycleandproducecorticalcylinders.
Asaresultofthediminishingsequenceintheactivityofsuccessive
cambialrings,thevascularringsexhibitagradualnarrowingfrom
thefirstvascularringtotheoutermostringofthevascularcylinder.
Thustheinnermostvascularringisalmostasthickasprimary
vascularringandtheoutermostmaybethinnest.
Theconcentricringsareseparatedfromeachotherbyafewlayers
ofparenchymatouscells.Secondaryringsarecomposedofthe
sameelementsasfoundintheprimaryringsofvascularbundles.
Vascularbundlesofsecondaryrings,whichdifferfromtheleaf
tracebundlesonaccountoftheabsenceofProtoxylem,are
traversedbybroadmedullaryrays,leaftraces,mucilagecanalsand
corticalbundlesrunningthroughoutfromthefirstringtothe
cortex.
Occasionally,thearcsofinvertedvascularbundlesmayalsooccur
betweenthenormalrings.Thusveryoldstemshowspolycyclic
condition.
27

28. RLS through wood
HLS of wood
Bordered pits
Sieve tube
Cycasstem wood anatomy
Tracheid with
Bordered pit
28

29. ANATOMY OF RACHIS
InaT/Srachismayappearbiconvex,cylindricalorflatteneddepending
upontheregion.Apairofdepressionsofarmsoneitherside,passinginto
theleaflets.
Thick-walledandsinglelayeredepidermiswithrectangularcellsis
coveredwithcuticle.Stomataareirregularlyarrangedandsunken.
Groundtissuesaredifferentiatedintothreedefinedzones.
Thehypodermalregionisdifferentiatedintooutergreen
chlorenchymatousandinnerthickwalledcompactsclerenchymatous.
Anarrowzoneofchlorenchymatouscellsoccursjustbelowthe
epidermis
Asclerenchymatouszoneconsistsof2-3celllayersthickinadaxialsurface
andpoorlydevelopednearthedepressions
29

30. Ground plan of Rachis TS
A vascular bundle
30

31. Anumberofmucilagecanalsandafewcellswithstar-shaped
Crystalsofcalciumoxalatealsooccurinthiszone.
Avaryingnumberofvascularbundlesarefoundembeddedin
theparenchymatoustissue.
Thevascularbundleswhicharemoreorlessovalareconjoint,
collateralandopenandarearrangedintheformofGreekletter
omega(u)orhorseshoe.
Thearrangementandthenumberofvascularbundles,
however,variableinthedifferentpartsofoneandthesame
rachis.
Eachvascularbundleisenclosedbyathick-walled
sclerenchymatousbundlesheath.
31

32. Pericycleisonetoseverallayeredthickenclosesxylem,phloem,athin
stripofinactivecambiumandparenchyma.
vascularbundlesarecharacterizedbythepresenceoftwotypesof
xylemsarrangedintwopatchesofendarch(centrifugal)xylemand
exarch(centripetalxylem)andarecalled"pseudomesarch"or
"diploxylic"
Phloemoccursbythesideofthecentrifugalxylem.
Athinstripofcambiumoccursinbetweenthephloemandxylem.
Bundlespresentatthetwoopenends,asapparentintheiromegashaped
arrangement,passoutandenterthetwopinnae.Thecontinuousentryof
thevascularbundlesinthepinnae,frombasetotheapexoftherachis,
graduallyreducestheirnumber.Ultimatelyterminalbundlestooenter
onebyoneintheterminalpinnaeoftheleaf.
32

33. ANATOMY OF CYCASLEAFLET
Whole mount/section
A portion is magnified
33

34. •Inaverticalsectionleafletsorpinnaerevealxerophytes
characters.(presenceofcuticle,hypodermis,sunkenstomata,
bundlesheath)
•Theepidermisissinglelayeredwiththickwalledcells.
•Itiscontinuousontheuppersideandisinterruptedonthelower
sidewithminutepitsatthebottomofwhichliesunkenstomata(
haplocheilictype)
•Eachstomataopensoutsideinto"epistomatalcavityandinside
intoa"hypostomatalchamber".
•Eachstomaconsistsoftwoguardcellsthataresurroundedbya
pairofpapillatedorprojectedsubsidiaryorbuffercellsthat
regulatetheopeningandclosingofthestomatalaperture.
34

35. •Internaltotheupperandlowerepidermisliessinglelayeredthick
sclerenymatoushypodermis.Whileinthemidribregionitistwoormore
layeredthick.
•Besidesprovidingmechanicalstrength,thehypodermisactsasaheat
screenandprotectstheleafletsfromoverheatingandexcessive
transpiration.
•Nexttohypodermisisazoneofchlorophyllousmesophyllcells.
•Themesophyllisdifferentiatedintoverticallyelongatedpalisadetissue
situatedtowardsuppersideandlooselyarrangedspongyparenchyma
tissuetowardstheSide,Palisadeactsasanassimilatorytissueand
consistsofsinglelayeroflinearlyarrangedchloroplasts.
35

36. •Spongyparenchyma,anassimilatoryandaeratingzoneliesabovethe
lowerepidermisandconsistsofthepalisadelayerandspongy
parenchymaoccursathreeorfourthicktransfusiontissuecomposedof
long,colorlessparenchymacells.
•Thistissueisconnectedwithcentripetalxylem,thebundlesheathand
runslaterallyfromthemiddletotheedgesoftheleaflet.
36

37. •The cell walls of this tissue are provided with reticulate or bordered pitted
thickenings. A few tracheidal cells, looking like the colorless lignified tracheids,
are also found distributed in between the mesophyll cells on either side of the
midrib.
•They are provided with bordered pits on their radial walls.
•These constitute the primary transfusion tissueand appear close to vascular
bundle.
•The transfusion cells are arranged at right angles to the axis of the leaf and
constitute the so called radial parenchyma, secondary transfusion tissue,
hydrosterom or accessory transfusion tissue.
37

38. •Some of the parenchymatous cells around the
stele are provided with star shaped crystals of
calcium oxalate and are called
sphaeroraphides.
•In the midrib region, there is a single large
vascular bundle surrounded by a jacket of
thick-walled cells. Phloem lies towards the
lower surface of the leaf. Protoxylem is
diploxylic or pseudomesarch.
•The centripetal Xylem is directed towards the
upper surface of the leaf and the centrifugal
xylem towards the lower leaf surface.
Centrifugal xylem consists of a few tracheidal
elements.
A single vascular bundle
38

39. Cycas–Sexual Reproduction
The male cone structure &
The female megasporophylland ovule structures
ThesexualreproductioninCycasoccursafterattaining
certainagei.e..about5-10yearsold.Bulbilsarerarely
producedasthevegetativepropagulesinsomespecies.
39

40. SexualReproduction
•Sexualreproductionisthemostcommontypeof
reproductioninCycas.
•Ittakesplacebytheformationthelargeorconspicuous
reproductivestructurescalledthemaleconesandthe
femalemegasporophylls.
•TheplantsofCycasareheterosporousandinvariably
dioecious,i.e.maleandfemalereproductiveorgansoccur
onseparateindividuals.
40

41. Male sex organs of Cycas
Male plant with a single cone
41

42. Malecone/strobilus:Amaleconeisanovalor
conicallargestructurewithnumerousspirallyarranged
microsporophylls,whichalmostperpendicularlyattachedto
theconeaxis.
Thenewstemapexappearsterminalanddevelopsfresh
crownsoffoliageleavesandscaleleaves.Anothermale
conedevelopsatthetipofsuchlateralbranch.
Themalestrobilusisshortlystalked,long,compactand
fusiformorovalstructuresurroundedbyacrownofyoung
leaves.
Maturestrobiliappearwoodyintextureandareabout20to
60cmlong.
Thesesometimesattainlengthofabout80cmasreported
insomeplantsofC.circinalis,
42

43. Male
cone
Microsporophyll
Central axis
Sorusof microsporangia
A microsporophyll with sori
Cycasmale reproductive structures
43

44. Themalestrobilusconsistsofanumber of
microsporophylls(stamens)closelyarrangedinspirals
acropetallyonthecentralconeaxis.
Inasurfaceviewmicrosporophyllsappeartobearranged
inverticalrows.
Theyoungmicrosporophylls aresoftandfleshy,
becomehardandwoodyatApophysis
Theyaresmalltowardsthebaseandtheapexofthe
strobilusandattaintheirmaximumlength(30to35mm
long)inthemiddle.
Someofthemicrosporophyllssituatedattheextreme
topandatthebasemaybesterile.
44

45. Microsporophyll:Eachmicrosporophyllarisesasa
smalloutgrowthonthecentralconeaxisandappearsas
awedge-shapedmoreorlesstriangularandflattened
structureabout3to3.5cminlengthatmaturity.
Thebasalpartofeachmicrosporophyll,arisingatright
angletothecentralaxis,isnarrowand'sterile.
Itgraduallybecomesbroader,andfinallyterminatesinto
anexpandedsterilediscwithashortupwardprojection,
theapophysisthatoverlapsothermicrosporophyllsat
higherlevel.Inbetween,thebasalpartofthe
microsporophyllandapophysisliesthefertilepart.
45

46. Microsporangia (pollensacs)areborneontheabaxial
(lower)surfaceofthemicrosporophyll.
Theyoccurinquitelargenumber(about700inC.circinalis
and1,000inC.revoluta)andarearrangedindefinitesoriin
twodepressedzonesoneithersideofamedianridge.
Thenumberofthesporangiaineachsorusvariesfrom2to6.
Unicellularorbicelledhairsoccurintermixedwithsporangia.
Thedevelopmentofthemicrosporangiumisofeusporangiate
type.
46

47. Maturemicrosporangium :
Afullymaturemicrosporangiumlookslikeashortstalked
ovalsac.
Atmaturitymicrosporangiumpossessasinglelayered
wall,enclosingwithinitthespores.
Thetapetumandouterwallcellsgetdisintegrated.
Dehiscence of microsporangia: The ripen
microsporangiadehiscebylongitudinalslitsalongthe
thickenedcellsontheoutermostlayerofthewall.
Thesecellsloosewaterandshrink.Becauseofelongation
ofcentral'coneaxis,microsporophyllsgetseparatedand
exposetheboatshapedopenmicrosporangia.
47

48. Atthisstage,themicrosporophyllsaswellascentral
axisofthemaleconebecomehardduetothelossof
theturgidityanddryingofresins.
Whendry,resinbecomesinelasticandhard.Sincethe
ripeningdependslargelyondrying,theyoungest
microsporophyllpresentatthetopofthestrobilusand
farthestfromwatersupply,ripentheirpollengrains
first.
Consequently,sheddingofthepollengrainsstarts
fromthetopmostsporophyllsandproceedstowards
thebase.
48

49. LS of microsporangium
49

50. Cycas-FemaleReproductivestructures
Megasporophylls.
Cycasisuniqueinthattheredoesnotpossesfemalecone/strobilus,
Themegasporophyllsoccurinclosespiralsaroundthestemapexof
thefemaleplant.
Theyarelooselyarrangedinacropetallysuccessionandgivean
appearanceofarosette.
Theapicalmeristemisleftunaffectedandcontinuestogrow
throughouttheplantlifeandformsthefutureleavesandsporophylls.
Thedevelopmentofthemegasporophyllsstartssoonaftertheplantis
somewhatmature.
Usuallythemegasporophyllsareformedonceayearandoccupytheir
positioninbetweensuccessivewhorlsoffoliageresemblecloselywith
thefoliageleavesandarisefromtheplaceswhere,underleaves.
50

51. Megasporophyllsatfirstbendinwardsandcoverthegrowing
pointbutsoonwiththeelongationofthestemtheygetpushed
awayandassumeahorizontalordroopingposition.
Eachmegasporophyllisdifferentiatedintothree,moreor-less
distinctparts:
I.Aproximalshortorlongstalklikeaxis,
II.Amiddleportionbearingtworowsofoppositeand
alternatelyarrangedovules,and
III.Adistalmoreorlessexpanded,dorsoventrallyflattened
sterileportionofvaryingshapes.
Inmostcases,megasporophyllsarecoveredwithyellowor
brownwoolyhairscalledramantae.g.C.revoluta.
51

52. ThelargestovuleisintheCycasthourasii(7cmheight)followedby
C.circinalis(6cm)intheentirePlantkingdom
52

53. C. swamyi
Variation in the structure of megasporophyllsand ovules in the Indian
Cycasspecies
53

54. Megasporangium /Ovule:TheovulesofCycasarelarge,
orthotropous,shortlystalked,unitegmicandperhapsthelargest
insize.Thenumberofovulespermegasporophyllvariesusually
from2to12(1to6oneitherside).
Structureoftheovule:AmatureovuleofCycasshows
thefollowingstructures.
Theovuleisofconsiderablesize(about6cminlengthand4
cmindiameter)andcanbeseenbynakedeyes.
Whenyoung,itisgreenandcoveredwithhairsbuton
maturityitappearsorangeorredandlosesthehairs.
Eachovuleisanovalorsphericalstructurewithacentral
massofparenchymatoustissuecallednucellus.
54

55. Thenucellusissurroundedbyasingle,about1cmthick
integumentwhichisdifferentiatedintothreelayers;
(i)Outerfleshylayer(outersarcotesta),
(ii)Middlestonylayer(middlesclerotesta)and
(iii)Innerfleshylayer(innersarcotesta)
Egg
55

56. Integumentprotectsthemegasporangium.
ItgetstransformedintoOuterFleshylayer,MiddleStonylayer
andInnerFleshylayer,
Middlelayerpossesses2to3ridges.Innerfleshylayeris
composedofparenchymatouscellsexceptattheapex,Itis
consumedduringthedevelopmentoftheovuleandpersists
asathinpaperylayer.
Further,itishavingthepollenchamber,Archegonia,Nucellus,
Femalegametophyte,Vascularstrands,
Integumentremainsinclosecontactwiththenucellusexcept
attheapicalregionwhereitformsalongmicropyle.
56

57. Theapicalportionofthenucellusformsthenucellarbeak
thatforcesitswaysintothemicropyle.
Inthecentreofthebeakandbelowthecellsofthenucellus
breakdownandformthepollenchamber
Theovuleissuppliedwithapairofvascularbundlesatthe
base.
Oneofthebranchofeachvascularbundleentersintothe
outerfleshyandtheotherintoinnerfleshylayer.
Aftertheactofpollinationovulegetsconsiderably
enlarged.
57

58. THEGAMETOPHYTES (Male&Female):
Cycasisaheterosporousplant,i.e.bearingmicrospores(pollen
grains)andmegaspores(embryosacs).
Thedevelopmentofthemaleandfemalegametophytestakesplace
frommicrosporesandmegasporesrespectively.
Bothmicrosporeandmegasporemothercellsrepresentthelast
stagesofthesporophyticgeneration.
Theydividemeioticallywhilestillpresentwithinthemicrosporangia
andmegasporangia,respectively.
Microsporangium producesalargenumber offunctional
microspores,whilethemegasporangium producesonly3to4
megaspores,ofwhichonlybasalonematurestoformfunctional
megasporeandothersdegenerate.
Microsporesandmegaspores representthefirststagesof
gametophyticgeneration. 58

59. MaleGametophyte :Themicrosporeistheinitialstageorunitof
malegametophyte.
Itisaunicelled,uninucleateandhaploidstructure.
Ithaslongitudinalslit.Eachmicrosporeisprotectedbytwowalls:
(i)Exine-theouterthickwalland
(ii)Intine-theinnerthinwall.
The microspore is subjected two asymmetric mitotic cell
divisions and thereby produce 3-celled ,haploid male
gametophyte or pollen grain.
It possess a vegetative cell and two flagellate , motile sperms or
antherozoids.
The Cycasmale sperms are the largest gametes (200µm) in the
plant kingdom
59

60. Pollen grain
Microspore
Metamorphosis
Antherozoid
60

61. 61

62. FemaleGametophyte :Thefemalegametophyteisthe
Archegoniumwhichisahaploid,flaskshapedand
multicellularstructure.Further,itcontainsapairofneck
cellsandaneggcellwithalargenucleus(500µm).
2-8archegoniaarepresentinanovulesurroundedby
endosperm.
Archegonium isdeveloped fromthefunctional
megasporeduetomeiosisofmegasporemothercell
(Megasporogenesis).
Themegasporemothercellisderivedfromthenucellus.
Someofthesecellsarealsometamorphosed intothe
haploidandnutritivetissuecalledEndosperm.
62

63. Pollination:Windpollination,butrarelyinsectpollination.
Thepollengrainsmaydepositintothepollenchamberof
theovule.
Restsforfewmonthsandgerminatelater
Fertilization:Siphanogamoustypeorzoodiogamoustype.
Thepollengraingerminatesandproducealongpollen
tubefromthevegetativecellandtwociliate-motile
gametes(antherozoids).
Thenucleusofthefunctionalmalegameteissuckedby
thearchegonium,whereitfuseswiththelargeegg
nucleusandproducesadiploidzygote.
Seed:isthefertilizedovuleandthematuredseedisfleshy,
red,orangeorbrownincolour.Itcontainsadicotyledon
embryoandothertissues
63

64. Pollen tube
Antherozoid
Embryo
PollenDiagramatic
life cycle of
Cycas
64