2. SYNOPSIS
1. Historical background.
2. General characters of order Gnetales.
3. Classification saga of Gnetales.
4. Systematic position & Distribution.
5. IUCN Red List
6. Characteristic features of Gnetum.
7. Anatomy of root (young & old)
8. Anatomy of stem (young & old)
9. Anatomy of leaves.
10. Reproduction – micro & megasporogenesis.
11. Pollination, fertilization & post fertilization.
12. Embryo formation & germination of seed.
13. Resemblance b/w Gnetum & Angiosperms
14. Resemblance b/w Gnetum &Gymnosperms.
15. Images of some Indian Gnetum species
16. Reference
3. HISTORICAL BACKGROUND
Chamberlain (1935)- There are only three genera
Ephedra, Welwitschia, Gnetum.
Florin (1933) - Advocated strongly for splitting
Gnetales into three independent orders viz.
Ephadrales, Welwitschiales, Gnetales.
Foster & Gifford (1959)- He classified all three genera
into single family Gnetaceae under Gnetales.
Maheshwari & Vasil (1961)- He considered the
Ephedra Welwitschia, Gnetum to be highest evolved
among Gymnosperms.
Sporne (1965)- divided Gnetales into three unigeneric
families, Gnetaceae : Gnetum, Ephedraceae:Ephedra,
Welwitschiaceae: Welwitschia
4. GENERAL CHARACTERS OF ORDER GNETALES
Mostly woody, some species are trees (Gnetum gnemon), many lianas or
shrubs &a few are stumpy turnip like (Welwitschia mirabilis)
Leaves are simple elliptical or strap shaped or sometimes reduced to
minute scales. They are generally opposite or whorled.
Vessels are present in the secondary wood.
Flowers are unisexual usually dioecious & rarely monoecious.
The male flowers are surrounded by a perianth.
In each female flower a single erect orthotropous ovule is present.
Nucellus of the ovule remains surrounded by the two or three
envelopes.
Pollen tube contains two male nuclei.
A unicellular primary suspensor is present in the embryo.
Two cotyledons are present in the embryo.
6. SYSTEMATIC POSITION
Kingdom Plantae
Sub kingdom Tracheobionta
Superdivision Spermatophyta
Division Gnetophyta
Class Gnetopsida
Order Gnetales
Family Gnetaceae
Genus Gnetum
DISTRIBUTION
Represented by 40 species, confined to Tropical & Humid region.
According to BHARADWAJ 1957, mainly 5 species of Gnetum in India
G.gnemon- Shrubby plant; found in Assam, Naga Hills region.
G.contractum- Scandent shrub; found in Kerala & Nilgiri hills.
G.latifolium-Climber; found in Andaman & Nicobar Island.
G.ula-Woody Climber with swollen nodal branches; found in Kerala,
Andhra pradesh, Orissa
G.montanum-Climber with slender branches, woven at nodes; found in
Assam, Sikkim,& some parts of Orissa.
7. IUCN RED LIST
Species are classified by the IUCN Red List into various groups
namely,
Extinct (EX) – No known individuals remaining
Extinct in the wild (EW) – Known only to survive in captivity, or as
a naturalized population outside its historic range
Critically endangered (CR) – Extremely high risk of extinction in
the wild
Endangered (EN) – High risk of extinction in the wild
Vulnerable (VU) – High risk of endangerment in the wild
Near threatened (NT) – Likely to become endangered in the near
future
Least concern (LC) – Lowest risk (Does not qualify for a more at-
risk category; widespread and abundant taxa are included in this
category.)
Data deficient (DD) – Not enough data to make an assessment of its
risk of extinction
Not evaluated (NE) – Has not yet been evaluated against the criteria
G.ula, G.montanum, G.gnemon, G.latifolium, are under LC.
G.contractum is under Vulnerable category
8.
9. CHARACTERISTIC FEATURES OF GNETUM
Most species are climbers except
few being shrubs & trees.
Several stems are articulated.
BRANCHES- 2 types-
(i) Branches of limited growth
(ii)Branches of Unlimited
growth (This type of distinction is
absent in shrub or tree type)
Climbing species have branches
of limited growth (short shoot) ,
unbranched with foliage leaves.
LEAVES- Dicot like : large & oval
with entire margins 9-10 in pairs
arranged in ducussate fashion
with reticulate venation.
10. ANATOMY OF ROOT
In Young Roots-
Layers of starch filled cortical
cells.(polygonal, parenchymatous)
4-6 layers of pericycle; primary xylem visible.
Roots may Di-arch & Ex-arch.
In endodermis casparian strips are
present.(angio character)
In Older Roots-
Primary xylem indistinguishable due to
secondary growth.
Consists of tracheids, vessels & xylem
parenchyma.
Phloem consists of sieve tubes & phloem
parenchyma.
“Bars of sanio” present in tracheids; absent
in vessels.
In Tracheids uniseriate bordered pits are
present.
Is vessels multiseriate bordered pits are
present
11. ANATOMY OF STEM
In Young Stem-
Resembles typical dicot stem.
Sunken stomata present.
Cortex:-1st few layers-Chlorenchymatous cells
2nd few layers- Parenchymatous cells.
3rd few layers- Sclerenchymatous cells.
Endodermis & Pericycle indistinguishable.
Vascular Bundles- Conjoint, Collateral, Open & End-arch
in a ring.
Xylem- comprises Tracheids & Xylem vessels.
Phloem- comprises Sieve cells & Phloem parenchyma.
In Old Stem-
Primary cambium ephemeral(short lived).
Sclerotic cells present.
2ndry cambium in different parts of cortex form rings
2ndry wood comprises Tracheids (bordered pits)&
vessels(simple pits)
Phloem Sieve cells have oblique perforated sieve plates.
Medullary rays consists of Polygonal parenchyma cells.
12. TANGENTIAL LONGITUDINAL SECTION OF STEM
Periderm is thin possess
lenticels which functions
as a pore & provide
pathway for direct
exchange of gases.
(Presence of lenticels are
characteristic feature of
angiosperms.)
13. ANATOMY OF LEAVES
They resemble dicot leaves with
cuticularised epidermis, stomata on
both sides except on veins.
Mesophylls differentiated into single
layered Palisade, well developed
Spongy parenchyma.
A ring of thick walled Stone cells
present just outside the phloem & Latex
tubes are present in mid-rib region.
Transfusion tissues are absent.
Vascular Bundles- (i) Arch/curve from in
mid-rib region
(ii)Conjoint, Collateral type.
(iii)Xylem; Tracheids,Vessels,Xylem
parenchyma.
(iv)Phloem; Sieve cells, Phloem
parenchyma.
14. REPRODUCTION
Gnetum- dioecious; Cones/Strobili reproductive structures.
Cones- (i)Organised into Inflorescence, Panicle type.
(ii)Cone axis with two opposite bracts at the base.
(iii)Whorls of circular bracts present on main axis form cupules &
collars.
(iv)Flower like structures in collars formed of bracts.
MALE CONE & MALE FLOWER
o Flowers arranged alternatively in definite rings above each collar on
the nodes.
o Each flower contains 2 coherent bracts which form perianth.
o Above the rings of male flower there is a ring of abortive ovule.
o At maturity, stalk elongates such that anther comes out of perianth
sheath
FEMALE CONE & FEMALE FLOWER
o Single rings of 4-10 ovules above collar.
o Only few ovules develops into mature seeds.
o Ovule consists of nucellus surrounded by 3 envelopes.
o Nucellus (contain female gametophyte) consists of central mass of
cells
o Inner envelope elongates to form micropylar tube.
o Stomata, sclereids & laticiferous cells formed in other two
envelopes.
o Ovule is orthotropus
16. MICROSPOROGENESIS
Two archesporial cells divide to form
multicelled archesporium.
Archesporium divides periclinally
forming:-
Outer layer- Parietal cells
Inner layer- Sporogenous cells.
Parietal cells forms the wall layer &
tapetal cells periclinally.
Sporogenous cells divide irregularly
forming MMC.
Tapetal cells get bi-nucleated.
MMC forms haploid microspores by
meiosis; soon, Wall cells& Tapetal cells
of tapetum disorganise.
As soon as the anthers dehisce (along
the double row of small cells which
extends from tip towards the base), the
microspores are released
18. MEGASPOROGENESIS
Two Archesporial cells divides periclinally
forming- (i)outer parietal cells. (ii) inner
sporogenous cells.
Primary parietal cells & Epidermal cells
divides periclinally & Anticlinally many
times forming Nucellus.
Sporogenous cells by repeated division
forms MMC.
MMC by meiosis forms tetrasporic
embryo sacs (out of which 1 remains
functional other degenrates)
Finally, female gametophyte is formed
which broader at micropylar end tapering
at chalazal end
In Gnetum, Normal archegonia & megaspore
tetrad are present.
20. POLLINATION , FERTILIZATION & POST FERTILIZATION
Pollination- Pollen pollinated by anemophily; get entangled to
fluid secreted by micropylar tube thereby male gametes reach
egg chamber.
Fertilization-Pollen tubes breaks into ovule releasing male cells.
One cell enters egg cells.male & female nuclei lie side by side for
sometime, then fuse to form zygote.
Endosperm formation-In Gnetum,cellular endosperm formed
after fertilization although cell formation starts after
fertilization(usually in gymnosperm endosperm develop before
fertilization.)
After Fertilization wall formation starts from base & proceeds
upwards such that cytoplasm divides into many compartment
containing many nuclei, which fuse to form a single nucleus.
The upper portion usually remains free nuclear while lower
portion become cellular having multinucleated compartment. The
vice-versa may also be possible& in some rare cases no wall
formation takes place
Triple fusion characteristic of angiosperms is absent.
21.
22. EMBRYO
In Gnetum, different species shows variable embryonal development. (A/c
to Maheshwari & Vasil (1961), the first division of zygote in
angiosperm is accompnied by wall formation while in gymnosperms
[except sequoia spp.]free nuclear division.Thus Gnetum in this
respect forms a link b/w gymnosperms & angiosperms as it shows
both free nuclear division & cell wall formation)
A/c to Madhulata(1960), In G.gnemon, out of 2-4 zygotes, 1 remains
functional& develops 1-3 small tubular outgrowths, out of which only one
receives nucleus while other disintegrates. The surviving outgrowths or
PST primary suspensor tubes coiled around each other. A small cell
cuts off at the tip of PST, divides 1st transversely & then longitudinally
giving 4 cells which divides to from cell mass. Some of its cells
elongates/divides to form SST secondary suspensor tube while other
cells form Embryonal mass EM .The primary & secondary suspensors
helps in pushing embryo to endosperm.
At the end of pushing, stem tip with 2 lateral cotyledons formed at the tip
of EM while root tip with root cap is formed at opposite side; thereafter
feeder develops b/w the root & shoot portion such that the resultant
mature Embryo consists of stem tip, two cotyledons root tip & root
cap in respective sequence.
23.
24. GERMINATION OF SEED
Seeds in Gnetum-(i)Oval
shaped, green to red in colour.
(ii)Surrounded by 3 layer
envelope enclosing Embryo &
Endosperm-
(a)Inner envelope-
Parenchymatous
(b)Middle envelope- Hard
protective differentiated into 3
layers(i)Parenchymatous (ii)
Palisade (iii)Fibrous.
(c) Outer envelope- Fleshy,
Parenchymatous, coloured.
Seed Germination –
(i) Epigeal type
(ii)Hypopcotyl elongates
bringing cotyledons out of soil.
(iii) First pair of foliage leaves
produced by developing
plumule.
25. ECONOMIC IMPORTANCE OF GNETUM:
Seeds of Gnetum gnemon, G. latifolium and G.ula are eaten
after boiling or roasting. Sometimes they are fried in oil and
eaten.
Young leaves and inflorescence of Gnetum gnemon and
G. latifolium are cooked as vegetable.
A fiber of high tensile strength is obtained from the bark of
Gnetum gnemon and G. latifolium, and used for making fishing
nets and ropes. Ropes prepared from this fiber are light,
strong.
Paper is made from the pulp produced from the bark of
Gnetum gnemon.
A fixed oil, used for illumination and also effective in
rheumatism, is obtained from the kernels of Gnetum ula. It is
used widely in Kerala and Assam.
Gnetum montanum possesses pesticidal properties.
27. RESEMBLANCE B/W GNETUM & ANGIOSPERMS
Reticulate venation in leaves.
Presence of xylem vessels.
Tetrasporic development of Female
gametophyte.
Absence of Archegonia.
Dicot nature of Embryo.
28. RESEMBLANCE B/W GNETUM & GYMNOSPERMS
Presence of wood with Tracheids.
Presence of Anemophily mode of pollination.
Presence of naked ovule.
Absence of ovary thus fruit absent.
Development of prothalial cells.
Presence of cleavage polyembryony.
