All about Gnetum (Gymnosperm)

Gnetum is a link between Gymnosperms and Angiosperms as it shows characteristics of both

1. PRESENTED BY- RATNAKAR UPADHYAY AN ACCOUNT OF ORDER GNETALES ( Gnetum )

2. Synopsis Historical background. General characters of order Gnetales . Classification saga of Gnetales . Systematic position & Distribution. IUCN Red List Characteristic features of Gnetum . Anatomy of root (young & old) Anatomy of stem (young & old) Anatomy of leaves. Reproduction – micro & megasporogenesis . Pollination, fertilization & post fertilization. Embryo formation & germination of seed. Resemblance b/w Gnetum & Angiosperms Resemblance b/w Gnetum &Gymnosperms. Images of some Indian Gnetum species 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.

5. classification saga of gnetales

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. 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.

9. 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

10. Anatomy of stem In Young Stem- Resembles typical dicot stem. Sunken stomata present. Cortex:- 1 st few layers- Chlorenchymatous cells 2 nd few layers- Parenchymatous cells . 3 rd 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.

11. 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.)

12. 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.

13. 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 Flowers arranged alternatively in definite rings above each collar on the nodes. Each flower contains 2 coherent bracts which form perianth . Above the rings of male flower there is a ring of abortive ovule. At maturity, stalk elongates such that anther comes out of perianth sheath FEMALE CONE & FEMALE FLOWER Single rings of 4-10 ovules above collar. Only few ovules develops into mature seeds. Ovule consists of nucellus surrounded by 3 envelopes. Nucellus (contain female gametophyte) consists of central mass of cells Inner envelope elongates to form micropylar tube. Stomata, sclereids & laticiferous cells formed in other two envelopes. Ovule is orthotropus

14. Male cone & female cone

15. 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

16. Male gametophyte

17. 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.

18. Female gametophyte The Female gametophyte is Tetrasporic in development. The Archegonia are absent in Gnetum

19. 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.

20. 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 1 st 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.

21. 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.

22. 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.

23. life cycle of gnetum

24. 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.

25. 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 .

26. Gnetum ula Gnetum gnemon Gnetum latifolium Gnetum montanum

27. Reference Bhatnagar,S P, Moitra , Alok,Gymnosperms,1996, new age international limited publisher,Delhi . Sharma,O.P , Gymnosperms,1999, Pragati prakashan , meerut . Vashishta,P.C , Sinha,A.K , Kumar,Anil,Gymnosperms,1976 (reprint 2016), S.chand & Company Pvt.Ltd ., Delhi.

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