Class : Gnetopsida
Order : Gnetales
Family : Gnetaceae
Genus : Gnetum
Distribution :
The genus includes 30 -35 species of woody trees, shrubs and even climbers. They are found in the tropical and humid regions of the world, mostly in the rain forests. Gnetum is represented in India by 5 species.
Gnetum contractum : confined to southern regions - Kerala, Nilgriri hills and Coonoor.
G. gnemon : Naga Hills and Golghat areas of Assam
G. montanum : Sikkim, Assam
G. latifolium : woody climber - Andaman and Nicobar islands
G. ula : woody climber - Western Ghats (Khandala), Coorg, Kerala, Nilgiri hills, Godavari district of Andhra pradesh.
Habit:
They may be woody trees, shrubs or woody climbers (lianas). Majority of the Indian speices are woody climbers (G.ula, G. latifolium, G. montanum). a few are scandant shrubs (G. contractum), whereas G.gnemon is a medium sized tree.
G. trinerve is a parasitic species found in Cinchona forests in Chimboraze.
External Morphology:
Plant Body:
The plant body is sporophyte and resembles a dicotyledonous plant. The sporophyte is differentiated into root, stem and leaves.
Root: It is profusely branched tap root which does not grow very deep. This is soon replaced by adventitious roots.
Stem : The stem is woody and branched. Branches are of two types (dimorphic)
Dwarf shoots or branches of limited growth : they are divided into nodes and internodes and bear foliage leaves that arise in opposite decussate pairs. The number of leaves on a dwarf shoot may be 9 or 10.
Long shoots or shoots of unlimited growth : bear reduced and cup-like pairs of scale leaves. The nodes are swollen and branches become articulated and has prominent joints. The joints consists of two portions, one above the node and the other below the node, and the two are demarcated by an annular groove. Internodes break at these joints and fall to the ground and appear like bones.
In G. gnemon there is no distinction into two types of branches. the branches are all of one type and are distinctly jointed.
Leaves: Leaves are also of two kinds (dimorphic) - scale and foliage leaves.
Scale leaves : Reduced and cup-like scale leaves are found only on long shoots.
Foliage leaves : dorsiventral or bifacial borne in an opposite and decussate manner on the dwarf shoots. The leaves are subsessile or shortly petiolate. The shape of the lamina may be oblong, elliptical or even lanceolaate. The lamina is large, leathery with reticulate venation (typical of angiosperm), exstipulate, has entire margins and acute apex. The foliage leaves bear axillary buds.
Anatomy
Root:
The root resembles a dicot root in its primary structure. The young root is almost circular in a transverse section and is differentiated into epidermis or epiblema, cortex and stele.
Epiblema : it is single layered, parenchymatous covered by cuticle. Some cells five out root hairs.
Cortex: it is multilayered and parenchymatous. The cells are filled with starch grains. Groups of sclerenchymatous cells occurs among the cortical cells.
A single layered endodermis consisting of cells filled with starch grains is quite distinct. Casparian strilps are seen in endodermis.
Stele : the pericycle is made up of 4-6 layers of parenchymatous cells.
The vascular region is radial, diarch or triarch and exarch. Primary xylem is made of tracheids and vessels and primary phloem is made of sieve tubes and parenchyma. Primary xylem losses its identity completely after secondary growth and becomes indistinguishable.
Stem:
A transverse section of the young stem is roughly circular outline and shows epidermis, cortex and stele.
Epidermis : it consists of a single layer of rectangular cells. The cells bear papillate outgrowths. The cuticle is thick. The outer walls of the epidermal cells are also very thick. The continuity of the epidermis is interrupted by the presence of sunken stomata.
Cortex: it consists of three distinct regions:
1. Outer Chlorenchymatous cortex : made up of 5-7 layers of polygonal or rounded cells that contain chloroplasts
2. Middle Parenchymatous cortex : consists of few layers of thin-walled cells. The cells are compactly arranged. Many fibrous cells are also present which mainly store food materials.
3. Inner Sclerenchymatous ring: consists of 2 to 5 layers of thick-walled cells.
Endodermis and Pericycle are not clearly demarcated.
Vascular Region: consists of a variable number ( 20 -24) oval vascular bundles that are arranged in the form of an interrupted ring. The bundles are conjoint, collateral, endarch and open.
The xylem consists of tracheids and vessels. Xylem fibres are absent. Phloem consists of sieve cells and phloem parenchyma.
The vascular bundles are seperated by broad parenchymatous regions. these are the medullary rays.
Pith: it occupies the centre of the stem. It is extensive and entirely parenchymatous.
Leaf:
It resembles to a dorsiventral dicot leaf. A transverse section of the leaf shows epidermis, mesophyll and vascualr region.
Epidermis: the leaf is dorsiventral and consists of distinct upper(dorsal) and lower(ventral) epidermis. The epidermal cells have undulate walls.
A thick cuticle covers the upper epidermis. Cuticle is comparatively thinner over the lower epidermis. the stomata are confined to the lower epidermis. Stomata may be syndetochelic (G. gnemon) or haplocheilic (G. ula)
Syndetochelic stomata : guards cells and subsidiary cells develop from the same epidermal mother cell.
Haplocheilic stomata : guards cells and subsidiary cells develop from different epidermal mother cells.
Mesophyll : consists of distinct pallisade and spongy parenchyma.
The pallisade parenchyma is composed of a single layer of elongated cells that are full of chloroplasts.
Spongy parenchyma consists of loosely arranged and lobed cells that contain chloroplasts and are thin-walled.Stellate sclereids with thick and lignified walls occus in the spongy tissue near the lower epidermis.
Scattered fibres and latex tubes are also found scattered around the midrib region.
Vascular Region: consists of an arc of vascular bundles. The bundles are conjoint, collateral, endarch.
The phloem lies towards the lower epidermis whereas xylem lies towards upper epidermis. The phloem consists of sieve cells and phloem parenchyma. The xylem consits of trahceids, vessels and xylem parenchyma.
Each vascular bundle is surrounded by a sheath of thick walled cells.
Reproduction:
The Gnetum is dioecious and bears male and female reproductive organs on seperate plants. The reproductive organs are organised into well-defined cones or strobili. These strobili are organised into well-defined inflorescences. Inflorescences are usually axillary in position, but terminal inflorescences are also found.
The conses arise in the axils of paired and decussate scale leaves. These leaves are fused at their base to form a boat shaped structure. They are also called bracts.
Male Strobilus and Male Flowers:
It consists of an elongated axis that is distinguished into nodes and internodes. The nodes of the axis bear scaly bracts arranged in whorls. These nodal bracts fuse together to form a cup-like structure called the cupule or the collar. So, every node of the axis bears a collar of fused bracts.
Each axis possess 10-25 collars.
Each collar bears above it 3-6 rings or whorls of male flowers. Each whorl possess a number (20-25) of male flowers. The male flowers in the whorls are arranged alternatively.
Above the whorls of male flowers there is one ring of abortive female flowers or ovules.
Structure and Develoment of Male Flower:
Each male flower is enclosed within a perianth that appears sheath-like. It consists of a stalk that bears two anthers at its apex. In G. gnemonoides the stalk bears only one anther lobe. Each anther has a single locule or a microsporangium. At maturity and just before dehiscence the stalk elongates and projects beyond the perianth.
The male flowers are interspersed with uniseriate and multicellular hair.
Each anther lobe contains a distinct epidermis. Below the epidermis of anther two archesporial cells distinguish. They divide and redivide to form many celled archesporium. The outermost layer of the archesporium divides by periclinal divisions to form an outer layer of parietal cells and an inner layer of sporogenous cells.
The parietal layer undergoes another periclinal division to form an external wall layer beneath the epidermis and an internal tapetal layer. The wall layer may become two layered by further division.
The sporogenous cells undergo a few divisions to form the microspore mother cells. They contain dense cytoplasmic contents and distinct nuclei.
At this time, the tapetal cells become conspicuous and large in size and have dense cytoplasmic contents. They become bi-nucleate.
The microspore mother cells undergo meiosis and form isobilateral, decussate or tetrahedral tetrads of haploid microspores. The microspores enlarge and become free after breaking the original microspore mother cell wall. The microspores are uni-nucleate.
Meanwhile the wall cells and tapetal cells undergo degeneration and ultimately they become disorganized.
The dehiscence of the anther takes place along a double row of small cells that extend from the tip of the anther downwards its base.
Female Strobilus :
Female strobilus or cone has more or less the same organization as that of male cone with a few differences. There is a single ring or row of four to ten ovules above each collar. Several uniseriate hairs are interspersed among the ovules. During younger stages of development all the ovules are of the same size. Later some ovules grow bigger while others reamin smaller and may fall down. The ovules may not develop in the uppermost collars.
Ovule or Female flower:
Each ovule consists of a central mass of cells called the nucellus. The nucellus is surrounded by three envelopes. The outermost envelope is orange red and flesh and is called as perianth. The middle is designated as the outer integument.
The innermost envelope is fused with nucellus in its lower part and is free above and is called the inner integument. The innermost envelope grows beyond the middle integument and forms a narrow and cylindrical tube called the micropylar canal or style.
A rudimentary pollen chamber is present at the nucellar apex. There is no nucellar beak in Gnetum.
The nucellus contains the female gametophyte. The cells of the nucellus lying below the female gametophyte divide to form a fan-shaped tissue made up of several rows of cells. This tissue is called as pavement tissue. This tissue is later absorbed.
Gametophyte:
Male Gametophyte:
Microspore or pollen gran is the first cell of the male gametophyte. It is spherical in shape when young. It is uni-nucleate and is enveloped by a thick and spiny exine and a thin intine. The microspore nucleus divides into two daughter nuclei – an antheridial (generative) nucleus and a tube nucleus. According to Thompson there is no prothallial cell in the male gametophyte of Gnetum.
The antheridial nucleus divides once again and forms a stalk nucleus and a body nucleus. At this three nuclei stage (stalk, body and tube nucleus) the pollen grains are released.
The pollen grains are carried by wind – anemophilous. The micropylar tube of the ovule secretes a drop of fluid to which pollen grains get attached. The liquid later evaporates and the pollen grains are withdrawn into the tube down to the pollen chamber.
The exine ruptues and intine grows into a pollen tube. The tube nucleus migrates first into the pollen tube. The stalk and body nucleus also migrates into the pollen tube. The stalk nucleus divides into two unequal male gametes.
Female gametophyte or Megagametophyte:
A distinct epidermal layer develops in the nucellus called the nucellar epidermis. A group of two or four archesporial cells develop below the epidermis. The archesporial cells divide periclinally to cut off outer primary parietal cells and inner sporogenous cells.
The nucellar epidermis and and the parietal cells undergo further periclinal and anticlinal divisions to form the massive nucellus and push the sporogenous cells deep into tis tissue. The sporogenous cells divide once or twice to from 8-16 sporogenous cells arranged in linear rows. They function directly as megaspore mother cells.
All of them may undergo meiosis to form tetrasporic embryo sac, i.e., the mother cell undergoes meiosis without wall formation so that 4 haploid nuclei lie free within the wall of the mother cells. All the four haploid nuclei participate in the development of the embryo sac.
Out of 8-16 megaspore mother cells formed, only one develops into an embryo sac and rest degenerate. In the functional megaspore mother cell, free nuclear divisions produce 256 to 1,500 nuclei that are peripherally arranged around a central vacuole. The embryosac assumes a spathulate form, i.e., broad above and narrow below. It has a big central vacuole and the free nuclei are arranged in the peripheral layer of cytoplasm.
Later, the nuclei lying towards the chalazal end get enclosed by walls to form distinct cells. The nuclei at the micropylar end remain free. So the gametophyte is partly cellular and partly free nuclear.
There are no archegonia in Gnetum. Some nuclei at the nicropylar end functions as eggs. They become bigger in size and can be distinguished from the other nuclei.
Fertilisation:
The pollen tube pierces through the membrane of the female gametophyte. The tip of the pollen tube bursts and releases the male cells, one of which enters the egg cell. The male and female nuclei fuse together to form the zygote.
Many zygotes are found in one female gametophyte.
Endosperm formation:
It has been observed that the lower part of the female gametophyte becomes cellular prior to the fertilization. After fertilization the upper part also becomes cellular so that whole of the gametophyte becomes cellular. Wall formation is by cleavage. This tissue which is formed is called the endosperm.
The cells of the endosperm become multinucleate and these nuclei later fuse with one another to form polyploidy cells. In some cells two nuclei fuse whereas in other three or even more nuclei fuse together. This is called gradate ploidy and nuclei vary from haploid to 12n in the same gametophyte.
Polyembryony:
Development of more than one embryo in a seed is called as polyembryony. In Gnetum polyembryony is found, because there are many zygotes formed. Each zygote forms many tubes, the tip of each tube develops into an embryo.
Economic importance of Gnetum
In Malaya and Indonesia the seeds of Gentum specially those of G. latifolium, G. gnemon and G. ula are eaten after roasting or cooking. Seed kernel are smashed, washed and dried in the sun and moulded into biscuits or cakes.
The young leaves and strobili of G.gnemon are cooked as vegetable. The seed or kernel oil of G. ula is used for illumination, edible purpose and also as a massage in rheumatism.
The bark of G. gnemon and G. latifloum yields fibre of high tensile strength. These are used for making ropes, fishing nets. The bark is also used as pulp in paper making. The plant of G. montanum is used as fish poison.
Resemblance of Gnetum with Angiosperms:
1. Leaves with reticulate venation.
2. Opposite and decussate phyllotaxy.
3. Xylem has vessels
4. Presence of perianth in flowers.
5. Male gametophyte without prothallial cell.
6. Archegonia are absent.
7. Tetrasporic female gametophyte development.
8. Embryo is dicotyledonous.
Stem:
A transverse section of the young stem is roughly circular outline and shows epidermis, cortex and stele.
Epidermis : it consists of a single layer of rectangular cells. The cells bear papillate outgrowths. The cuticle is thick. The outer walls of the epidermal cells are also very thick. The continuity of the epidermis is interrupted by the presence of sunken stomata.
Cortex: it consists of three distinct regions:
1. Outer Chlorenchymatous cortex : made up of 5-7 layers of polygonal or rounded cells that contain chloroplasts
2. Middle Parenchymatous cortex : consists of few layers of thin-walled cells. The cells are compactly arranged. Many fibrous cells are also present which mainly store food materials.
3. Inner Sclerenchymatous ring: consists of 2 to 5 layers of thick-walled cells.
Endodermis and Pericycle are not clearly demarcated.
Vascular Region: consists of a variable number ( 20 -24) oval vascular bundles that are arranged in the form of an interrupted ring. The bundles are conjoint, collateral, endarch and open.
The xylem consists of tracheids and vessels. Xylem fibres are absent. Phloem consists of sieve cells and phloem parenchyma.
The vascular bundles are seperated by broad parenchymatous regions. these are the medullary rays.
Pith: it occupies the centre of the stem. It is extensive and entirely parenchymatous.
Leaf:
It resembles to a dorsiventral dicot leaf. A transverse section of the leaf shows epidermis, mesophyll and vascualr region.
Epidermis: the leaf is dorsiventral and consists of distinct upper(dorsal) and lower(ventral) epidermis. The epidermal cells have undulate walls.
A thick cuticle covers the upper epidermis. Cuticle is comparatively thinner over the lower epidermis. the stomata are confined to the lower epidermis. Stomata may be syndetochelic (G. gnemon) or haplocheilic (G. ula)
Syndetochelic stomata : guards cells and subsidiary cells develop from the same epidermal mother cell.
Haplocheilic stomata : guards cells and subsidiary cells develop from different epidermal mother cells.
Mesophyll : consists of distinct pallisade and spongy parenchyma.
The pallisade parenchyma is composed of a single layer of elongated cells that are full of chloroplasts.
Spongy parenchyma consists of loosely arranged and lobed cells that contain chloroplasts and are thin-walled.Stellate sclereids with thick and lignified walls occus in the spongy tissue near the lower epidermis.
Scattered fibres and latex tubes are also found scattered around the midrib region.
Vascular Region: consists of an arc of vascular bundles. The bundles are conjoint, collateral, endarch.
The phloem lies towards the lower epidermis whereas xylem lies towards upper epidermis. The phloem consists of sieve cells and phloem parenchyma. The xylem consits of trahceids, vessels and xylem parenchyma.
Each vascular bundle is surrounded by a sheath of thick walled cells.
Reproduction:
The Gnetum is dioecious and bears male and female reproductive organs on seperate plants. The reproductive organs are organised into well-defined cones or strobili. These strobili are organised into well-defined inflorescences. Inflorescences are usually axillary in position, but terminal inflorescences are also found.
The conses arise in the axils of paired and decussate scale leaves. These leaves are fused at their base to form a boat shaped structure. They are also called bracts.
Male Strobilus and Male Flowers:
It consists of an elongated axis that is distinguished into nodes and internodes. The nodes of the axis bear scaly bracts arranged in whorls. These nodal bracts fuse together to form a cup-like structure called the cupule or the collar. So, every node of the axis bears a collar of fused bracts.
Each axis possess 10-25 collars.
Each collar bears above it 3-6 rings or whorls of male flowers. Each whorl possess a number (20-25) of male flowers. The male flowers in the whorls are arranged alternatively.
Above the whorls of male flowers there is one ring of abortive female flowers or ovules.
Structure and Develoment of Male Flower:
Each male flower is enclosed within a perianth that appears sheath-like. It consists of a stalk that bears two anthers at its apex. In G. gnemonoides the stalk bears only one anther lobe. Each anther has a single locule or a microsporangium. At maturity and just before dehiscence the stalk elongates and projects beyond the perianth.
The male flowers are interspersed with uniseriate and multicellular hair.
Each anther lobe contains a distinct epidermis. Below the epidermis of anther two archesporial cells distinguish. They divide and redivide to form many celled archesporium. The outermost layer of the archesporium divides by periclinal divisions to form an outer layer of parietal cells and an inner layer of sporogenous cells.
The parietal layer undergoes another periclinal division to form an external wall layer beneath the epidermis and an internal tapetal layer. The wall layer may become two layered by further division.
The sporogenous cells undergo a few divisions to form the microspore mother cells. They contain dense cytoplasmic contents and distinct nuclei.
At this time, the tapetal cells become conspicuous and large in size and have dense cytoplasmic contents. They become bi-nucleate.
The microspore mother cells undergo meiosis and form isobilateral, decussate or tetrahedral tetrads of haploid microspores. The microspores enlarge and become free after breaking the original microspore mother cell wall. The microspores are uni-nucleate.
Meanwhile the wall cells and tapetal cells undergo degeneration and ultimately they become disorganized.
The dehiscence of the anther takes place along a double row of small cells that extend from the tip of the anther downwards its base.
Female Strobilus :
Female strobilus or cone has more or less the same organization as that of male cone with a few differences. There is a single ring or row of four to ten ovules above each collar. Several uniseriate hairs are interspersed among the ovules. During younger stages of development all the ovules are of the same size. Later some ovules grow bigger while others reamin smaller and may fall down. The ovules may not develop in the uppermost collars.
Ovule or Female flower:
Each ovule consists of a central mass of cells called the nucellus. The nucellus is surrounded by three envelopes. The outermost envelope is orange red and flesh and is called as perianth. The middle is designated as the outer integument.
The innermost envelope is fused with nucellus in its lower part and is free above and is called the inner integument. The innermost envelope grows beyond the middle integument and forms a narrow and cylindrical tube called the micropylar canal or style.
A rudimentary pollen chamber is present at the nucellar apex. There is no nucellar beak in Gnetum.
The nucellus contains the female gametophyte. The cells of the nucellus lying below the female gametophyte divide to form a fan-shaped tissue made up of several rows of cells. This tissue is called as pavement tissue. This tissue is later absorbed.
Gametophyte:
Male Gametophyte:
Microspore or pollen gran is the first cell of the male gametophyte. It is spherical in shape when young. It is uni-nucleate and is enveloped by a thick and spiny exine and a thin intine. The microspore nucleus divides into two daughter nuclei – an antheridial (generative) nucleus and a tube nucleus. According to Thompson there is no prothallial cell in the male gametophyte of Gnetum.
The antheridial nucleus divides once again and forms a stalk nucleus and a body nucleus. At this three nuclei stage (stalk, body and tube nucleus) the pollen grains are released.
The pollen grains are carried by wind – anemophilous. The micropylar tube of the ovule secretes a drop of fluid to which pollen grains get attached. The liquid later evaporates and the pollen grains are withdrawn into the tube down to the pollen chamber.
The exine ruptues and intine grows into a pollen tube. The tube nucleus migrates first into the pollen tube. The stalk and body nucleus also migrates into the pollen tube. The stalk nucleus divides into two unequal male gametes.
Female gametophyte or Megagametophyte:
A distinct epidermal layer develops in the nucellus called the nucellar epidermis. A group of two or four archesporial cells develop below the epidermis. The archesporial cells divide periclinally to cut off outer primary parietal cells and inner sporogenous cells.
The nucellar epidermis and and the parietal cells undergo further periclinal and anticlinal divisions to form the massive nucellus and push the sporogenous cells deep into tis tissue. The sporogenous cells divide once or twice to from 8-16 sporogenous cells arranged in linear rows. They function directly as megaspore mother cells.
All of them may undergo meiosis to form tetrasporic embryo sac, i.e., the mother cell undergoes meiosis without wall formation so that 4 haploid nuclei lie free within the wall of the mother cells. All the four haploid nuclei participate in the development of the embryo sac.
Out of 8-16 megaspore mother cells formed, only one develops into an embryo sac and rest degenerate. In the functional megaspore mother cell, free nuclear divisions produce 256 to 1,500 nuclei that are peripherally arranged around a central vacuole. The embryosac assumes a spathulate form, i.e., broad above and narrow below. It has a big central vacuole and the free nuclei are arranged in the peripheral layer of cytoplasm.
Later, the nuclei lying towards the chalazal end get enclosed by walls to form distinct cells. The nuclei at the micropylar end remain free. So the gametophyte is partly cellular and partly free nuclear.
There are no archegonia in Gnetum. Some nuclei at the nicropylar end functions as eggs. They become bigger in size and can be distinguished from the other nuclei.
Fertilisation:
The pollen tube pierces through the membrane of the female gametophyte. The tip of the pollen tube bursts and releases the male cells, one of which enters the egg cell. The male and female nuclei fuse together to form the zygote.
Many zygotes are found in one female gametophyte.
Endosperm formation:
It has been observed that the lower part of the female gametophyte becomes cellular prior to the fertilization. After fertilization the upper part also becomes cellular so that whole of the gametophyte becomes cellular. Wall formation is by cleavage. This tissue which is formed is called the endosperm.
The cells of the endosperm become multinucleate and these nuclei later fuse with one another to form polyploidy cells. In some cells two nuclei fuse whereas in other three or even more nuclei fuse together. This is called gradate ploidy and nuclei vary from haploid to 12n in the same gametophyte.
Polyembryony:
Development of more than one embryo in a seed is called as polyembryony. In Gnetum polyembryony is found, because there are many zygotes formed. Each zygote forms many tubes, the tip of each tube develops into an embryo.
Economic importance of Gnetum
In Malaya and Indonesia the seeds of Gentum specially those of G. latifolium, G. gnemon and G. ula are eaten after roasting or cooking. Seed kernel are smashed, washed and dried in the sun and moulded into biscuits or cakes.
The young leaves and strobili of G.gnemon are cooked as vegetable. The seed or kernel oil of G. ula is used for illumination, edible purpose and also as a massage in rheumatism.
The bark of G. gnemon and G. latifloum yields fibre of high tensile strength. These are used for making ropes, fishing nets. The bark is also used as pulp in paper making. The plant of G. montanum is used as fish poison.
Resemblance of Gnetum with Angiosperms:
1. Leaves with reticulate venation.
2. Opposite and decussate phyllotaxy.
3. Xylem has vessels
4. Presence of perianth in flowers.
5. Male gametophyte without prothallial cell.
6. Archegonia are absent.
7. Tetrasporic female gametophyte development.
8. Embryo is dicotyledonous.
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