Morphology, anatomy and reproduction of Gnetum (Gnetales):

Gnetum is a gymnospermic plant. They are similar with dicot plants. 40 types of species found all over the world. 5 species of Gnetum are found in India. The plants found in Gnetum are woody and climber type, also some plants are shrubs and one or two three types and one species found in Gnetum, which is parasite. They are found in tropical and humid region of the world. Tropical means those type of hot environments. Here in India, it is found in Western Ghats near Khandala, Nilgiris, Andhra Pradesh, Orissa, Assam, Tamil Nadu, Sikkim, Andaman and Nicobar Islands.

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Gnetum : Morphology, anatomy and reproduction

Species of Gnetum which are found in India are-

Gnetumula

Gnetum contractum

Gnetum gnemon

Gnetum mentanum

Gnetum latifalium

These 5 species are found in India.

Fig: Gnetum

2 types of branches found in plants of Gnetum.

(1) Limited growth of branches

( (2) Unlimited growth of branches

The plants are look like dicot species but they are developed species. Plants are divided in node and internode; small petioles are found too.

In unlimited growth of branches, we found scale leaves.

Anatomy of stem:

* Rough circular structure is found. The outermost layer is thick cuticle layer. Which covered single layer of epidermis.

* Sunken stomata also found in upper epidermis. Sunken stomata show zero fatigue character. Less transpiration occurs in Sunken stomata. Which helps to reduce the extra loss of water.

* Cortex layer is found inside of epidermis. It is divided in 3 parts-

(a) Outer cortex (5-7 layers), made of chlorenchymatous cells. Chlorophyll is found inside of it. Inter cellular space found.

(b) Middle cortex, made of parenchymatous cells. Too many fibre cells found in middle cortex, in which food materials are stored.

In Gnetum, Xylems are present in vessel. Tracheid’s are also present, made of lignin. Cellulose is present in xylem parenchyma.

Secondary growth in Stem:

* Periderm is thin.

* Cambium divided into 2 parts-

(a) Secondary xylem

(b) Secondary phloem

* Cambium formed in the outermost layer of phloem.

* One side is thin and one side is woody (wide), called eccentric.

* Ring of sclerotic cells found.

* Uniseriate and multiseriate medullary rays found.

* Starch is present inside of medullary rays.

Internal structure of roots:

* Single layer epidermis, covered by thick cuticle layer.

* Cortex region found, where so many starch are present.

* Casparian strips found in endodermis.

* 4, 6 layers of pericycle found.

* Upper epidermis and inner epidermis found.

* Protoxylem and metaxylem found.

Anatomy of leaf:

* Upper epidermis and lower epidermis covered by thick cuticle layer.

* Xylem is present in the upper structure of the leaf and phloem is present in the lower structure of the leaf.

* In middle portion cambium is present.

* Xylem is covered by sclerenchyma cells.

* Sunken stomata are present.

Reproduction of Gnetum:

Gnetum is a dioecious plant, but bisexual strobili also occur rarely. Gnetum reproduces sexually. Gnetum is dioecious and both the male and female strobili are compound. The inflorescence composed of connate bracts at the base and a series of cup-like bracts called cupules or collars that are superposed one above the other.

The strobilus becomes compact at young stage. The collars of a young strobilus appear to be continuous. At maturity, the axis becomes elongated and the collars get separated.

Male strobilus:

The male strobilus is compound and has a long slender axis, bearing 10-25 whorl of bracts (collars). About 12-25 male flowers are arranged in 3-6 rings above each collar. A single ring of 7-15 imperfect female flowers is present just above the male flowers.

Male flower:

A male flower has two unilocular anthers enclosed in a small sheathing perianth. Its stalk is called antherophore, which elongates at maturity. Consequently, the anther emerges beyond the collar through a slit in the perianth. The number of anthers in a male flower may also vary.

Female strobilus:

The female strobilus is very much similar to that of the male strobilus in the young stage. Like male strobilus, the female strobilus consists of an axis bearing several whorls of collars one above the other. A ring of 4 -10 female flowers is present above each collar. The male flowers are not found in the female strobilus.

Microsporangium:

Two groups of hypodermal archesporial cells form multicellular archesporium

by repeated divisions.

* The outermost layer of the archesporial cells divides to form the primary

parietal and the sporogenous cells.

* The parietal layer, by periclinal division, gives rise to a wall layer towards

outside and tapetum towards inside.

* The tapetal cells become densely cytoplasmic and are normally binucleate

(rarely multinucleate). The nuclei may fuse and become polyploidy. The tapetal

cells start degenerating after meiosis.

* The sporogenous cells divide and increase in number, the last cell generation of

which differentiates into microspore mother cells(2n).

* Broad cytoplasmic channels interconnect microspore mother cells or meiocytes

forming a syncytium.− As the mother cells enter meiosis, it is surrounded by a thick layer

of callose.

* Meiosis results in the formation of decussate, tetrahedral or

isobilateral tetrads of microspores(n) still embedded in the callose

cover.

* The callose covering is soon absorbed releasing individual haploid

microspores.

* The microspore/pollen wall has an outer thick exine with minute

spines and an inner thin intine.

Megasporangium:

* The integumented megasporangium is called ovule.

* The ovule is stalked in G. ula, but may be subsessile or even sessile.

* The ovules are orthotropous, crassinucellate and are protected by

three envelopes.

* The outer envelope is thick and succulent at maturity. It is considered

to be the perianth.

* The middle and the inner envelopes are actually the integuments. The

middle envelop is called the outer integument which is anatomically

similar to the outer envelope.

* The inner envelope, i.e., the inner integument, elongates for beyond the

apical cleft of the perianth and forms a long micropylar tube or the so called ‘style’. The inner integument is free from the nucellus except at the chalazal end. Two sets of vascular bundles are formed which the outer integument and the other to the inner integument.

Fertilization:

* Both the male nuclei are discharged into the egg cell. Of these one enlarges

slightly and moves towards the egg nucleus.

* The other male nucleus, which is non-functional, remains in the periphery of

cytoplasm and degenerates. Only the nucleus of the male gamete enters the egg;

the sheath is left outside.

Gnetum (Gnetales) : Morphology : Anatomy : Reproduction