Morphology, anatomy, reproduction of Gnetales
Morphology, anatomy, reproduction of Gnetales:-
Introduction:- Gnetales is an order of the gymnospermic seed plants. The members of the order Gnetales are characterized by the presence of compound staminate strobili, opposite or whorled leaves. The vessels are present in the secondary wood and absence of resin canals.
Distribution:- The members of Gnetales are widely distributed in Western Africa. In India, they are found in the hills regions. Examples: Gnetum ula, G. montanum, G. gnemon, G. latifolium etc.
Morphology:-
External morphology:-
Stem:- The stem is woody, branched in Gnetum. The branches are of two types- dwarf shoots (short branches of limited growth) and long shoots (long branches of unlimited growth). Nodes and internodes are present on the dwarf shoots.
Leaves:- In Gnetum, leaves are simple, exstipulate, and shortly petioled. The leaf blade (lamina) is oval-shaped, large, and entire with pinnately reticulate venation. Leaves (about 9 and 10) are arranged in opposite decussate pairs on each branch.
Root:- In Gnetales, roots are normal tap roots type and they are branched.
Internal morphology:-
Stem:- In Gnetum, three types (external, middle, and internal) cortex is present. The outer cortex is made up of chlorenchymatous tissues. The middle cortex consists of parenchymatous tissues and the internal cortex contains sclerenchymatous tissues. The primary vascular bundles vary in number from 20 to 24, they are arranged in the form of a ring. Vascular bundles are collateral, open and endarch. Vessels are large and pith is present.
Root:- In one layer of epiblema is made up of root hairs. Cortex is a simple parenchymatous. Starch is present in Gnetum roots. Endodermis and pericycle are present. Xylem is diarch. Secondary growth is normal.
Leaves:- In Gnetum, the upper epidermis of the leaf is thickly cuticularised. On the lower epidermis, a large number of stomata are present. Vascular bundles are collateral and endarch. Fibers and latex tubes are present.
Reproduction of Gnetales:- All members of Gnetales are mostly dioecious. Flowers are unisexual. In Gnetum, male and female flowers are borne in clusters on male and female plants respectively forming cones or strobili.
Male strobilus:-
> In Gnetum, the male cones are compact and slender axis-like structures, which are up to 6 cm in length. Each cone is generally a penicle that is either solitary and axillary or fascicled at the apex. The axis of the strobilus bears opposite and connate bracts. Bracts are arranged one above the other to form collars or cupules. There are 6-25 cupules are present, each cupule, staminate flowers are arranged in several definite rings, usually 3 to 6 in number.
> Each mature microsporophyll consists of a stalk bearing two unilocular microsporangia (anthers). The stalk is invested at the base by a perianth.
> In Gnetum ula, 2-4 anthers are there which are formed from microsporangial initial cells. It divides periclinally and anticlinally to form a primary cell wall, tapetal cell, and sporogenous cells. Sporogenous cells later differentiate into spore mother cells. Microspore mother cell forms microspores (pollen grains) through meiosis.
> The wingless pollen grains are liberated by the longitudinal dehiscence of the anthers.
Female strobilus:-
> In Gnetum, female flowers bear 4-10 ovules. Some ovules enlarge and they have three layers of integuments. Only inner integuments form micropyle. While middle and outer integument forms the outer cover. The whole structure is covered by perianth.
> A single hypodermal archesporial cell is differentiated in the nucellus of the ovule. The nucellus is bitegmic and is surrounded by 2 envelopes. Archesporial cell forms parietal cell and megaspore mother cell which forms 4 haploid megaspores by meiosis.
> The lowermost megaspore cell is the functional megaspore, while the other three facing the micropyle degenerate.
Gametophytic generation Gnetales:-
Male gametophyte:-
> Microspore (pollen grain) is the first cell of the male gametophyte. The outer exine is thick-walled and the inner intine is thin-walled.
> In Gnetum, a microspore consists of a prothallial cell, a generative cell, and a tube nucleus. The generative cell divides within the pollen tube into two non-motile naked male gametes.
Female gametophyte:-
> The functional megaspore is the first cell of the female gametophyte.
> In Gnetum, the megaspore mother cell takes part in the development of the gametophyte. It’s containing four megaspore nuclei and forms a female gametophyte (embryo sac). The four megaspores divide continuously and form thousands of female nuclei. The embryo sac is tetrasporic, in which 256-1500 nuclei are found.
> At this time cell wall is formed from the chalazal end. A few free nuclei in this part become larger and differentiate as eggs. Archegonia are not formed in Gnetum female gametophyte.
Fertilization:- In Gnetum, the pollen tube comes in contact with the female gametophyte and the male gamete enters the embryo sac. One gamete attaches to the female nucleus and forms a zygote. While the second gamete attaches to the other nucleus and as a result double fertilization occurs. At this time, the endosperm is formed due to wall formation at the micropylar end.
Embryo development:- In Gnetum, the zygote divides and forms two cells. Both the cells enlarge and form tubes. Each tube divides and forms suspensor and embryonal cells.
Seed germination:- Seeds are endospermic. Process of germination of the seed is hypogeal in Gnetum.