Gnetophytes are considered as the evolutionary link between
gymnosperms and angiosperms. Today only there genera represents the Gnetophytes
such as Welwitshchia, Gnetum and Ephedra. These there genera differ greatly from one
another in form, structure and function. From those three genera, Welwitschia and Gnetum show evolutionary closer relationship to angiosperm. Ephedra
is evolutionary more closely to gymnosperms.
. Gnetophytes are the only gymnosperms who have
vessels in the secondary xylem as in angiosperm. Vessels are relatively
efficient water and minerals conducting cell type. The size of gametophyte
generation is further reduced. Sporophyte generation becomes further dominant. Their sperm is non-motile. They possess
separate female and male strobilus. Their seeds are comprised of two integuments.
Inner integument exudes pollen droplets which draws the pollen grain to ovule.
Outer integument is not a true one. Pollen tube of Gnetophytes is siphonogamous.
Their pollen tube conveys their sperm directly to the archegonia of Ephedra or female gametophyte in Welwitschia
and Gnetum.
Ephedra is the most primitive form of
Gnetophytes. It inhabits in arid zones and sandy places. It is a highly branched shrub. It has a jointed stem with whorls of tiny
scale like leaves at each node. Depending
on the species some are monoecious while some are dioecious. Their pollen grains have sterile cells. They
possess archegonia. Its female gametophyte has monosporic development. Free nuclei division without cell wall
accompanying is present in female gametophyte formation. Their female cone is
resembled as brightly colored berrylike fleshy structures. Their male cone is appeared as spike like
clusters.
Welwitschia possess a ring shape stem.
It anchors to the soil aid in huge taproot with surface roots. Its leathery
leaves rise up from the stem and continue the growth. It bears reproductive
structures which rise at the base of the stem. They produce separate female and
male plants. Sterile cells are absent in
the pollen grains of Welwitschia. Its female gametophyte has tertasporic
origin. Archegonia are entirely absent. It has free nuclei period in embryogenesis.
Pollen cones and ovulate cones are produced in separate plants. They are borne at the end of the branched
systems of plants. Insects are attracted to pollinate the pollen grains by
secreting nectar. Only one seed is produced per a cone. Seeds bears fly like
structures in order to disperse them aid in wind.
Although Gnetum and Welwitschia share common characters,
only Gnetum has been proposed as the
probable ancestor of Angiosperms because distribution of Welwitschia is
restricted to desserts. But Gnetum is
widely distributed all over the world.
Gnetum successfully grow in humid and
tropical regions. Most species of Gnetum
are in the form of climber. Some species are in the form of shrubs and trees.
They possess complex microsporangiate and megasporangiate cones. They are
similar in inflorescences in the angiosperms. They bear board leaves which are
similar in form, structure and venation to those found in Eudicots of
angiosperms. They secret
nectar which attracts insects for their pollination. Their seeds are enclosed
in a false fruit. It helps to disperse
their seed aid in animals. Archegonia are entirely absent. Their pollen grains
are lack of sterile cells. They only carry generative cells which produce two
sperm cells and tube cells which produce large tube cells. Their female
gametophyte is formed as a result of tertasporic development. In both Welwitschia and Gnetum, female gametophyte formation is associated with free nuclei
division without accompanying cell wall accompanying formation.
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