In this article we will discuss about Gnetales. After reading this article you will learn about: 1. Taxonomic Arrangements of Gnetales 2. General Characteristics of Gnetales 3. Classification.
Taxonomic Arrangements of Gnetales:
Foster and Gifford (1959) described Gnetales as “a small group of gymnosperm-like plants” while Maheshwari and Vasil (1961) Mentioned that ” the order Gnetales, formerly included three genera, Ephedra, Welwitschia and Gnetum which were considered to be highest evolved among the gymnosperms and believed to show an approach to the angiosperms”.
But mainly due to the presence of naked ovules and also because of the absence of true sty le and stigma in Ephedra, Welwitschia and Gnetum, these members can only be treated under gymnosperms and not under angiosperms.
Formerly, these genera (Ephedra, Welwitschia and Gnetum) were considered to be the only living representatives of family Gnetaceae of order Gnetales. But Sporne (1965) and others divided Gnetales into three families, each having a single genus, viz. Gnetaceae (Gnetum), Welwitschiaceae (Welwitschia) and Ephedraceae (Ephedra).
Due to the heterogenous nature of this order several workers advocated strongly for a split of the Gnetales into three independent orders viz. Ephedrales, Welwitschiales and Gnetales. Each order contains only one family and only one genus.
Chamberlain (1935) opined about this controversy very clearly and mentioned, “It seems to be a matter of taste whether the Gnetales should be put into one family, with three genera, or into three families, with one genus in each. All agree that there are only three genera, Ephedra, Welwitschia and Gnetum”.
In the present text, the classification proposed by Sporne (1965) for Gnetales has been followed. Delevoryas (1962) opined that except for the fossil remains of Ephedra and Welwitschia- like pollens, the fossil-history of Gnetales is almost unknown.
General Characteristics of Gnetales:
Gnetales, believed by some botanists to be the ancestors of flowering plants or angiosperms, are the highly evolved members of gymnosperms and show following characteristics:
1. These are woody plants, of which some species are trees (Gnetum gnemon), many are lianes or shrubs and a few. are stumpy turnip-like (e.g. Welwitschia mirabilis).
2. Leaves are simple elliptical or strap-shaped or sometimes reduced to minute scales. They are generally opposite or whorled.
3. Vessels are present in the secondary wood.
4. ‘Flowers’ are unisexual, usually dioecious and only rarely monoecious as in some species of Gnetum.
5. ‘Flowers’ are arranged in compound strobili or ‘inflorescences’.
6. The male flowers are surrounded by a perianth. Each male flower contains an antherophore with one to eight synangia.
7. A single erect orthotropous ovule is present in each female flower.
8. Nucellus of the ovule remains surrounded by two or three envelopes.
9. The micropyle of each ovule remains projected in the form of a long bristle-like tube.
10. At the time of fertilization the pollen tube contains two male nuclei.
11. A unicellular primary suspensor is present in the embryo.
12. Two cotyledons are present in the embryo.
Classification of Gnetales:
Sporne (1965) divided Gnetales into following three unigeneric families:
(i) Gnetaceae: Gnetum
(ii) Ephedraceae: Ephedra
(iii) Welwitschiaceae: Welwitschia
The plant body is sporophytic and resembles remarkably with a dicotyledonous plant, specially when it is not in the fruiting stage. Most of the species are lianes or climbers with twining stem, except a few which are shrubs or trees, e.g., Gnetum gnemon and G. costatum.
Ephedra, the lone genus of family Ephedraceae of order Gnetales, is represented by some 40 species. It is distributed in mountainous or rocky places, or in sandy desert regions throughout the world. Most of the species are shrubs, and a few are lianes.
Ephedra triandra, a South American species, usually reaches up to a diameter of 30 cm. and a height of several metres and appears like a tree. Ephedra is known for ‘ephedrin’ a well-known drug, obtained from its various species of Asiatic origin. Khoshoo (1961) states that haploid chromosome number in Ephedra is 7 and 14.
Welwitschia bainesii (= W. mirabilis) is the only species of genus Welwitschia, the sole representative of family Welwitschiaceae of Gnetales. This is the most strange or bizarre of all gnetalean plants showing a very restricted distribution in a narrow coastal belt of about 1000 Km long in south-west Africa.
The specific name (Welwitschia mirabilis) of this African genus “is very appropriate because the adult sporophyte is unlike that of any known plant on the earth”. According to the recent nomenclatural changes, however, the correct and valid name is Welwitschia bainesii. The common name “desert octopus” is often riven to this genus because of its peculiar habit.