The gnetophytes are a strange group of plants. They include approximately 70 species of gymnosperms that each exhibit a number of flowering characteristics which have led people to believe they may be the predecessors to the flowering angiosperm plants.
The three different families of Gnetophyta each have a single genus – Gnetum, Ephedra and Welwitschia. They can grow as trees, shrubs or vines and are found in a range of areas around the world although many species have limited ranges.
Diversity and taxonomy
There is currently around 70 living species of gnetophytes on Earth and all but one belong to the genera Gnetum and Ephedra. There have been many more species that have gone extinct over the past 100 million years.
The gnetophytes sit within the class Pinopsida which includes all gymnosperm plants and belong to the sub-class Gnetidae. The sub-class Gnetidae has three families: Gnetaceae, Ephedraceae and Welwitschiaceae; and each family has a single genus: Gnetum, Ephedra and Welwitschia.
The gnetophytes grow in a number of forms including as trees, shrubs and lianes. The Ephedra plant species grow commonly as shrubs with many long, slender stems that bear infrequent, small leaves. Plants from the genus Gnetum grow mostly and woody vines known as lianes but some species also grow as trees and shrubs.
The single species from the genus Welwitschia has one of the most unusual growth forms in the plant Kingdom. It has two giant, succulent leaves that can grow up to 9 m long and no trunk but a large stem that remains only slightly above ground level.
Distribution of gnetophyta
Gnetophyta plants are distributed around much of the world mostly in tropical and temperate climates. The genus Gnetum has a tropical distribution as is found in Southeast Asia, northern South America, the South Pacific and western Africa.
Ephedra plants are typically found in temperate climates in all continents excluding Antarctica and Australia. The Welwitschia lives only along the south-west coast of Africa in harsh desert environments.
Evolution of gnetophyta
The gnetophytes potentially evolved way back in the Permian or Triassic Periods more than 200 million years ago. However, the earliest definite fossils that have been found and assigned to the gnetophytes are from the Cretaceous Period between 145-66 million years ago.
Within the gnetophytes, Welwitschiacae and Gnetaceae are more closely related to each other than they are to the family Ephedraceae. These plant species are thought to have hardly changed in the past 100 million years and be relics of an ancient flora in which they were far more competitive and common.
The evolution of the gnetophytes is placed alongside other gymnosperm groups and they are thought to be more closely related to the conifers than any plants although they do exhibit a number of angiosperm traits. It is possible that angiosperms evolved from gnetophytes.
The 28 species from the genus Gnetum consist of a few trees and shrubs accompanied by a number of woody vine species. They can be difficult to identify because the distinguishable characteristics are often hidden up in the canopy and only the woody stem of the vine can be seen from ground level.
They bear evergreen broad-leaves with a net-like pattern of veins which gives an overall design that is very similar to angiosperm leaves. They also have a similar vascular tissue to angiosperms. Genetic analysis has however shown that they are gymnosperms and are more closely related to the pines.
The male and female reproductive structure resemble flowers and are grown on separate trees making the Gnetum plants dioecious. They are insect pollinated and have scented cones which attract animals to eat their seeds.
The Ephedra genus includes around 40 species of mostly small, shrubby plants that live in dry habitats. They have heavily reduced leaves and complete the most of their photosynthesizing via their green stems.
They are mostly wind pollinated and different species are wind or animal dispersed. Many species have seeds with a fleshy outer layer, much like Ginkgo, which attract animals in to disperse seeds. The fleshy fruit-like seeds are typically red or orange.
The Ephedra plants are well-known medicinal plants for its decongestant properties. They have been cultivated in China for over 5000 years.
Welwitschia mirabilis is the sole remaining species from the Welwitsciaceae family and it is arguably the weirdest plant in the world. It has no trunk but a large stem that barely grows above ground and two extremely large, succulent leaves that can grow up to 9 m long and 2 m wide. As the leaves grow they split into multiple strips.
The plant is found only in the Namibian desert and grows on dry, gravelly ground. It has an impressive ability to withstand extreme temperature fluctuations. Night time temperatures can only drop to as low as 7°C and day time temps can rise to as high as 50°C.
Yearly rainfall is also a constant stress. A typical year in the Namibian desert brings 1-10 cm of rain. In order to collect water in such a dry habitat, Welwitschia mirabilis absorbs water from fog and dew, and has a long tap root which can access water deep beneath the ground’s surface. Despite such tough living conditions, Welwitschia mirabilis can live for up to 1500 years.
Welwitschia mirabilis plants are either male or female. Male cones are salmon orange in color; female cones are larger and a grey/green color. Both male and female cones produce nectar to attract insects for pollination. Their seeds are white and require several days of heavy rain before they are able to germinate.
Last edited: 16 January 2016