Natalie SarracinoValentina Cipriani

02/27/11 P: 8

A gnetophyte is one of four gymnosperms. A gymnosperm is a seed bearing plant

whose seed is not enclosed in plant tissue. In simple terms a gnetophyte is a woody

plant that produces seeds. From here gnetophytes are classifies into

three genera which include, Ephedra, Gnetum, and Welwitschia.

Ephedra- Found in dry areas over wide areas, mostly in the northern hemisphere all the way from southwestern North America to north africa.

Can also be located in the southern hemisphere in south america.

Gnetum- Can be found in Asia and Africa near tropical rainforests.

Welwitschia- Located in the deserts of southwest Africa.

Ephedra comes in the form of shrubs and has small leaves and joined stems.

Contains 65 species. Joined at the bottom of the plant to form a

protective covering over the area from which the stem emerges.

Fertilization may occur just 10 hours after pollination.

Come in the form of vines, trees, and shrubs, and also contain leathery leaves.

Contain 30-35 species. Contain wood vessels in the xylem which is

one of the two transport tissues in the plant. Some species have tiered branches that are

spread apart and include wide, glossy, dark green leaves.

Grows from a short woody trunk with only two leaves.

Eventually these leaves continue to grow until they reach 2-4m.

The leaves are then split apart by wind and sand and torn into sections.

Are considered endangered because of their slow growth and because they become depleted by collectors.

Reproduce with seeds that are exposed. The reproductive structure are contained in cones. Most species are dioecious meaning that one separate plant produces either pollenproducing or seed producing cones. Gnetophytes plant their cones meters away. Just before pollination occurs some cells from the ovule fall off and a

sticky solution falls through an opening. A pollination drop is formed at the tip of the shell which is very high in sugar and attracts insects that pollinate the plant along with other factors such as wind.

Gnetophyta is often unrecognizable from angiosperms due to their angiosperm like leaves (in having reticulate veination).

However, what is the most advanced is their reproductive structure, which is currently being argued over in plant systematics as to whether or not it is a flower.

Gnetophytes have played an important role in the formation of phylogenetic hypotheses.

The issue is whether the Gnetophyta are the sister group of angiosperms, or whether they are sister to, or nested within, other extant gymnosperms.

From the early twentieth century, the anthophyte hypothesis was the prevailing explanation for seed plant evolution, based on shared morphological characters between the gnetophytes and angiosperms .

In this hypothesis, the gnetophytes, along with the extinct order Bennettitales, are sister to the angiosperms, forming the “anthophytes”. Some morphological characters that were suggested to unite the anthophytes include vessels in wood, net-veined leaves (only in Gnetum ), lignin chemistry, the layering of cells in the apical meristem, pollen and megaspore features (including thin megaspore wall), short cambial initials, and lignin syringal groups.

Various genetic studies have rejected the anthophyte hypothesis because they have suggested that the gnetophytes and angiosperms have independently derived characters, including flower-like reproductive structures and tracheid vessel elements, that appear shared but are actually the result of parallel evolution.

In the gnetifer hypothesis, the gnetophytes are sister to the conifers, and the gymnosperms are a monophyletic group, sister to the angiosperms.

The gnetifer hypothesis first formally emerged in the mid-twentieth century, when vessel elements in the gnetophytes were interpreted as being derived from tracheids with circular bordered pits, as in conifers. It did not gain strong support, however, until the emergence of molecular data in the late 1990s.

The gnepine hypothesis is a modification of the gnetifer hypothesis, and suggests that the gnetophytes belong within the conifers as a sister group to the Pinaceae .

According to this hypothesis, the conifers as currently defined are not a monophyletic group, in contrast with molecular findings that support its monophyly.

Some partitions of the genetic data suggest that the gnetophytes are sister to all of the other extant seed plant groups.

However, there is no morphological evidence nor evidence in the fossil record to support the gnetophyte-sister hypotheses.

Knowledge of fossils of the gnetophytes has increased greatly since the 1980s.

There are fossils from the Permian, the Triassic, and the Jurassic which may belong to the gnetophytes, but this is uncertain.

The fossil record is richer starting in the early Cretaceous, with fossils of plants as well as seeds and pollen which can be clearly classified to the gnetophytes.