The gnetophytes are reduced to three far-flung and remarkably different genera (each genus in its own order).  They have very few vegetative synapomorphies.  Like the flowering plants, they have vessels in the wood.  Also, they have dicot-like leaves that are decussate.  These features seem to point to a connection with the flowering plants.  

The nature of the ovule and the apparent occurrence of double fertilization in Ephedra caused Friedman (1990a, 1990b, 1992, 1994) to consider the gnetophytes as a sister group to the flowering plants.  Tudge (2000) follows this view.  However, molecular taxonomies of the seed-bearing plants paint a very different picture.  Chaw et al (1999) and Bowe et al. (2000) both compared sequences from the nucleus, the mitochondrion, and the chloroplast.  They consistently point to the Pinaceae as the sister group to the gnetophytes.  If interpreted strictly, the gnetophytes would be a group of conifers.

Friedman and Floyd (2001) discuss the problem of the two phylogenies and the relative importance of morphological and molecular evidence.  Because of the uncertainty, I follow the classical view of Bold et al. (1987).  I offer it as a tentative taxonomy of 1 classes with 3 orders.

I. PHYLUM CHARACTERISTICS

A. Structure

Habit: The gnetophytes are seed plants with decussate leaves. They tend to be dioecious with both staminate and ovulate strobili being compound.

Pollen: Pollen walls are variable. The microgametophyte has 1 or 2 prothallial cells, a stalk cell, 2 nonflagellate sperms and a tube cell.

Microstrobilus: Compound strobilus, bract-like appendages surround the microsporangia (pollen sacs).

Seeds: Ovules with extra integuments (or reduced bracts) around the nucellus. The micropyle projects as a long tube. Archaegonia develop at the micropylar end of the megagametophyte. There is evidence for double fertilization. The embryo is dicotyledonous.

Megastrobilus: Compound.

Stems: Wood contains vessels.

Leaves: Decussate (opposite). The leaves of Gnetum resemble those of dicots.

Roots: Like many of the conifers.

B. Ecology: Ephedra and Welwitschia are adapted to arid conditions and occur in North America and South-western Africa, respectively. Gnetum is a vine or shrub which inhabits tropical forests. They have a fossil record which dates to the Permian.

II. TAXONOMY: As I have defined this phylum, the GNETOPHYTA has a single class, GNETOPSIDA, with a three orders.

ORDER EPHEDRALES

These plants are highly branched, green-stemmed shrubs with small leaves in whorles or opposite. Mostly dioecious; strobili solitary, paired or grouped, terminal or on branches arising from the axils of bracts; male strobilus with 1 stamen (These are grouped into compound strobili).

example: Ephedra.

ORDER GNETALES

Usually vines (rarely shrubs or trees); with opposite, net-veined leaves. Compound strobili spike-like with strobili in the axils of united scale-like bracts; male strobilus with 1 stamen.

example: Gnetum.

ORDER WELWITCHIALES

Plants with short stem and large tap root; 2 broad, continuously growing, strap-like, opposite leaves with parallel veins. Compound strobili cone-like and growing from the axils of bracts.; male strobilus with 6 stamens and rudimentary ovule.

example: Welwitchia.

revised: 04/15/2003