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Gnetales is composed of three genera, Ephedra, Gnetum, and Welwitschia, all of which differ significantly in terms of morphological, ecological, and molecular properties. The phylogenetic position of the group is unknown. Previously related to angiosperms, Gnetales is now associated with conifers. Ephedra and Gnetum are trees, shrubs, vines, and highly-branched, crossed, or swirling vines. Welwitschia is unique in that it has a compact, unbranched stem and two permanent leaves that remain on the plant through its presence. The pronunciation or phonetic spelling of Gnetales is gne-tales.

Gnetales characteristics 

  • The members of this division show arrangement flowers in compound strobili, a feature common to angiosperms and gymnosperms.
  • Ovules and pods are present around the anterophore.
  • The outer skin has micropyramidal ridges called “tubules” that produce pollinated droplets.
  • Blood vessels are present in the secondary xylem. 

Gnetales

Based on their morphology, Gnetales is a diverse group of three families–Ephedra, Gnetum, and Welwitschia. It consists of approximately 70 to 75 species collectively in all three families. Although initially thought to be linked with angiosperms, it is now categorised with conifers. Ephedra and Gnetum are trees, shrubs, or vines, with many branches. Welwitschia has a compact, unbranched stem with two permanent leaves. Plants belonging to this division are usually dioecious. 

Ephedra Family

  • The majority of Ephedra species are perennial and dioecious shrubs. 
  • Some are 4m high climbers or 2m high small trees. The thin, spear-shaped leaves are arranged laterally or spirally on the nodes. 
  • When fully grown, the leaves are 2 to 15 (40) mm long, but when vegetative buds do not grow, they become non-functional.
  • The apex of each blade is exposed, but the basal region is integrated into the sheath and the degree of fusion is a species-specific characteristic. 
  • Ephedra species are found in deserts, semi-deserts, desert meadows, and seasonally arid environments such as evergreen or deciduous forests and subtropical thorn bushes in both Old and New World Mediterranean climate zones.
  • Ephedra anatomical features include the presence of blood vessels that increase conduction efficiency compared to the tracheal system of non-magnetic gymnosperms.
  • Ephedra’s female cone (ovulatory strobili) consists of bracts arranged in crossed or triphasic phyllotaxis (as a vertical variant), with each distal pair/swirl wrapped in a seed coat. They surround 1–3 sown seeds. 
  • The male cone consists of two lateral bracts with 2–3 sterile bracts at the base, followed by 2–8 fertile bracts, with two median bracts each. It surrounds the stem-shaped bracts.

Gnetum (Gnetophytes) 

  • Gnetum have wide pinnate leaves and their pinnate veins resemble the simple leaves of many dicotyledonous dicotyledons.
  • The tree blood vessels of Gnetophytes are much wider than Maou in both ryanoid and tree species, but Gnetophytes are significantly smaller than ryanoid flowering plants because they are partly found in shaded undergrowth. 
  • Gnetum ovulation cone has a bulging collar and contains a single large seed wrapped in yellow or red fleshy or corky shells. Staminated cones are often several centimetres long, with elongated internodes separating the nodes containing the microspore sac.
  • Gnetum pollen grains are spherical, fine (1220 mm), have no openings and microexcites, and have thin, granular infrastructure lids and layered endexins. 

Welwitschia

  • Welwitschia is a strange seed plant found only in the Namib Desert of Namibia and Angola.
  • It has two strap-shaped leaves that grow endlessly from the basal meristem and fray at the edges over time. 
  • Straight roots are several meters long and located on the ridges of short, unbranched trees. 
  • Welwitschia has typical desert adaptations, such as a thin ship with a simple perforated plate and trachea. 
  • The female Welwitschia cone consists of 90–100 oval units. Bracts surround each reproductive unit, and the integument extends beyond the bracts into micropyral tubes.
  • In contrast to Ephedra and Gnetophytes pollen, Welwitschia pollen is oval, large (51 mm), monosulfate, and polyprelicate with psilate preca. 

 

Fertilisation in Different Families of Gnetales

 

  • In Ephedratrifurca and E. nevadensis from the Ephedra family, the second sperm nucleus fuses with the cytoplasmic ventral canal nucleus of the oocyte. In the Gnetum family, the Gnetum-free nucleus gathers around the pollen tube. 
  • Welwitschia mirabilis from the Welwitschia family does not have a second fertilisation event. 
  • Ephedramega gametophytes still contain archegonia and pollen chambers (like other gymnosperms), but female Gnetum and Welwitschia gametophytes lack archegonia and develop through the nucleus and downward. It has a tubular extension (Protalia tube) that accesses the growing pollen tube.

Conclusion

Gnetales comprises three morphologically diverse genera (Ephedra, Gnetum, and Welwitschia). All three genera show different morphological and anatomical characteristics. They also differ in terms of reproductive parts and fertilisation. As the morphological characteristics of seeds differ in these three genera, the seed dispersal mode also differs.