7.3: Gnetophytes

Gnetophytes represent an anatomically and genetically difficult group to classify. They have several traits in common with angiosperms, such as vessel elements in the xylem, double fertilization, and a covering over their seeds. Even their leaves are angiosperm-like, with netted venation. However, these traits are convergently evolved, meaning that angiosperms and gnetophytes each evolved these traits separately. Genetically, recent studies have placed the gnetophytes as a sister group to the Pinaceae (pine family) within the conifers. This would mean that pines, firs, and spruces are more closely related to strange gnetophytes like Ephedra than they are to other conifers like redwoods, cedars, and Pacific yew. However, the true nature of this evolutionary relationship remains murky and contentious.

Characteristics of Gnetophytes

• Angiosperm-like features: vessel elements, double fertilization, fruit-like ovule coverings
• Dioecious. Female plants have covered ovules, while male plants have pollen cones.
• Leaves xerophytic with opposite arrangement
• Primarily insect pollinated; brightly colored seeds are dispersed by birds

Notable Gnetophytes

Welwitschia mirabilis

This strange plant grows in the desert of Namibia. It has two large leaves that grow from a basal meristem.

Figure $\PageIndex{1}$: In this small Welwitschia mirabilis, the two large leaves can be seen extending from the central, flat stem. The leaves grow from a basal meristem. The tips of the leaves are ragged, as these are the oldest parts. The leaves are shiny and the setting is dry, indicating their xerophytic nature. In the center, where the two leaves meet, branching stalks terminate in megastrobili. Photo by Waldier, CC-BY-NC.

Figure $\PageIndex{2}$: A larger, older Welwitschia mirabilis. The leaves have split many times and have piled atop themselves. Photo by Alex Dreyer, CC-BY-NC.

Figure $\PageIndex{3}$: Megastrobili of the female Welwitschia mirabilis. These magastrobili are composed of tightly overlapping megasporophylls. Seeds are produced within these structures. Note the short, woody stem at the base of the plant. Photo by Judd Kirkel, CC-BY-NC.

Figure $\PageIndex{4}$: A closer view of a megastrobilus. Note the opposite arrangement of the megasporophylls. Photo by Christoph Moning, CC-BY-NC.

Figure $\PageIndex{5}$: Male Welwitschia mirabilis plants produce microstrobili. These are smaller, a bit more red-to-pink, and thinner than the megastrobili. Photo by Luis Querido, CC-BY-NC.

Figure $\PageIndex{6}$: Microstrobili of a male Welwitschia mirabilis. These cones are composed of many overlapping microsporophylls. Pollen is produced within microsporangia held on the microsporophylls. Photo by Peter Weston, CC-BY-NC.

Ephedra spp.

Figure $\PageIndex{7}$: A thin stem of Ephedra aspera showing opposite leaf arrangement of two scale-like leaves. Photo by Fred Melgert / Carla Hoegen, CC-BY-NC.

Figure $\PageIndex{8}$: Megastrobili of a female Ephedra distachya. These structures are swollen and red, making them appear fruit-like. Seeds are produced within these structures. Photo by Ramazan_Murtazaliev, CC-BY-NC.

Figure $\PageIndex{9}$: Microstrobili of a male Ephedra californica. These small structures look like inflorescenses with anthers emerging. From between the microsporophylls, branching structures emerge, topped with yellow pollen. Photo by Fred Melgert / Carla Hoegen, CC-BY-NC.