A Rare Plant Treat

July 6, 2010

I was in Seattle recently and enjoyed a visit to the greenhouses at the University of Washington. I was delighted to find a collection of Welwitschia plants there, one of which was forming cones. This bizarre plant of the Namibian Desert belongs to the gnetophyte lineage of seed plants. It is one of three wildly different branches of this clade.

Welwitschia plants in the UW greenhouse have tall pots to accommodate their long tap roots.

The gnetophytes are one of five extant lineages of seed plants. The others are the cycads, the ginkgo, conifers, and the angiosperms, aka flowering plants. The angiosperms greatly outnumber the others, but that has become the case in the Cenozoic Era. In the Mesozoic Era, there were many other types of gnetophytes, but presently the three lineages are the genus Ephedra, genus Gnetum, and genus Welwitschia.

Although there are about 60 species of Ephedra and about 30 of Gnetum, there is only one species of Welwitschia, W. mirabilis. Ephedra species are native to arid environments in North and South America, Africa, and Eurasia. They have greatly reduced leaves and look like a bush of tough, narrow stems. Gnetum is the odd one on environment. It grows in the tropics of Indonesia, the Philippines, and parts of Africa. Most of its members are vines, although a few are trees or shrubs. Their leaves look very much like those of angiosperms.

Ephedra, a relative of Welwitschia, growing in Utah. The leaves of this bush are reduced to tiny scales.

Gnetophytes were once thought to be closely related to flowering plants, but the DNA told a different story. They are now considered to be more closely related to the conifers. Characteristics that all gnetophytes share include opposite leaves and having staminate and ovulate reproductive structures on separate plants, i.e. they are dioecious.

Welwitschia grows only two leaves, not counting its two seed leaves, in its whole life time. The leaves arise from a woody stem that has a sort of upside down cone shape. The plants are estimated to live about 1000 years, judging from their growth rates and the length of the leaves on wild plants. The leaves grow from their bases and often split so that it isn’t easy to see that there are only two.

The stem of Welwitschia is woody and has a roughly inverse cone-shape.

The two leaves of this Welwitschia plant are split into several strips. The top of its woody stem is visible.

One of the plants in the UW greenhouse had formed cones. It is a male plant, and there are stamens showing near the bottom bracts. The branch that bears the cones grows from the top of the woody stem, near the base of the leaves. I had read about their cones, but never seen them “in person” before, so finding them was a real treat. 

These are staminate cones of Welwitschia. Their stalk grows from the top of the woody stem.