An Afternoon at Yale

Marsh Botanical Garden cactus bed.

 

 

 

 Last weekend, the Connecticut Cactus and Succulent Society met at Yale University's Marsh Botanical Garden. The greenhouse complex at Yale includes a large modern structure with houses for botanical research, teaching and public displays, plus a couple of small legacy greenhouses that are mainly used for overflow space. One of the newer glass houses includes an open area with chairs and tables--used during the week for classes--where the CCSS held their meeting.

A pair of mature or nearly mature Welwitschia mirabilis plants at Marsh Botanical Garden.

 The cactus club was lucky enough to have a presentation by Dr. Michael J. Donoghue of Yale's Department of Ecology and Evolutionary Biology. Michael Donoghue is one of the best known plant evolutionary biologists active today. His talk for the CCSS covered Welwitschia, the peculiar xerophytic gymnosperm (cone-bearing seed plant) from arid northern Namibia and southern Angola. The fascinating lecture ranged from the history of Welwitschia's introduction to botanical science, with independent discoveries in 1861 by the Austrian medical doctor Friedrich Welwitsch as well as the artist Thomas Baines, to the latest findings from molecular phylogenetics, and finds of fossil Welwitschia-like pollen and cones.

Yearling Welwitschia seedling from the UConn teaching collections.

One point of interest from the talk, appropriate for the Darwin Day (February 12) season, concerned the evolutionary relationships of Welwitschia. The plant has long been known to be part of a small group of gymnosperms called the Gnetophyta, which also includes the genera Gnetum (broad-leaved tropical trees and lianas) and Ephedra (joint-stemmed shrubs of temperate deserts, the source of the stimulant ephedrine). The Gnetophytes as a whole were of somewhat uncertain affinities; an earlier idea was that they were the closest living relatives of the Angiosperms (flowering plants), based on details of their wood anatomy and pollination/fertilization process, as well as reproductive structures that resemble modified bisexual flowers. More recent evidence, primarily from DNA sequencing but also from the reinterpretation of anatomy and reproductive biology, indicates that Welwitschia and its allies are actually more closely related to the conifers (pines, cedars and their relatives).

Michael Donoghue at the February CCSS meeting.

Welwitschia Propagation

Female Welwitschia in cultivation at the University of Connecticut, mid-July. Note the pollination drops of sugary liquid at the tips of filaments protruding from between the cone scales.

Welwitschia mirabilis is a distant relative of the conifers, native to the Namib desert in Namibia and Angola. Welwitschia is notable for a number of reasons, most obviously its fat, stumpy trunk and the single pair of strap-like leaves that last for as long as the plant survives, which is thought to be up to 1000 years or possibly more. The trunk of a Welwitschia never produces branches (aside from ephemeral reproductive shoots) or offsets, and the only way to propagate the plants is by seed.

Welwitschia matures at around 10 years old under my conditions, although if given a more generous root run and warmer temperatures they can grow considerably more quickly. Adult plants are either male or female. From my observations, male plants seem to be more common than females, but that's based on a small sample size, and I haven't run across any surveys of sex ratios in the wild. In both cultivation and their southern hemisphere native range, Welwitschia cones are initiated in the spring and become fully formed and receptive in mid-summer.

Male Welwitschia with pollen-producing cones. Male cones are smaller that the females, only a little larger in diameter than a pencil.

Welwitschia seems to be pollinated by insects and not by wind, which is somewhat unusual for a cone-bearing plant. During the early afternoon the female cones secrete a drop of nectar from whisker-like extensions of their ovules. The male cones bear little whorls of pollen-producing microsporangia, and at the center of each whorl is a nectar gland, which may itself be a modified, vestigial ovule. The cones of cultivated plants in Connecticut attract a variety of insects collecting nectar and pollen, including flies, wasps, bumblebees and yellow jackets. In Namibia, various flies (including the common housefly, which also shows up on my greenhouse plants), bees and wasps have been observed transferring pollen from male to female plants. Neither wind-dispersed pollen nor the Welwitschia Bug (Probergrothius sexpunctatis), both of which were once suspected of being important to Welwitschia reproduction, actually seem to be involved in pollination of plants in the field (Wetschnig and Depisch, 1999).

A large wasp visiting male cones on a cultivated Welwitschia.

Successfully pollinated female cones ripen in autumn, about two months after pollination. Ripe cones shatter into a pile of papery scales and winged seeds, which seem to be adapted to wind dispersal.

Female cones in September releasing winged seeds.

I've had good luck sowing the seed right after it was shed, in September or October, in a sunny, warm spot, in a pot of well-drained soil mix for desert plants. I mostly use biodegradable fiber pots, which can be sunk into the soil of a larger pot when the seedling needs more room, minimizing root disturbance. The seed germinates well within a few weeks in moist soil. The young seedlings are very susceptible to damping off, for some reason; once they make it past their first few months they seem to become resistant to such problems. I've tried the usual cultural remedies for damping off--strong sun, allowing the soil surface to dry as much as possible--without much success, and will probably resort to the use of a fungicide the next time I sow a batch of Welwitschia.

Welwitschia seedling at a few days old. The cotyledons (seedling leaves) start out a bronzy-yellow color, and take some time to turn green.

Welwitschia seedling at age six months, with its first and only pair of true leaves emerging at right angles to the cotyledons.

Reference: Wetschnig, W. and  B. Depisch. 1999. Pollination biology of Welwitschia mirabilis Hook. f. (Welwitschiaceae, Gnetopsida). Phyton 39: 167-183.

Welwitschia seeds!

The Welwitschia mirabilis plants at UConn produced abundant fully-formed cones for the first time, this past summer. On warm afternoons in midsummer, the mature female cones produced "pollination drops," or droplets of nectar at the end of hair-like extensions of the ovules. The male cones have nectaries, as well, which probably attract insect pollinators in the wild in Namibia.

I placed pollen from the males into the pollination drops of female cones for several days running back in July, and two months later the seeds are starting to mature. Some of the seeds are very light and thin, and are probably duds, but quite a few look like viable Welwitschia seed. I may plant a few now to see what happens, though it probably isn't the best time of year.

Female cones with pollination drops, back in midsummer.

Welwitschia Cones

Welwitschia mirabilis at the University of Connecticut.

Welwitschia mirabilis is a gymnosperm (cone bearing, non-flowering seed plant) endemic to the Namib Desert in southwestern Africa. Welwitschia is in a category of its own in the plant kingdom, morphologically speaking: seedlings produce exactly two foliage leaves before the shoot apex aborts. The resulting stubby trunk with pair of strap-like leaves can live for centuries, gradually expanding by growing from the region where the leaves are attached.

There is an extensive mythology surrounding the cultivation of Welwitschia, but the plants aren’t as difficult as one might be led to believe. They don’t actually need to be grown in tall, skinny drainpipes, and may in fact benefit from planting in a wide container (or in a ground bed in a greenhouse), which allows room for an extensive network of surface roots to develop. It is possible to transplant them, though Welwitschia roots are a bit on the delicate side. They can also grow fairly quickly: the large mature plants at the University of Connecticut in the photos are only about 12 years old. For the past several summers, these plants have produced cones.


Welwitschia plants are either male or female (i.e., they are dioecious). So far here at UConn, we only have had fully formed cones on male plants like the one in the photo above, but the production of seed should be possible, eventually, as more of our plants reach maturity.

Certain aspects of the reproductive biology of Welwitschia and its relatives in the plant order Gnetales are similar to reproduction in flowering plants, and for a time Welwitschia and the flowering plants were considered to be fairly closely related. More recent information on the evolutionary biology of the vegetable kingdom has pretty well sunk this idea, though, placing Welwitschia and friends much closer to pines and other conifers. Likely fossil relatives of Welwitschia, with similar leaves and reproductive structures, are known from North and South America. Some of the fossil species were apparently tree-like, with branches.