Welwitschia - Biology

Biology

The species grows readily from seed, which may be purchased from specialty seed dealers. The seeds undoubtedly display orthodox behaviour and naturally have an extremely low water concentration. Removal of the outer seed coverings enhances germination performance suggesting that the seeds may display non-deep physiological dormancy. The seed must be kept moist, but not immersed in water, for the first couple of weeks of cultivation. Seeds collected from the wild are often heavily infected or contaminated with spores of the mold Aspergillus niger var. phoenicis, which causes them to rot shortly after they germinate. The fungal inoculum infects the growing cones of W. mirabilis early during their development, and a sharp increase in infection occurs coincident with the appearance of the pollination drops, through which the fungal spores may gain access to the interior of the developing seed. Seeds in the wild may therefore be obliterated through fungal action even before they are fully developed. Seeds from botanical gardens, or other cultivated sources are much cleaner and less likely to rot. The fungicide tebuconazole may be useful in controlling limited A. niger seed infection.

After germination, the cotyledons grow to 25–35 mm in length, and are followed shortly afterward by the appearance of two permanent leaves. The growth of the true leaves outstrips that of the cotyledons after about four months, after which the growing tips of the two cotyledons die. The true leaves are produced at right angles to the cotyledons, and continue to grow in length throughout the entire life of the plant; they are the only leaves that the plant will bear, since the stem is acephalous; its apical meristem dies after the first two leaves are produced. Those two leaves grow continuously from a basal meristem in the plant's terminal groove. They eventually grow to a length of 2–4 m and usually become split, frayed and shredded into several well-separated strap-shaped sections by the distortions of the woody portions surrounding the apical slit, and also by wind and adventitious external injuries. The largest specimens may be no more than 1.5m tall above ground, but the circumference of the leaves at contact with the sand may exceed 8m.

Welwitschia has an elongated, but relatively shallow root system and a woody, fibrous unbranched stem. Bornman appears to have investigated the matter more intimately at first hand than most authors, and describes the root system as "rather shallow and simple, consisting of a tapering taproot with one or more non-tapering extensions, some pronounced lateral roots, and a network of delicate spongy roots." He offers the guideline that the roots extend to a depth roughly equal to the span of the living leaves from tip to tip.

The species is dioecious, with separate male and female plants. Fertilization, that is, the transfer of the pollen from the male to the female strobili, is carried out by insects. The insects responsible include flies and true bugs. The commonest of the true bugs attending Welwitschia is a member of the family Pyrrhocoridae: Odontopus sexpunctatus. The genus is also known as Probergrothius and the matter of which name is correct is still under discussion. Though much associated with Welwitschia mirabilis however, Odontopus sexpunctatus may not be a Welwitschia specialist; there are claims that it also occurs on other plants, such as Adansonia, though it is not clear how well-supported these claims might be. Infrequently, wasps and bees also play a role as pollinators of Welwitschia. At least some of the pollinators are attracted by "nectar" produced on both male and female strobili.

W. mirabilis has been classified as a CAM plant (crassulacean acid metabolism) after reconciliation of some initially contradictory and confusing data.

The age of the plants is difficult to assess, but they are very long-lived, living 1000 years or more. Some individuals may be more than 2000 years old.