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Welwitschia And Ephedra
Remarkable Genera Of Gymnosperms
One of the truly remarkable members of the Kingdom Plantae belongs to the Division Gnetophyta. Its scientific name is Welwitschia mirabilis, and it only grows in a remote region of southwestern Africa in the Namib and Mossamedes Deserts. Welwitschias are extraordinary plants and certainly rank among the top ten list of "most bizarre plants on earth." Old, mature plants have a short, woody trunk or crown (caudex) that protrudes above the ground. Below the ground, the caudex extends into a large taproot. The woody trunk (stem) gives rise to two, wide, strap-shaped leaves that may reach two meters in length. Like grass leaves, their meristematic region is at the base so they continually grow even though their tips get worn off by abrasion. In fact, the leaves lie on the ground and as they flap about in the wind they become split and frayed. Welwitschia plants are dioecious, with pollen-bearing and seed-bearing cones (strobili) produced on separate male and female plants.
According to K.S. Stern (Introductory Plant Biology, 1997), welwitschias have CAM photosynthesis. CAM (Crassulacean Acid Metabolism) photosynthesis is also found in cacti and succulents, including the Crassula Family (Crassulaceae). During the hot daylight hours their stomata are tightly closed; however they still carry on vital photosynthesis as carbon dioxide gas is converted into simple sugars. During the cooler hours of darkness their stomata are open and CO2 enters the leaf cells where it combines with PEP (phosphoenol pyruvate) to form 4-carbon organic acids (malic and isocitric acids). The 4-carbon acids are stored in the vacuoles of photosynthetic cells in the leaf. During the daylight hours the 4-carbon acids break down releasing CO2 for the dark reactions (Calvin cycle) of photosynthesis inside the stroma of chloroplasts. The CO2 is converted into glucose through a series of complicated reactions involving ATP (adenosine triphosphate) and NADPH2 (nicotinamide adenine dinucleotide phosphate), the latter two compounds which were synthesized during the light reactions of daylight in the grana of chloroplasts. The adaptive advantage of CAM photosynthesis is that plants in arid regions can keep their stomata closed during the daytime, thereby reducing water loss from the leaves through transpiration; however, they can still carry on photosynthesis with a reserve supply of CO2 that was trapped during the hours of darkness when the stomata were open.
According to George Lindsay (Pacific Discovery Vol. 35, 1982), welwitschias also absorb moisture in the form of dense fog that flows over the Namib Desert. Although the desert receives little or no rain, the fog drip is apparently sufficient to sustain these remarkable plants that grow nowhere else on earth.
Ephedra: Another Remarkable Gnetophyte
Other members of the Gnetophyta include Gnetum, a genus of broad-leafed, climbing, tropical vines, and Ephedra, drought-resistant shrubs with jointed stems and whorls of minute scale-like leaves in 2's or 3's at each joint (node). They are often called joint firs or Mormon tea in North America, referring to their jointed stems which can be boiled for a tea. One very unusual characteristic of the Gnetophyta is that many members possess vessels in their xylem tissue, an angiosperm trait that is not found in most gymnosperms. In addition, species of Ephedra have double fertilization, where two sperm are involved in the fertilization process. Double fertilization was once thought to be a strictly angiosperm characteristic. Some older references have suggested that the Gnetophyta may represent a "missing link" in the evolution of flowering plants, but others say that vessels and double fertilization are examples of parallel evolution. Considering all their amazing similarities, it seems quite plausible that the Gnetophyta and angiosperms may have had a common ancestor.
The chinese species Ephedra sinica was the original source of the alkaloid ephedrine, a common decongestant in popular allergy and hay fever remedies. Since ephedrine has a chemical structure similar to epinephrine (adrenalin), it works like a powerful cardiac stimulant that may cause cardiac arrest in infants and heart patients. New synthetic drugs based on the ephedrine/epinephrine ring structure are now marketed as effective and safer bronchodilators. Pseudoephedrine, an isomer of ephedrine, also occurs in species of Ephedra, and may be produced synthetically. Compared to ephedrine, it causes fewer heart symptoms such as palpitation, but is equally effective as a bronchodilator. It is used in over-the-counter drugs such as Sudafed©. E. sinica and other species are also marketed under the name of "ma-huang," a popular herbal stimulant and decongestant.
Like several other potent alkaloids, such as the alkaloid mescaline found in the peyote cactus, ephedrine contains a nitrogen atom that is not contained in a carbon ring. Mescaline has a chemical structure similar to the brain neurotransmitter dopamine. It is also structurally similar to the neurohormone norepinephrine (noradrenalin) and to the stimulant amphetamine. In the peyote cactus, mescaline is formed in a complex pathway from the amino acid tyrosine. A similar pathway in humans produces epinephrine (adrenalin) and its demethylated precursor norepinephrine from tyrosine. Dopamine and its precursor L-dopa are also derived from a tyrosine pathway.
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