The Ephedra, the Gnetum and the Welwitschia. Genus
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MODULE I UNIT 4 THE PHYLUM GNETOPHYTA This phylum consists of three genera; the Ephedra, the Gnetum and the Welwitschia. Genus: Ephedra There are about 100 known species of gnetophytes. They are unique among the gymnosperms in having vessels in the xylem. More than half of the gnetophytes are species of joint firs in the genus Ephedra. These shrubby plants inhabit drier regions of southwestern North America. Their tiny leaves are produced in twos and threes at a node and turn brown soon after they appear. The stems and branches, which are often whorled, are slightly ribbed; they are photosynthetic when they are young (Fig. 14). The leaves are little more than scales; therefore, most photosynthesis is conducted by the green stem. Before pollination, the ovules of Ephedra produce a small tubular extension resembling the neck of a miniature bottle extending into the air. Sticky fluid oozes out of this extension, which constitutes the micropyle, and airborne pollen catches in the fluid. Male and female strobili may be produced on the same plant or on different ones, depending on the species. Figure 14: Joint fir (Ephedra) 1 Economic Importance of Ephedra Joint fir is the source of the drug ephedrine, an alkaloid that constricts swollen blood vessels and also a mild stimulant. An overdose can cause death. It is also used as a tea in Chinese herbal medicine. Genus: Gnetum The members of Gnetum occur in the tropics of Africa, South America, and South Asia. Most are vine-like, with broad leaves similar to those of flowering plants (Fig.15). It is a tropical genus of 30 species, which include lianas (climbing vines), shrubs, or trees. The leaves are nearly indistinguishable from those of broad-leafed flowering plants, and unless reproductive structures are present, Gnetum is difficult to identify. Figure 15: A climbing Gnetum africanum (Okazi leaf or Afang leaf or Eru) 2 Economic Importance of Gnetum Fresh leaves of Gnetum africanum are widely used as a vegetable. In Nigeria, the plant is used for the treatment of piles, high blood pressure, used against enlarged spleen, sore throat and as a purgative. In some African counties, the leaves are eaten raw to treat nausea, chewed to mitigate the effect of drunkenness and to ease childbirth and menstrual pain. They are also used to treat boils and fungal infections on the fingers. The supple stem is sometimes used as rope. Apart from the leaves, the tuber and seeds can be cooked and eaten as food. The pedicles can be cut, crushed and mixed with soap, which is used for washing the hair in other to stimulate hair growth. The vine is believed to contain a high level of iodine Traditionalists use the leaf for preparing medicines used in treating children suffering frommeasles. Genus: Welwitschia This genus has only one species, which is confined to the temperate Namib and Mossamedes deserts of Southwestern Africa. In these areas, the average annual rainfall is only 2.5 cm, in some year, it does not rain at all. The plants carry on Crassulecean Acid Metabolism (CAM) photosynthesis, and their stomata are open at night. The plants obtain most of their water from dew and condensate from fog that rolls in off the ocean at night. In Welwitschia plants, the stem rises only a short distance above the grounds surface and is in the form of a large shallow cup that tapers at the base into a long taproot. The plants may live to be 100 years old and at maturity have crusty, barklike covering on the surface of the stem cup. The stems may be more than 1 m in diameter (Fig. 16). Welwitschia is dioecious, with both male and female strobili being produced on axes that emerge from the axils of the leaves. This makes the strobili appear to be growing around the rim of the stem cup. 3 Figure 16: Welwitschia mirabilis plants Economic Importance of Gnetophytes Ephedra shrub species produce the compound, ephedrine, which is medicinally used as a decongestant. RECOMMENDED TEXTBOOK 1. Vasishta P.C., Sinha A.K. and Kumar A. Botany for Degree Students Gymnosperms. Publisher: S Chand & Co Ltd 2. Stern K.R., Bidlack J.E., and Jansky S. (2010) Stern's Introductory Plant Biology. McGraw- Hill Education Publisher - Europe 12th edition 4 Differences between Gymnosperms and Angiosperms 1. The gymnosperms are not herbaceous, whereas majority of angiosperms are herbaceous 2. Gymnosperms are mostly perennial with few exceptions, whereas angiosperms may be annual, biennial or perennial 3. Pollination and dispersal is anemophilous among gymnosperms, whereas anemophily, entomophylly, hydrophylly, etc. are all prevalent among angiosperms 4. Ovules are naked among gymnosperms, but protected by carpels in angiosperms 5. Gymnosperms possess archegonia, while angiosperms lack these structures 6. The endosperm is formed before fertilization in gymnosperms, but after fertilization in angiosperms 7. Gymnosperms are unisexual and may be monoeciuos or dioecious, whereas angiosperms may be bisexual as well as unisexual, monoeciuos or dioecious. 8. No double fertilization occurs in gymnosperms, which is a distinct feature of angiospermic plants. 9. Pollen grains fall/settle directly on the micropyle among gymnosperms, whereas they fall on the stigma for onward transfer to the ovary and ovules in angiosperms 10. The endosperm is haploid among gymnosperms, but triploid among angiosperms. 5 .