UNIT 4 DIVISION GYMNOSPERMA Learning objectives At the end of the class, students should be able to:- Classify the gymnosperms that you have studied. - identify the general features of gymnosperms. - Identify features of species studied - discuss the life cycle of gymnosperms. Classification of Gymnosperms • Four major phyla of gymnosperms exist;• Phylum 1: Coniferophyta, Order coniferales. Example Pinus, Conifers. • Phylum 2: Cycadophyta, Order Cycadales Example Cycads. • Phylum 3: Gnetophyta, order Gnetales Example Gnetum. • Phylum 4: Ginktophyta, order Ginkgoales Example ginkgo. Gymnosperms are flowerless plants with naked seeds. The word "naked" means the seeds are not enclosed in an ovary. General characteristics • The gymnosperms are woody, evergreen, perennial trees. • They are the tallest and oldest of all the trees on the planet (Earth). • Gymnosperms were the first plants to have seeds. • They are mostly found in temperate region but cycads and gnetales thrive in warm dry climate. • They are heterosporous. • Sporophylls are spirally arranged to form strobili. • Ovules are naked, pollination are anemophilous • Double fertilization is absent. • Siphonogamous occur in few species of gymnosperms. Endosperm is formed before fertilization. • Embryo development is meroblastic i.e develops from a small part of the zygote. Alternation of generation is present. • Sporophyte are differentiated into root, stem and leaves. • They are inconspicuous and endosporic i.e develop within the spores. • Leaves are dimorphic, plants posses taproot system. • Possess xerophytic traits i.e presence of thick bark, thick hypodermis, thick cuticle, scale leaves, sunken stomata and needle-like leaves. • Gymnosperms are a group of plants which produce seeds that are not contained within an ovary or fruit. • The seeds are open to the air and are directly fertilized by pollination. • “Gymnosperm”, from the Greek, gymnos, “naked” and sperma, “seed”, develop their seeds on the surface of scales and leaves, which often grow to form cone or stalk shapes, contrasting in characteristics from the angiosperms, flowering plants which enclose their seeds within an ovary. • The gymnosperms consist of • the conifers, • the cycads, the gnetophytes; and the sole extant species of the Gingkophyta division, the Gingko biloba. • Conifers • Conifers, in the division Pinophyta or Coniferophyta, are the most numerous of the gymnosperms; woody and with vascular tissue, these are cone bearing trees and shrubs. • Conifers can be found growing in all parts of the world, although they most notably dominate the boreal forests of the northern hemisphere. Many are adapted to cold climatic conditions, with downward facing branches, which help to shed snow, and specific biochemical properties that provide resistance to freezing. • Examples of conifers include pines, yews, redwoods, spruces, firs and cedars. • The conifer forests of the world cover huge areas of land and provide the largest terrestrial carbon sink. Conifers are also valued economically; their softwood is used for the production of paper and timber, they are used to cultivate pine nuts, and the berries of the juniper bush are used to flavor gin. • Gnetophytes • The gnetophyta are distinguishable within the gymnosperms because they have vessel elements, a system of channels mostly found in the angiosperms, which transport water within the plant. • Cycads • The appearance of the cycads (division Cycadophyta) typically constitutes a single, stout, cylindrical, woody trunk and a crown of large, hard and stiff, evergreen compound leaves, which grow directly from the trunk in a rosette formation. The cycads are dioecious, meaning that each individual plant is either male or female. • The cycads are partly xerophyte, which means they are adapted to survive in areas with very little water. • The cycads were much more numerous in the past than today, peaking in the age of the cycads'- the Jurassic period. • There are only three extant families within the cycads today: the Cycadaceae, Stangeriaceae and Zamiaceae. Ginkgo • The closest relatives to the cycads, Gingko is a genus of gymnosperm of which Gingko biloba is the sole extant species. • Gingkos are large, slender, shade-intolerant trees, growing up to 160ft with distinctive fan-shaped leaves. They are deep rooted and resistant to damage from wind and snow. They are also resistant to disease and insect damage, owing to an exceptionally large genome, which enables antibacterial and chemical defense mechanisms. Diagram of Ginkgo biloba Gingko first appears within the fossil record in the Permian period, 270 million years ago, and the Gingko biloba remains largely unchanged today, earning it a classification as a ‘living fossil’. Ginkgo biloba is cultivated globally for use in various traditional medicines and as a food source. Gymnosperm life cycle • Gymnosperms reproduce with an alteration of generations, meaning their reproductive cycle has both haploid and diploid phases. • As in all other vascular plants, gymnosperms have a sporophyte dominant life cycle (the sporophyte is the diploid multicellular stage, which comprises of the body of the plant, i.e., a leafy tree). The gametophyte phase is relatively short, and sees gametes produced on the reproductive organs, which are usually cones. • The female ovulate cone, or megasporophyll, bear the megasporangium, diploid cells, which undergo meiosis to produce four haploid spores. • Of these haploid spores, only one survives as the megaspore. • The surviving megaspore then, through mitosis, develops into the female gametophyte. • Within the female gametophyte there is an egg and an endosperm mother cell; the endosperm mother cell creates endosperm, which eventually ‘feeds’ the embryo. • The male cone, called the microsporophyll, is a small, spongy, leaf-like organ which bears the microsporangium. • The microsporangium contains the male microspores, which undergo meiosis to generate the male gametophyte, pollen • The pollen grain contains the pollen tube cell and the generative cell (which contains two sperms, although one dies). • When the pollen reaches the egg cell, either by wind or by animal through pollination, the pollen grain releases the single sperm. • The nuclei of the female and the male then fuse to create a diploid zygote. The endosperm, a haploid nutritional tissue, is released from the endosperm mother cell, and surrounds the zygote to form a seed. The seeds appear as the ‘scales’, which are visible on the cones of gymnosperms; these scales are then dispersed to form a new sapling sporophyte, which grows into a mature sporophyte, and the cycle continues. • Female cones are larger and woodier than male cones and are usually positioned higher up on the tree, • although in dioecious species, such as the cycads, the male and female cones are borne on separate tree
