CORE COURSE II PLANT BIODIVERSITY II ( III Unit- GYMNOSPERMS ) Unit III : A general account of the characteristic features of Gymnosperms. Origin of Gymnosperms. Classification of Gymnosperms (Sporne, 1965). General structure and interrelationships of Pteridospermales, Bennetittales, Pentoxylales and Cordaitales. 1. General account on characteristic features of Gymnosperms. The word “Gymnosperm” comes from the Greek words “gymnos”(naked) and “sperma”(seed), hence known as “Naked seeds.” Gymnosperms are the seed- producing plants, but unlike angiosperms, they produce seeds without fruits. These plants develop on the surface of scales or leaves, or at the end of stalks forming a cone-like structure. Gymnosperms belong to kingdom ‘Plantae‘ and sub-kingdom ‘Embryophyta’. The fossil evidence suggested that they originated during the Paleozoic era, about 390 million years ago. Basically, gymnosperms are plants in which the ovules are not enclosed within the ovary wall, unlike the angiosperms. It remains exposed before and after fertilisation, and before developing into a seed. The stem of gymnosperms can be branched or unbranched. The thick cuticle, needle-like leaves, and sunken stomata reduce the rate of water loss in these plants. The family of gymnosperms consist of conifers, the cycads, the gnetophytes, and the species of Gynkgophyta division and Ginkgo biloba. Following are the important characteristics of gymnosperms: 1. Habit: Gymnosperms are a small group of seed plants which are represented by only 900 living species.The living gymnosperms are woody, evergreen (except Larix Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 1 and a Taxodium) perennials grow as trees or shrubs. Tallest trees are Sequoia sempervirens (366ft) and S. gigantia (342ft). 2. Occurrence: Gymnosperms are more ancient than the angiosperms. They formed dominant vegetation on earth some 200 million years back in mesozoic era. Today they are dominant only in cold areas, where instead of rain; snow is the source of water. The living members are founding in colder regions of earth where snow (not rain) is the source of water. Only the members of cycadales and gnetales thrive in warm dry climate. 3. Plant body: The dominant plant body is sporophyte (2n) which may be dioecious or monoecious. Gametophytes are inconspicuous and endosporic i.e. develop with the spores.Sporophyte differentiated into — root, stem and leaves. 4. Root system: Tap root system is exarch and diarch to polyarch. Besides tap root, coralloid roots (in cycads) and mycorrhizal root (in coniferals) present. 5. Stem: Erect, generally branched (Cycas is un-branched) 6. Leaves: Dimorphic i.e. 2 types, foliage and scale leaves. 7. Flowers: Flowers are absent. Two types of sporophylls, microsporophyll’s and megasporophylls are usually aggregated to form distinct cones or strobili, pollen cones (male cones) and seed cones (female cones) respectively. Seeds are not formed inside a fruit. They are naked. 8. Xerophytic Traits: Gymnosperms are xerophytes in nature due to presence of thick bark, thick hypodermis, thick cuticle, scales leaves, sunken stomata, transfusion tissue, etc. Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 2 In some cases leaves modified into needle-like, scale like or small leathery. These are the adaptations to combat water stress in air and colder regions. 9. Xylem: The xylem composed of xylem parenchyma and tracheids with bordered pits. Vessels are absent (exceptin Gnetales). 10. Phloem: The phloem composed of sieve cells and phloem parenchyma but companion cells absent. 11. Heterospory: The gymnosperms are heterosporous, means 2 types of spores produced i.e. haploid microspores and megaspores. Microspores produced within micro- sporangia while megaspores produce within megasporangia (nucellus) of ovules. Both types of sporangia are formed on special leaf-like structures called sporophylls (microsporophylls and megasporophylls). 12. Cones or Strobili: Sporophylls are spirally arranged along an axis to form compact cone or strobili i.e. male or pollen cones and female or seed cones but in Cycas female cone is loosely arranged called lax. 13. Ovules: A distinction of ovary, style and stigma is absent.The ovules are naked, sescile, geneiallyorthotropous, and unitegmic or bitegmic (in Gnetum). Ovules are orthotropous and sessile. Each ovule is surrounded by a 3-layered integu•ment. 14. Pollination: Pollination is direct as a stigma is absent and the pollen grains directly reach the micropylar ends of ovules. Pollination is usually accomplished by wind (anemophily). 15. Fertilization: Fertilization is siphonogamous i.e. male gametes carried to female gametes by means of a pollen tube. Double fertilization, a feature unique to angiosperms, is Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 3 absent in gymnosperms, but found in Ephedra. After fertilization, Zygote develops into embryo and ovules become seed. 16. Endosperm or female gametophyte: Endosperm or female gametophyte formed before fertilization and is always haploid. (But triploid in angiosperms.) Female gametophyte contains archegonia. Seeds contain a food laden tissue or endosperm for future growth of embryo into seedling. The tissue represents the female gametophyte. 17. Male gametophyte: Male gametophyte produces only two male gametes or sperms. Generally one of them is functional. External water is not required for transport of male gametes. Instead, a pollen tube is formed by the male gametophyte for effecting fertilization (siphonogamy). 18. Embryo development: Embryo development is meroblastic i.e. develops from a small part of zygote. Most members show polyembryony i.e. development of more than one embryo, but only one survives at the end. Cotyledons 2 (in Cycas) or many (in Pinus 2- 14). 19. Connecting Links: These plants have vascular tissues which help in the transportation of nutrients and water. The lower gymnosperms like Cycadales resemble pteridophytesin absence of vessels and wood fibers whereas higher gymnosperms like Gnetales and Coniferales resemble angiosperms. Like pteridophytes, xylem does not possess vessels except in some geophytes. Phloem is without companion cells and sieve tubes. Sieve cells are not arranged end to end in rows. Vascular tissues are arranged into vascular bundles just like angiosperms. Vascular bundles of stem are open so that secondary growth is quite common Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 4 Origin of Gymnosperms Gymnosperms were the first seed plants to have evolved. The earliest seedlike bodies are found in rocks of the Upper Devonian Series (about 382.7 million to 358.9 million years ago). During the course of the evolution of the seed habit, a number of morphological modifications were necessary. First, all seed plants are heterosporous, meaning that two kinds of spores (microspores and megaspores) are produced by the sporophyte. Hence, it is assumed that the ancestors of seed plants were heterosporous. Secondly, sporangia of seedless plants typically lack an integument, which forms the seed coat in gymnosperms. Fossil ovules discovered in Scotland suggest that integuments originated during the Mississippian subdivision of the Carboniferous Period (about 358.9 million to 323.2 million years ago). The ovules of Genomosperma kidstonii, for example, consisted of an elongated megasporangium with one functional megaspore and featured eight elongated fingerlike processes that loosely surrounded the megasporangium. In a related species, G. latens, those eight fingerlike processes were fused at the base into a cup and covered the megasporangium rather closely. The extinct division Progymnospermophyta is thought to be ancestral to seed plants. The best-known progymnosperm is the Devonian Archaeopteris, originally assumed to be a fern, with wedge-shaped subdivided leaflets known as pinnules and sporangia borne on appendages in between the pinnules. Its wood was like that of many conifers, consisting of tracheids and vascular rays, with closely spaced circular bordered pits on the radial walls of the tracheids. Pits were clustered, separated from other clusters by an area of the wall lacking pits. At least some species are known to have been heterosporous. Archaeopteris had many of the features to be anticipated in a seed-plant ancestor and likely gave rise to more than one group of gymnosperms. Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 5 Earliest gymnosperms The earliest recognized group of gymnospermous seed plants are members of the extinct division Pteridospermophyta, known as pteridosperms or seed ferns. These plants originated in the Devonian Period and were widespread by the Carboniferous. In habit, seed ferns resembled some progymnosperms in that they were small trees with fernlike leaves (the equivalent of a progymnospermous flattened branch) bearing seeds. Secondary vascular tissues were common in stems of seed ferns, though the wood was composed of thin-walled tracheids and abundant vascular rays, suggesting that stems were fleshy like those of cycads. Pteridosperm seeds were very similar to those of cycads and were often large, with a soft outer seed coat and a harder inner seed coat. Within a mature ovule was a massive female gametophyte with several archegonia. Some fossils suggest that the transport of the sperm through a pollen tube (siphonogamy) was in existence as far back as the Paleozoic. Pollen-bearing organs were variable among the pteridosperms; in many cases the microsporangia were elongated and fingerlike and were produced in clusters