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General Characters of Gymnosperms.pptx

Gymnosperms, coined by Theophrastus, are plants with naked seeds that originated in the Devonian period and dominate as a major forest type, crucial for timber and wood pulp. Comprising about 650 species, gymnosperms display a range of adaptations such as xerophytic traits, distinct reproductive structures like cones, and a lifecycle featuring sporophyte dominance. They bridge the evolutionary gap between pteridophytes and angiosperms, with examples including conifers, cycads, and ginkgo.

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General Characters of Gymnosperms
INTRODUCTION  The term ‘Gymnosperm’ was coined by Theophrastus (372 - 286 B.C) for plants whose seeds are naked and unprotected.  Gymnosperms (Gymnos = naked, Sperma = seed) include the small group of plants with naked seeds.  The Gymnosperms originated in the Devonian period of the Paleozoic Era and formed the supreme vegetation in the Mesozoic Era.  At present, Gymnosperms are represented by about 72 genera and about 650 species and are widely distributed from north temperate to tropical regions of the world.
 Gymnosperms form intermediate group between pteridophytes and angiosperms. i.e., they are more advanced than pteridophytes but primitive than angiosperms.  Gymnosperms seeds are not enclosed in a ovary. Gymnosperm seeds are with a single integument. Double fertilisation is completely absent (except Ephedra).  Gymnosperms form one of the major forest types of our planet having the greatest economic importance as a source of timber, lumber and wood pulp.
MAIN CHARACTERISTIC FEATURES • HABIT - Gymnosperms are predominantly woody plants, evergreen perennials, represented by trees, shrubs or rarely climbers – Ex: Larix and a Taxodium. • They are usually xerophytic, some of them are deciduous while others are evergreen. • Sequoia sempervirens (California or Coast red wood) is tallest living tree reaching a height nearly 112 m and attaining a growth of 15 m. Smallest gymnosperm is Zamia pygmaea, it grows upto 25 cm tall. • Taxodium maxicanum has a trunk with the enormous diameter of 17 meter. The bristlecone pines (three species of pines i.e., P. aristata, P. longaeva and P. balfouriana) are thought to reach an age greater than that of any other single organism known, upto nearly 5000 years.
• OCCURENCE - 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. • In warmer areas only a handful of gymnosperms can be observed, e.g., Cycas circinalis in South India, Aracauria heterophylla native of South America, New Zealand and East Australia. • XEROPHYTIC TRAITS - Gymnosperms are xerophytes in nature due to presence of thick bark, thick hypodermis, thick cuticle, scales leaves, sunken stomata, transfusion tissue, etc. • 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. • Plant body is sporophytic and can be differentiated into root, stem and leaves. The lifecycle of gymnosperms shows heteromorphic alternation of generation. • The dominant plant body is sporophyte (2n) which may be dioecious or monoecious. Gametophytes are inconspicuous and endosporic i.e. develop with the spores.
• ROOT SYSTEM - Generally the plants possess well developed tap root system. In some gymnosperms the roots show symbiotic relationship e.g., coralloid roots of Cycas with algae and mycorrhizal roots of Pinus with fungi. Tap root system is exarch and diarch to polyarch. • STEM - It is erect, woody and branched (unbranched in Cycas and tuberous in Zamia). Presence of leaf scars on the stem is the characteristic feature of gymnosperms. • LEAVES - The arrangement of the leaves on the stem may be spiral or cyclic. They may be of one kind (monomorphic) or two kinds (dimorphic) - foliage leaves and scale leaves. • Foliage leaves are green, simple, may be small (microphyllous e.g., Pinus) or large (megaphyllous e.g., Cycas). Their main function is photosynthesis. Scale leaves are present around the reproductive structures and apex. They are mainly protective in nature. • XYLEM - Composed of xylem parenchyma and tracheids with bordered pits. Vessels are absent (except in Gnetales). • PHLOEM - Composed of sieve cells and phloem parenchyma but companion cells absent.
• 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. • 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. • Female gametophyte contains archegonia. Male gametophyte produces only two male gametes or sperms. Generally one of them is functional. • POLLINATION - Pollination is anemophillous. A distinction of ovary, style and stigma is absent. Pollination is direct as a stigma is absent and the pollen grains directly reach the micropylar ends of ovules. An external water is not required for transport of male gametes.
• FERTILIZATION - Fertilization is siphonogamous. Double fertilization is absent in gymnosperms, but found in Ephedra. After fertilization, Zygote develops into embryo and ovules become seed. • OVULES - Naked, sessile, generally orthotropous and unitegmic or bitegmic (in Gnetum). • EMBRYO - Embryo development is meroblastic i.e. develops from a small part of zygote. • VASCULAR BUNDLES - Vascular tissues are arranged into vascular bundles just like angiosperms. Vascular bundles of stem are open so that secondary growth is quite common. • COTYLEDONS - Cotyledons 2 (in Cycas) or many (in Pinus 2-14). • CAMBIUM - Cambium form secondary wood which is monoxylic (soft and porous) or pycnoxylic (hard and compact) • CONNECTING LINKS - The lower gymnosperms like Cycadales resemble pteridophytes in absence of vessels and wood fibres whereas higher gymnosperms like Gnetales and Coniferales resemble angiosperms.
CYCADOPSIDA  It is smaller group of gymnosperms which have palm-like habit and fern-like foliage. Leaf bases are often persistent.  In stem the vascular tissues constitute only a narrow patch though secondary growth occurs.  Plants are dioecious, that is, microsporophyll and megasporophyll develop on separate plants.  Megasporophylls are often leaf-like and possess ramenta. Sperms are motile.  The plants are mostly xerophytic and occur in warm weather.
CONIFEROPSIDA  They comprise more than 500 living species of cones bearing gymnosperms which have withstood competition from angiosperms.  The plants are evergreen with dense and massive vascular tissues and non-motile gametes. Conifers are usually monoecious.  Cones or strobili are compact and woody. Both the types of cones are borne on the same plant.
• Conifers have monopodial. This gives a conical or ex-current appearance to the plant just like a ‘Christmas Tree’. • Dimorphic Branch - Long and dwarf. Long branches have indefinite growth. Leaves are mostly borne on the dwarf branches. The dwarf branches have a limited or definite growth • The plants usually possess an aromatic antiseptic sticky semifluid substance called resin. It is secreted by special channels or ducts present in wood, bark, leaves, roots, etc. • Araucaria - a tall and majestic conifer of South America, New Zealand and East Australia, grows in warm weather in plains as well as foot hills. • Other conifers form vast and dominant woodlands in mostly north temperate regions of Europe, Asia, North America, etc. • They are abundant on the upper timber-line zone of mountains including the Himalayas, e.g., Pinus (Pine), Cedrus (Cedar), Picea (Spruce), Abies (Fir), Juniperus (Juniper), Thuja, Larix, Cupressus.
GINKGOALES - GINKGO  Ginkgopsida is represented by single species maiden hair tree (Ginkgo biloba).  Primitive gymnosperms which possess deciduous leaves with furcate venation.  Dimorphic branches, unisexual plant, catkin like male inflorescence with each microsporangiosphore having 2-12 microsporangia.  Multiciliate male gametes and clusters of megasporangiate structures each with a long stalk and two ovules.
GNETOPSIDA  Gnetopsida contains Gnetum, Ephedra and Welwitschia. These 3 are known as Monogeneric families.  These are nearer to flowering plants in having flower like arrangement of sporophylls and possessing primitive vessels in xylem.