Plant Kingdome - GYMNOSPERMS| | NCERT concept | Neet Concept | Biology

Plant Kingdome - GYMNOSPERMS| NCERT concept | Neet short material| Biology

GYMNOSPERMS

Gymnosperms are plants with naked seeds , having freely exposed ovules on megasporophylls . They are referred to as phanerogams without ovary and act as connecting link between pteridophytes and angiosperms . The plant body is differentiated into root , stem and leaves .

• The plants possess well developed tap root system . In some cases , the roots are symbiotically associated with algae ( e.g. , coralloid roots of Cycas ) or with fungus ( e.g. , mycorrhizal roots of Pinus )
• The stem is erect , aerial , solid , woody and branched .
• The gymnosperms include world's tallest tree - Sequoia sempervirens measuring about 115m in height and 8.9m in girth . The smallest gymnosperm is Zamia pygmaea , which has underground tuberous stem . It reaches only upto 25cm in height .
• The vascular tissues , xylem and phloem are present . They are arranged in vascular bundles . The vascular bundles of stem are conjoint , collateral and open . Xylem vessels are absent .
• Xylem vessel is mostly absent .
• Secondary growth occurs and annual rings are distinct in most of the gymnosperms .
• Gymnosperms are heterosporous . The two types of sporangia are borne on special leaf - like structures called sporophylls . The microsporangia are borne on microsporophylls ( stamens ) and the megasporangia on megasporophylls ( carpels ) .
• The sporophylls are aggregated in the form of cones or strobili
• The microsporangium produces numerous light pollen grains .
• Pollination is anemophilous . Some of the pollen grains come in direct contact with the micropylar end of ovules and reach upto the nucellus .
• Fertilisation occurs by siphonogamy , i.e. , the male gametes reaches the female gamete by forming a tube .
• The diploid zygote , develops into an embryo while still enclosed within the ovule and the ovule becomes a seed .
• During seed development , the endosperm ( haploid ) supplies nutrients to the developing embryo .
• The seeds of gymnosperm are naked and remain attached to the megasporophyll .
• Polyembryony is common in Pinus . The number of cotyledons may be one or two or a whorl of many .
• Gymnosperms can be classified into following types :

Cycadopsida

• Primitive group , containing plants with palm like habit and fern like foliage . Plants are dioecious .
• Wood is manoxylic .
• Male cones are large with compactly arranged microsporophylls .
• Megasporophylls are loosely arranged and do not form a cone .
• Plants are mostly xerophytic . Examples : Cycas , Zamia , etc.

Coniferopsida

• Includes larger dominant gymnosperms .
• Monoecious .
• Stem branches are dimorphic , i.e. , with long and dwarf shoots . Wood is pycnoxylic .
• Sporophylls form cones .
• Examples : Pinus , Ginkgo , Taxus .

Gnetopsida

• Includes advanced gymnosperms .
• Branches are dimorphic ( long and dwarf shoots ) as well as leaves are also dimorphic ( scale and foliage leaves ) .
• Xylem shows vessels . Ovules are orthotropous .
• Examples : Gnetum , Ephedra , Welwitschia .

• Ginkgoales ( Under coniferopsida ) is represented by only one living member , Ginkgo biloba ( living fossil ) . Plants have dimorphic branches .
• Leathery , fan shaped leaves present having dichotomously branched veins . Plants are dioecious . Male gametes are multiflagellate and motile .

Importance of Gymnosperms

• Food : Seeds of Pinus gerardiana are edible and are eaten after roasting .
• Timber : The softwood of gymnosperms is used in preparation of light furniture , plywood , packing cases , etc.
• Paper : A wide range of gymnosperms are utilised in manufacture of paper e.g. , Picea , Pinus , Abies , etc.
• Resin : It is a semifluid secretion containing terpenes , resin , acids and esters which solidifies on exposure to air . It helps in sealing , is antiseptic and toxic to pests , thus , prevents microbial and insect attack . Resin is commercially extracted to obtain turpentine and rosin .
• Ephedrine : It is a drug obtained from Ephedra and used in curing respiratory ailment , including asthma .

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