Gymnosperms - General characteristics and Economic importance.pptx

A brief account about general characteristics and economic importance of Gymnosperms

1. Gymnosperms: General characteristics and Economic importance

2. General characteristics Gymnosperms ( Gymnos = Naked + Spermos = Seed ) commonly called naked seeded plants form a distinct sub- division of the Spermatophyta (seed plants ). These have been referred by Goebel as “ Phanerogams without ovary”. The plants of this group are regarded as most primitive of all the seed plants . The plants possess well developed taproot system. In some cases the roots are symbiotically associated with algae (coralloid roots of Cycas ) or with fungi ( mycorrhizal roots of Pinus ).

3. Habit Gymnosperms are predominantly woody, evergreen, perennial trees or shrubs. A few members are climbers e.g. ( Gnetum ula , Ephedra foliata ). Stem is branched except in Cycas . They are xerophytic in habit and thus possess xerophytic adaptations in different forms. The height of the plant varies from 3-5 cm ( Zamia pygmaea ) to 40 m( Sequoia wellingtonia ). The growth of the stem varies from few mm ( Ephedra ) to about 15 meters ( Sequoia sempervirens ).

4. Leaves The leaves present on the Gymnospermous plants may be of only one kind or of two kinds (dimorphic). The foliage leaves are green, simple, needle shaped or pinnately compound. The scaly leaves are usually minute and deciduous. In some taxa like Ephedra only scaly leaves are present.

5. Reproduction Gymnosperms are heterosporous , producing two kinds of spores microspores and megaspores produced by separate sporangia The two types of sporangia-microsporangia and megasporangia are found on special leaf-like structures called sporophylls Microsporangia occur on microsporophylls (stamens) and megasporangia occur on megasporophylls (carpels )

6. The sporophylls are usually aggregated to form structures called cones or strobili. Male cones (with microsporophylls ) and female cones (with megasporangia ) occur on separate plants (unisexual ) The microsporangium (pollen sac) produces numerous pollengrains which are anemophilous . The megasporongium (ovule) remains exposed on the megasporophyll 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).

7. The pollination drop (PD) is a characteristic feature of many wind-pollinated gymnosperms. Although accumulating evidence shows that the PD plays a critical role in the pollination process, the mechanism of PD withdrawal is still unclear . Pollination drops are ovular secretions that form a landing site for pollen in many gymnosperms .

8. The generative cell present in the pollen tube produces two male gametes. One of them unites with the egg cell in the ovule to form a zygote . The diploid zygote develops into an embryo which remains enclosed within the ovule. The ovule then transforms it self into a seed The seed remains attached to the megasporophyll and is not enclosed by a fruit The ripe and dry seeds are shed. Under suitable conditions the seeds germinate and develop into new plants

9. Vascular system Like pteridophytes , xylem does not possess vessels except in some of them. 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.

10. Economic importance The gymnosperms are an economically important group of plants spread all over the globe, primarily in the temperate (colder) and at higher elevations in the tropics (warmer) regions. The trees are used for landscaping, timber, building constructions, resins and for manufacture of paper and board. They are also used in medicines, perfumes, varnishes and as essential oils.

11. It gives valuable wood, resin, essential oils, gums, paper, turpentine, medicines, food, ornamentals and miscellaneous items. Aesthetic value - Gymnosperms are frequently used in parks, gardens because of their evergreen habit and symmetrical appearance .

12. Resins Resins are plant exudates which make the wood resistant to decay. Conifers are amongst the major resin yielders of the world. The resins are insoluble in water but soluble in organic solvents . The superior grade resin is used in paper sizing, varnishes, enamels, plasters, medicines and ointments.

13. Food In many parts of the world Cycads are used as a source of starch- either from the seed kernels or from stem pith. The stem starch is more popularly known as ‘ sago ‘ and is mainly obtained from Cycas circinalis , C. rumphii and C. revoluta and also form Zamia and Macrozamia . The pith of Encephalartos stem is used in Africa to make ‘ Kaffir bread ‘ .

14. Medicine Seed kernels of Gnetum ula yield an oil for illumination and massage in rheumatism. Leaf extract of Ginkgo biloba is useful in the treatment of cerebral insufficiency and vertigo. Ephedrine (alkaloid) extracted from Ephedra used in treating asthma, cough, cold, bronchitis etc. Tincture of Ephedra is a cardiac stimulant. Anti-cancerous drug called taxol , is obtained from the bark of Taxus . The juice is extracted from young leaves of Cycas revoluta is used for curing blood vomiting and flatulence.

15. Wood The wood of Abies alba , an important timber tree of Europe is used in general carpentry . Cedrus atlantica (Algeria, Moroco ) , C. deodara (India) and C. libani (Lebanon ) are much valued among conifers . The wood of Cedrus is in great demand as it is very durable , oily sweet scented and generally without resin ducts . Several species of Pinus yield wood which is used for building material, furniture, poles, match boxes and other such articles.

16. Essential Oils All conifers young branches and adherent leaves provide essential oils. Himalayan Cedar oil ( Cedrus deodara ) and Red Cedar Wood ( Juniperus virginiana ) are used cleaning tissues in histological work and also use with the oil immersion lens of the microscope. The oil obtained from Cedrus atlantica possess medicinal properties and used against bronchitis, tuberculosis, skin diseases and gonorrhea. The essential oils are used extensively in preparation of deodorants, room sprays, disinfectants, perfumery and medicine etc .

17. In Industry Gum- Cycas gum used as adhesive, antidote for snake bites and using malignant ulcers. Tannins- Tannins are used in leather industry and it is extracted from the bark of Araucaria , Sequoia etc. Canada balsam- it is turpentine obtained from Abies balsamea and used as a mounting medium in biological preparations.

18. Canada balsam The resin obtained from Abies balsamea has a very high refractive index nearing that of glass. This makes Canada balsam as the most preferred mounting medium for microscopic objects and a cement for lenses in optical work.

19. As ornaments Species of Cycas are used for decoration purposes. Ginkgo biloba possess beautiful ornamental leaves. Thuja , Pinus , Taxus etc are grown in parks