[Biology] Gymnosperms

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WHAT IS GYMNOSPERMS?

  • Gymno means Naked, and Sperma means Seed. So, Gymnosperms means naked seed.
  • Gymnosperms have evolved much earlier than flowering plants.
  • The seeds have exposed megasporophylls, which mean carpels are exposed.
  • Gymnosperms have evolved during Jurassic period and are said to be the first surviving seed plants.

General Characteristics of Gymnosperms:

  • The seeds of the Gymnosperms are not within the fruits, hence they are exposed.
  • The haploid endosperm is present in the seeds.
  • The root system is generally tap root system, though in Cycas coralloid roots are present which helps in nitrogen fixing bacteria.
  • The stem can either be branches, in case of Pinus or can be un-branched like in Cycas.
  • The leaves can either be simple as of Pinus or compound as in Cycas. The leaves have thick cuticle and are needle like in structure.
  • They are heterosporous the spores founded are microspores and megaspores.
  • Flowers are unisexual and are without perianth (except Gnetum); the male and female cones are present.
  • The female and male gametophytes are dependent on sporophyte. There are two gametes in male gametophyte of Gymnosperms, but only one of the gamete is functional.

Classification of Gymnosperms:

  • As per A Arnold (1948) and as per the Modification by Melchior and Pilger (1954), Gymnosperms can be categorised into three groups, namely:
  • Cycadopsida
  • Coniferopsida
  • Gnetopsida

Cycadopsida

  • They are individual plants with either male or female meaning they are dioecious.
  • Cycadopsida plants are plants bearing seeds.
  • Most of the plants of this group of Gymnosperms are though extinct now.
  • They have large compound leaves, thick trunks and small leaflets attached to single stem.
  • Monoxylic woods are found with bi-pinnately compound leaves.

EXAMPLE:

  • Cycas
  • Zamia

Coniferopsida

  • Coniferopsida are large trees.
  • The nature of the plants of this group of Gymnosperms is Sprophytic
  • They produce cones in their reproductive phase.
  • They do not shed their leaves in the winter season and hence are evergreen.

EXAMPLE:

  • Pinus
  • Cordailes

Gnetopsida

  • The plants of the group Gnetopsida are mostly extinct, but few of them are living.
  • They generally consist of tree, shrubs and tropical plants.

EXAMPLE:

  • Gnetum
  • Ephedra

Reproduction in Gymnosperms:

Gymnosperms can reproduce by both sexual and Vegetative method of reproduction.

  • SEXUAL REPRODUCTION: The life cycle of Gymnosperms is characterized by alteration of generations. The cones contain female and male gametophyte, and the leafy part of the plant consists of sporophyte which is green in colour.
  • VEGETATIVE REPRODUCTION: The vegetative reproduction in Gymnosperms is done by Bulbils. The bulbils get separated from the plants and then germinate into new plants.

Life cycle of Gymnosperms:

  • Microspores develop into male gametophytic generation which is further reduced and is known as pollen grain.
  • Pollen grains develops in the microsporangia
  • The cones which bear the megasporophylls with ovules are known as female strobilli or macrosporangiate.
  • The female or male cones can be found in the same tree like Pinus sp. or in different trees like Cycas sp.
  • The megaspore mother cell differentiates from the cells of nucleus, which is protected by ovule.
  • The ovules are found in megasporophylls and forms the female cones,
  • One megaspore develops into multi-cellular female gametophyte which bears either two or more archegonia, also known as female sex organs.
  • The multi-cellular female gametophyte can also be found in the megasporangium.
  • The Gymnosperms do not have free living existence, they remain within the sporangia.
  • The pollen grains gets released from microsporangium and are carried by air currents, following air pollination and comes in contact with the ovules present in the megasporophylls.
  • The pollen tube which bears the male gametes grows toward the archegonia in ovules and they discharge the contents near archegonia’s mouth.
  • After the process, the zygote gets developed into embryo and ovules following the process of fertilization.

Economic Importance of Gymnosperms:

There are various economic importances of Gymnosperms; few of them are as listed:

  • Food
  • The tuber and seeds of Cycas are used as food.
  • The seeds of Gnetum sp.
  • Chilgoza are taken from the Pinus sp.
  • In the study of evolution:
  • Both the extant and extinct species of Gymnosperms helps in studies and in research and to study the process of evolution.
  • Ornamentals:
  • Most of the Gymnosperms can be used for ornamental process.
  • Pinus excelsia are used in jewellery.
  • Industrial Products:
  • Paper is obtained from the Pinus sp. pulp.
  • Terpentine, Resin and Methyl alcohol are obtained from Pinus sp.
  • Pinus excelsia can be used in X-ray sheets.
  • Medicines:
  • Resin of Cycas rumphil can be used to treat ulcers.
  • Resin of Pinus sp. can be used to treat gonorrhoea and in treating problems of stomach.
  • Epherdrine from Ephedra, which is used to treat asthma.
  • Furniture:
  • Wood is obtained from Pinus sp.
  • Wood from Ephedra sp. and Gnetum sp. are used.

IMPORTANT POINTS/TERMS TO REMEMBER:

  • Mycorrhiza
  • Microsporophylls
  • Microsporangia
  • Male strobili
  • The cones bearing megasporophylls with microsporangia. The cones bearing megasprophylls Macrosporangiate or Female strobili.
  • Pollen grain
  • Megasporangia
  • Female strobili
  • Megasporangium
  • Archegonia, also known as female sex organs

REFERENCES

  • NCERT biology; textbook for class 11. Plant Kingdom. Page no: 38-39.
  • Arihant’s handbook of Biology. Plant Kingdom. Page no: 36-39.
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