Gymnosperms

Gymnosperms

Belong to the group Phanerogams i.e., seed bearing plants. They have naked seeds (not enclosed within a fruit). They are mainly xerophytes.

  • Have diplontic life cycle.
  • Plant body is the sporophyte & the gametophytic generation is very much reduced.
  • Sporophyte is well differentiated into root, stem & leaves.
  • Roots: Tap root system.
  • Stem: Thick, woody with scars of fallen leaves.
  • Leaves are 2 types: foliage leaves & scale leaves.
  • They are either monoecious or dioecious.
  • Secondary growth occurs.
  • They are heterosporous and bear male & female cones.
MALE REPRODUCTIVE PART:
  • Male cones are smaller in size & short lived.

They consist of spirally arranged microsporophylls.

Microsporophyll bear microsporangia inside which the microspores/pollens are produced after meiosis.

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Pollen/microspore starts dividing in the microsporangium.

They produce 1-2 prothallial cells, a generative cell & a tube cell.

Further development occurs after pollination.

The generative cell divides to form body cell & stalk cell. The body cell then produces 2 male gametes. Tube cell forms the pollen tube carrying the male gametes.

 

 

FEMALE REPRODUCTIVE PART:
  • Female cones are larger in size.

They consist of spirally arranged megasporophylls.

Megasporophyll bear megasporangia.

Megasporangia/ovule is orthotropous type.

Inside ovule develops megaspore mother cell that undergoes meiosis to form 4 megaspores, 3 of which degenerate.

The 4th functional megaspore forms the female gametophyte, that possesses 2-8 archegonia. Egg are produced in archegonia.

  • Pollination is anemophilous.
  • Fertilization occurs in archegonia.
  • Development of embryo is meroblastic.
  • The zygote divides to form the embryo having cotyledons, plumule & radicle.
  • Polyembryony occurs.
  • Ovule matures into seed & the female gametophyte forms the endosperm.

 

Divided into 4 classes:

Coniferopsida: e.g., Pinus, Abies, Cedrus.

Cycadopsida: e.g., Cycas, Zamia.

Ginkgopsida: e.g., Ginkgo.

Gnetopsida: e.g., Gnetum, Ephedra, Welwitschia.

PINUS:

  • Mycorrhizal roots occur.
  • Stem shows monopodial branching.
  • Simple, needle-shaped leaves.
  • Monoecious.
  • 2 microsporangia per microsporophyll.
  • Pollens possess 2 air sacs that act as wings.
  • Many cotyledons present.
  • pycnoxylic wood(densely packed).

CYCAS:

  • Coralloid roots occur.
  • Unbranched stem with a crown of leaves.
  • Leaves are pinnately compound.
  • Dioecious.
  • Microsporangia develop in groups called sori in the microsporophylls.
  • Female cone is not formed. Megasporophylls lie scattered.
  • Two cotyledons present.
  • They possess largest egg, sperm & ovule.
  • Wood is manoxylic (not compact).

Also Read Bryophyta and Alteration of generation.

 

 

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