Gymnosperms

  • The gymnosperms (gymnos : naked, sperma : seeds) are plants in which the ovules are not enclosed by any ovary wall and remain exposed, both before and after fertilisation.
  • The seeds that develop post-fertilisation, are not covered, i.e., are naked. Gymnosperms include medium-sized trees or tall trees and shrubs.
  •  One of the gymnosperms, the giant redwood tree Sequoia is one of the tallest tree species.
  •  The gymnosperms are vascular plants that produce seeds on scales of woody strobili called ‘cones’.
  • The roots are generally tap roots. Roots in some genera have fungal association in the form of mycorrhiza (Pinus), while in some others (Cycas) small specialised roots called coralloid roots are associated with N2- fixing cyanobacteria.
  •  The stems are unbranched (Cycas) or branched (Pinus, Cedrus). The leaves may be simple or compound.
  • In Cycas the pinnate leaves persist for a few years.
  •  Cycads have a terminal rosette of leaves and bear seeds in cones.
  •  All cycads have separate male and female plants.
  •  The leaves in gymnosperms are well-adapted to withstand extremes of temperature, humidity and wind.
  •  In conifers, the needle-like leaves reduce the surface area. Their thick cuticle and sunken stomata also help to reduce water loss.
  • The gymnosperms are heterosporous; they produce haploid microspores and megaspores. The two kinds of spores are produced within sporangia that are borne on sporophylls which are arranged spirally along an axis to form lax or compact strobili or cones.
  • The strobili bearing microsporophylls and microsporangia are called microsporangiate or male strobili.
  •  The reduced gametophyte is called a pollen grain. The development of pollen grains takes place within the microsporangia.
  •  The cones bearing megasporophylls with ovules or megasporangia are called macrosporangiate or female strobili.
  •  The microspores develop into a male gametophyte which is highly reduced and is confined to only a limited number of cells.
  •  The male or female cones or strobili may be borne on the same tree (Pinus) or on different trees (Cycas).
  • The megaspore mother cell is differentiated from one of the cells of the nucellus. The nucellus is protected by envelopes and the composite structure is called an ovule. The ovules are borne on megasporophylls which may be clustered to form the female cones.
  •  The megaspore mother cell divides meiotically to form four megaspores. One of the megaspores enclosed within the megasporangium (nucellus) develops into a multicellular female gametophyte that bears two or more archegonia or female sex organs.
  •  The multicellular female gametophyte is also retained within megasporangium.
  •  In gymnosperms the male and the female gametophytes do not have an independent free-living existence. They remain within the sporangia retained on the sporophytes.
  • The pollen grain is released from the microsporangium. They are carried in air currents and come in contact with the opening of the ovules borne on megasporophylls.
  •  The pollen tube carrying the male gametes grows towards archegonia in the ovules and discharges  their contents near the mouth of the archegonia.
  •  Following fertilisation, zygote develops into an embryo and the ovules into seeds. These seeds are not covered.
  • The four divisions of Gymnosperms are-
  • Cycadophyta (Cycas)
  • Ginkgophyta (Ginkgo)
  • Gnetophyta (Ephedra)
  • Coniferophyta (Pinus)
  •  The generic name comes from Greek Koikas, and means "a kind of palm".
  •  The best-known species is Cycas revoluta, widely cultivated under the name "Sago Palm" or "King Sago Palm“ due to its palm-like appearance although it is not a true palm.
  •  Divisions Ginkgophyta has only on living species- Ginkgo biloba.
  •  The leaves of Ginkgo biloba are lobed.
  • Ginkgos have separate male and female trees.
  •  Divisions Gnetophyta has 3 genera-Gnetum, Ephedra, Welwitschia.
  •  Coniferophyta is the largest and most diverse division of the gymnosperms.
  •  Most Conifers are evergreen – keeping their leaves year-round.
  •  A very few are deciduous – dropping all of their leaves at the same time.