Biology Today — December 2017

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.

Crown of foliage

leaves

Young male cone

Stem

Fig.: Cycas

Long shoot

Male cones

Female cones

Lateral bud

Dwarf shoot

Terminal bud

Fig.: Pinus

General Characteristics

• There are about 70 genera and 725 species of living gymnosperms distributed throughout the temperate and tropical regions of
  the world.


• Sporophyte is dominant and differentiated into root, stem and leaves. Usually tap roots are present. Roots of some taxa have symbiotic
  relationship with algae (e.g., coralloid roots of Cycas) or fungi (e.g., mycorrhizal roots of Pinus.) Stem is erect, aerial and woody. It may
  be branched or unbranched.


• Leaves are dimorphic i.e., foliage and scale leaves. Flowers are absent, instead sporophylls, i.e., microsporophylls and
  megasporophylls are aggregated to form distinct cones or strobili.


• Plants may be dioecious (e.g., Cycas) or monoecious (e.g., Pinus) and are heterosporous producing microspores and megaspores.


• Gametophytic generation is much reduced. The pollen grain forms the first male gametophyte while the female gametophyte is
  represented by functional megaspore.
  Microsporophylls
  Microsporangium

Microspores

Central axis

Scale leaf

Fig.:L.Sof male cone ofPinus

Ovuliferous

sc lea

Bract scale

Ovule

Central

axis

Fig.:L.S. of female cone ofPinus

• Pollination takes place by wind and pollen grains directly reach the ovules. The male gametes reach the female gamete by forming
  a tube i.e., siphonogamy, to affect fertilisation.


• Endosperm is gametophytic and derived from female gametophyte. Polyembryony is common in Pinus. Naked ovule develops
  into seed, the ovular integuments form seed coat. The number of cotyledons may be one or two or a whorl of many.


• Fruits are not formed due to absence of ovary.


• Xylem does not possess vessels (except in Gnetales) and phloem does not possess companion cells and sieve tubes.


• Vascular bundles are open in stem, thus secondary growth is present.