GYMNOSPERMS

 

Gymnosperms are naked seeded plants with their ovules freely EXPOSED. The Jurassic period perhaps the best time for Gymnosperms.

• The living Gymnosperms are widely distributed in the cold climates
• Stems are characterized by the presence of open vascular bundles arranged in a ring, with endarch xylem  ( meta xylem towards pith and para xylum away from pith).
• The vessels are absent in the xylem (except in Gnetales).
• In most of the living Gymnosperms reproductive organs are, Cones. Cones may either be male (microsporangiate ) or female (megasporangiate)
• Male cones are usually short lived and smaller than female cones.
• Microsporangia (pollen sacs) are borne on the abaxial(underside) surface of microsporophyllls.
• Megasporangia (ovules) are borne naked on the sporophylous scales.
• The plants are heterosporous, i.e., they bear two types of spores
• The male gameteophyte bears only one or two cells.
• The female gametophyte possesses archegonia. Each archegonium possesses single egg
• Pollination takes place by direct contact of pollen grains with the ovules
• Fertilization takes place by the fusion of male and female nuclei resulting in the formation of zygote.

Polyembryony

Development of several embryos in one seed out of which though only one survives, is of        common occurrence, e.g., Pinus.  It is marvelous future shown by pteridophytes.  Embryo is differentated into radical, plumule and cotyledons.  Since there is no closed ovary, true fruits like that of Angiosperms are not found in the Gymnosperm.