Orders Of Cycadophyta

According to the Classification of Sporne (1965), Gymnosperms are classified into three classes. Among them, first one is Cycadophyta. In this article, three orders under Cycadophyta will be discussed in detail.


ORDER 1

Cycadofilicales / Pteridospermales

  • The Pteridospermales or seed ferns are characteristic gymnospermous plants that  bore fern-like foliage.
  • They came into existence during Upper Devonian period and lived through Carboniferous and Permian periods and reached their climax in the Mesozoic era.

Characteristic features:

  1.  Stem erect, slender and weak.
  2. Leaves are large, pinnately compound and frond-like.
  3. Primary xylem is in the form of a solid or a medullated protostele and is usually mesarch.
  4. Secondary xylem/ wood was manoxylic.
  5. Tracheids of secondary wood bore mutiseriate bordered pits on their radial walls.
  6. Ovules lacked annulus and are surrounded by cupule like structures.
  7.  Ovules have integuments, either free or fused with nucellus and have a distinct pollen chamber and a micropyle.
  8.  The integuments are vascularized.
  9. Microsporangia have no annulus and are sometimes grouped into synangia.
  10. Pollen grains lack pollen tube.
  11. Male gametes are motile and discharged directly into the pollen chamber.
  12. Megaspore is surrounded by a thick wall.

Examples: Neuropteris heterophylla, Lyginopteris oldhamia

Lygniopteris oldhamia. Details
Cross Section of L. oldhamia. Details

ORDER 2

Bennettitales/ Cycadeoidales

  • Bennettitalesis a fossil group – flourished well during Triassic to Cretaceous periods of Mesozoic era.
  • Mesozoic era is called the ‘Age of Cycads’.

Characteristic features:

  1.  Stem columnar, and grow very slowly.
  2. Leaves are large and pinnately compound.
  3. The most striking character is the presence of biporangiate strobilus.
  4. Stem has large pith surrounded by a ring of collateral conjoint, endarch and open primary vascular bundles.
  5. Reproductive organs are bisexual in majority of the genera (e.g. Cycadeoidea, Williamsonia etc.).
  6. Presence of interseminal scale on the receptacle. Seeds and ovules are distributed among them.
  7. Flowers arise in the axils of leaves.
  8. Ovules and seeds are stalked (Cycadeoidea) or sessile (Williamsoniella).
  9. Flowers are surrounded by numerous hairy bracts that arise from the base of
    receptacle.
  10. The microsporophylls or pollen bearing organs arise in whorls, and may be pinnate or entire, free or fused.
  11. Pollen grains are borne in bilocular synangia that may be sessile or stalked.
  12. Seeds possess two cotyledons.

Examples: Cycadeoidea decotensis
                      Williamsonia spectabilis.

Williamsonia spectabilis. Details

ORDER 3

Cycadales

  • The Order Cycadales includes both living and extinct forms that originated in the Upper Triassic period of early Mesozoic era.
  • Cycadales reached its peak during Mid-Mesozoic times and flourishes during the Jurassic and Cretaceous periods and after that their decline started, and in the present time there are only 10 genera with approximately 117 species.

Characteristic features:

  1. They are slow growing plants.
  2. Primary root system is tap root system.
  3. Coralloid roots are present in almost all living genera of Cycads and contain blue green alga Anabaena and Nostoc.
  4. Stem cylindrical, slow-growing and unbranched, occasionally adventitious branches may arise (i.e. Cycas circinalis).
  5. In some cases, stems are tuberous and nearly spherical and irregularly branched (e.g. Zamia sp.)
  6. Leaves may be monomorphic or dimorphic. When dimorphic, two types of leaves are found, one is large foliage green leaves and another is small, brown, scale leaves.
  7. Foliage leaves are spirally arranged and may be pinnate or bipinnate.
  8. Leaves have circinate vernation (i.e. Cycas) or sub-circinate vernation (i.e. Stangeria, Ceratozamia etc)
  9. The primary vasculature/ [vascular bundle] consists of an interrupted ring of conjoint, collateral, open and endarch vascular bundles.
  10. All the living genera except Cycas bear compact female cones.
  11. Ovules consists of a nucellus surrounded by integument with a distinct micropyle at the distal end.
  12. Spermatozoids are motile.
  13. Embryo has two cotyledons.

Examples: Cycas pectinata                               Cycas circinalis; Zamia pygmaea

Cycas circinalis. Details

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Reference:

Class Lecture of  Dr. M. Oliur Rahman,

Department of Botany, University of Dhaka.

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