Many systems of classification of Gymnosperms have been proposed by different authors. According to the Classification of Arnold (1948) & Sporne (1965), Gymnosperms are classified into three classes. Among them, first one is Cycadopsida.
In this article, three orders under Cycadopsida will be discussed in detail.
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.
- Stem erect, slender and weak.
- Leaves are large, pinnately compound and frond-like.
- Primary xylem is in the form of a solid or a medullated protostele and is usually mesarch.
- Secondary xylem/ wood was manoxylic.
- Tracheids of secondary wood bore mutiseriate bordered pits on their radial walls.
- Ovules lacked annulus and are surrounded by cupule like structures.
- Ovules have integuments, either free or fused with nucellus and have a distinct pollen chamber and a micropyle.
- The integuments are vascularized.
- Microsporangia have no annulus and are sometimes grouped into synangia.
- Pollen grains lack pollen tube.
- Male gametes are motile and discharged directly into the pollen chamber.
- Megaspore is surrounded by a thick wall.
Examples: Neuropteris heterophylla, Lyginopteris oldhamia
- Bennettitalesis a fossil group – flourished well during Triassic to Cretaceous periods of Mesozoic era.
- Mesozoic era is called the ‘Age of Cycads’.
- Stem columnar, and grow very slowly.
- Leaves are large and pinnately compound.
- The most striking character is the presence of biporangiate strobilus.
- Stem has large pith surrounded by a ring of collateral conjoint, endarch and open primary vascular bundles.
- Reproductive organs are bisexual in majority of the genera (e.g. Cycadeoidea, Williamsonia etc.).
- Presence of interseminal scale on the receptacle. Seeds and ovules are distributed among them.
- Flowers arise in the axils of leaves.
- Ovules and seeds are stalked (Cycadeoidea) or sessile (Williamsoniella).
- Flowers are surrounded by numerous hairy bracts that arise from the base of
- The microsporophylls or pollen bearing organs arise in whorls, and may be pinnate or entire, free or fused.
- Pollen grains are borne in bilocular synangia that may be sessile or stalked.
- Seeds possess two cotyledons.
Examples: Cycadeoidea decotensis
- 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.
- They are slow growing plants.
- Primary root system is tap root system.
- Coralloid roots are present in almost all living genera of Cycads and contain blue green alga Anabaena and Nostoc.
- Stem cylindrical, slow-growing and unbranched, occasionally adventitious branches may arise (i.e. Cycas circinalis).
- In some cases, stems are tuberous and nearly spherical and irregularly branched (e.g. Zamia sp.)
- 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.
- Foliage leaves are spirally arranged and may be pinnate or bipinnate.
- Leaves have circinate vernation (i.e. Cycas) or sub-circinate vernation (i.e. Stangeria, Ceratozamia etc)
- The primary vasculature/ [vascular bundle] consists of an interrupted ring of conjoint, collateral, open and endarch vascular bundles.
- All the living genera except Cycas bear compact female cones.
- Ovules consists of a nucellus surrounded by integument with a distinct micropyle at the distal end.
- Spermatozoids are motile.
- Embryo has two cotyledons.
Cycas pectinata; Cycas circinalis;
Class Lecture of Dr. M. Oliur Rahman,
Department of Botany, University of Dhaka.