Primitive Cycas and Advanced Gnetum

Gymnosperms are seed producing plants that do not produce any covering surrounding the seed. Cycas and Gnetum are two example of Gymnosperms that are respectively considered as primitive and advanced species.

In this article, Primitive characteristics of Cycas and advanced ones of Gnetum will be discussed explicitly.


CYCAS

Cycas is a specis of Cycadaceae family under the order Cycadales. Cycadales includes both living and extinct forms that originated in the Upper Triassic period of early Mesozoic era. Cycas is considered as a primitive species because of it’s characteristics.

Cycas. Details

Systematic Position

Division: Gymnosperm
Class: Cycadophyta
Order: Cycadales
Family: Cycadaceae
Genus: Cycas L.

 Genetical Distribution 

  • Cycas is a genus of about 20 species and is most widely distributed genus of Eastern Hemisphere.
  • This genus, indigenous to the Orient, is found wild or cultivated in the tropical or subtropical regions of the world.
  • The species of Cycas are distributed in the southern part of Japan, India, China,
    Islands of Indian and pacific Oceans, and northern parts of Australia.
  • A few species grow in Madagascar, Mauritius, and Eastern coast of Africa.

 

General Characteristics of Cycas

Cycas is a slow-growing short lived evergreen genus. It’s characteristic features i.e. root, stem, leaf are given below:

ROOT

  • The primary root of Cycas is a short-lived tap root, which later on is replaced by adventitious roots.
  • Most of the lateral branches of primary root, that are known as normal roots,
    penetrate the soil.

CORALLOID ROOT

Some of the roots, which develop near the surface of the ground get infected with certain algae, bacteria and fungi, which are already present in the soil surrounding the root system. These roots grow horizontally first, and then vertically in the soil, and become swollen at their tips due to entry of algae, bacteria and fungi. They behave like aerotropic or negatively geotropic structures that come out in the air above the soil surface and branched dichotomously to form greenish-brown coral like mass. Therefore, they are called coralloid roots.

Coralloid root. Details
  • Coralloid roots are abundant in young plants and the plants grown in green house.
  • In coralloid roots, following microorganisms are found:-
  1. Blue green algae- Anabaena cycadeae, Nostoc punctiformae.
  2. Diatoms.
  3. N2 fixing bacteria- Psudomonas sp, Azotobacter sp.
  4. Certain fungi

STEM

  • Cycas Stem. Details

    The stem of Cycas is erect, stout, cylindrical and usually unbranched, however, in very few cases, stem may be branched.

  • The stem when young is tuberous, short and subterranean.
  • Most part of the older stem is covered with a number of large and small rhomboidal leaf bases.

LEAF

  • There are two types of leaves in Cycas, viz.

i) green, large foliage leaves, and
ii) brown, small, scale leaves.

  • A crown of spirally arranged leaves present around the stem apex makes the Cycas look like a palm tree.
  • Foliage leaves are pinnately compound, showy, fairly large from 1-3 m, very thick and leathery. They are attached to the stem by rhomboidal leaf bases with a long or short petiole and a long stout rachis.
  • Circenate venation. Details

    In very young leaves, the leaflets are circinately coiled like those of ferns, that is called circinate vernation.

  • Scale leaves are small, rough, triangular and thickly covered with brown hairs.

 

Main Characteristics of Cycas 

  • Plant body is sporophytic, i.e. into root, stem and leaves.
  • Coralloid roots are found in Cycas.
  • Leaves are pinnately compound and spirally arranged at the top of the stem.
  • Circinate vernation present in young leaves.
  • Microsporophylls (Male sporophylls) from strobilus and grouped into cones; but megasporophylls (Female sporophylls) do not form strobilus and never grouped into cones.
  • Ovules are naked and very large which are formed row-wise in two sides of
    megasporophylls.
  • Embryo bears two cotyledons.

Primitive Characters of Cycas 

  • Cycas. Details

    Presence of circinate vernation in young leaves.

  • Xylem lacks vessel and phloem lacks companion cells.
  • Archegonium is present.
  • Sperms are flagellate and motile.
  • Microsporangia form sori.

Xerophytic Adaptation of the Cycas leaf 

  • Tough and leathery texture.
  • Strongly cutinized thick walled epidermis.
  • Highly thickened hypodermis on both sides of the leaflet.
  • Sunken stomata restricted only to the lower surface.
  • Unbranched midrib.
  • Occurrence of primary and secondary transfusion tissue.

Gnetum

Gnetum is a Species of Gnetaceae family under the order Gnetales. Gnetales bear some Angiospermic characteristics. That’s why it is considered as the most advanced group among gymnosperm and Gnetum is advanced one.

Gnetum. Details

Systematic Position

Division: Gymnosperm
Class: Coniferophyta
Order: Gnetales
Family: Gnetaceae
Genus: Gnetum L.

 

 

Advanced characters of Gnetum

Gnetum resembles with angiosperms in the following respects:

  1. Gnetum is climber or tree-like and resembles angiosperms.
  2. Leaves are broad, green with reticulate vena tion. They show opposite decussate arrangement.
  3. Archegonia are altogether absent in female gametophyte of Gnetum.
  4. Development of female gametophyte (embryo sac) in Gnetum is of tetrasporic type.
  5. Occurrence of free nuclear division in the embryo sac (a character of
    angiosperm).
  6. The endosperm completes its development after the fertilization.
  7. Reproductive structure. Details

    Male and female reproductive organs of Gnetum are more similar to angiosperms than members of other groups of gymnosperms.

  8. Embryo has two cotyledons alike to dicotyledonous plants.
  9. Vessels present in xylem.
  10. Vessels, similar to those of angiosperms, occur together with tracheids in the secondary wood of Gnetum.
  11. Companion cells present in Phloem.

Reference

Class Lecture of Dr. M. Oliur Rahman,

Department of Botany, University of Dhaka.

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