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Primitive Characters of Cycas and Advanced features of Gnetum

A primitive Cycas. Source: http://plantsrescue.com/wp-content/uploads/2013/09/Cycas-revoluta-new-growth.jpg

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

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


CYCAS

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

 

General Characteristics of Cycas

Cycas is a slow-growing and short lived evergreen genus. It’s characteristic features such as root, stem and leaf are as follows:

ROOT
  • The primary root of Cycas is a short-lived tap root. Later on, adventitious roots replace this root.
  • Most of the lateral branches of primary root are known as normal roots. These penetrate the soil.
CORALLOID ROOT

Some of the roots develop near the surface of the ground. These 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. Thus become swollen at their tips due to entry of algae, bacteria and fungi.

However, they behave like aerotropic or negatively geotropic structures that come out in the air above the soil surface. They are 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.
  • Coralloid roots contain some microorganisms which are as follows:
  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. In very few cases, stem may be branched.

  • When the stem is young, it is tuberous, short and subterranean.
  • A number of large and small rhomboidal leaf bases cover up most of the older stem.
LEAF
  • There are two types of leaves in Cycas such as:

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

  • A crown of spirally arranged leaves are present around the stem apex. As a result, 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

    The young leaves have circinately coiled leaflets like those of ferns. Hence, this 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.
  • Cycas possess Coralloid roots.
  • Leaves are pinnately compound and spirally arranged at the top of the stem.
  • Circinate vernation is present in leaves which are young.
  • Microsporophylls (Male sporophylls) form strobilus and grouped into cones. On the contrary, megasporophylls (Female sporophylls) do not form strobilus and never grouped into cones.
  • Ovules are naked and very large. These 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 and thick walled epidermis.
  • Highly thickened hypodermis on both sides of the leaflet.
  • Sunken stomata restricted only to the lower surface.
  • Unbranched midrib is present.
  • 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. Here, Gnetum is the most advanced species.

Gnetum. Details
Systematic Position

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

 

 

Advanced characters of Gnetum

Gnetum resembles with angiosperms in some aspects. These are as follows:

  1. Gnetum is climber or tree-like and resembles angiosperms.
  2. Leaves are broad, green with reticulate venation. 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. Free nuclear division occurs 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 are present in xylem.
  10. Vessels which are similar to those of angiosperms, occur together with tracheids in the secondary wood of Gnetum.
  11. Companion cells are present in Phloem.

Related Questions people may ask:

  • Why are cycads so expensive?

Answer: Cycads are extremely important due to their ancient connections. In addition, they are bigger in size which says about their commercial value. These qualities make Cycads expensive in a large scale.

  • What is the most unique feature of Gnetum?

Answer: The most unique character of Gnetum is that it has a single straight trunk which is marked with conspicuous rings.

  • Is Gnetum a pteridophyta?

Answer: No, Gnetum is not a pteridophyta. But it shows some similiraties with Pteridophyta. For example, it possess vessels and companion cells in xylem and phloem like pteridophytes.


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About Saifun Nahar Smriti

Currently in 2nd year at Department of Botany, University of Dhaka. Curious about the vast Plant science world & wants to contribute some fruitful contents to Plantlet. E-mail: smritisaifunnahar@gmail.com. Minimum Monthly Resolution- Publish (1) Revise (2) Share (2)

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Somudro Mahbub Nasir
Somudro Mahbub Nasir
Admin
2 months ago

informative.

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