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Gymnosperms Definition and Examples

Introduction:

  • The term Gymnosperms was coined by Theophrastus.
  • The study of Gymnosperm is called Gymnospermology.
  • ‘Gymnosperm’ word comprises two Greek words which are ‘Gymnos’ means naked and ‘Sperma’ means seed. So, gymnosperms are plants in which ovules are exposed and not enclosed within the ovary. Thus, the seeds they produced are naked and not covered with a fruit wall.
  • These are seed-bearing plants that do not produce flowers.

Introduction:

  • These are perennial woody plants or ancient seed-bearing phanerogamic sporophytic plants without flowers, ovaries, and fruits.
  • They are popularly called naked seeded vascular plants.

Detailed Explanation:
 

Salient features:
 

1. Habit and Habitat:

  • Gymnosperms include medium-sized trees or tall trees (Sequoia) and shrubs (Ephedra).
  • The giant redwood tree Sequoia is one of the tallest tree species.
  • These are found mainly in cold temperate climates but cycads occur in warmer areas.
  • In India, Gymnosperms are found in the Himalayan mountains.

2. Morphological characters:
    a. Root:

  • The roots are generally tap roots.
  • Roots in some genera have fungal association in the form of mycorrhiza (Pinus), while in some others (Cycas) small specialized roots called coralloid roots are associated with N2-fixing cyanobacteria.

      b. Stem :

  • The stems are unbranched (Cycas) or branched (Pinus, Cedrus / Deodara).

      c. Leaf :

  • The leaves may be simple or compound.
  • In Cycas the pinnately compound leaves persist for a few years (Perennial leaves).
  • The leaves in gymnosperms are well-adapted to withstand extremes of temperature, humidity and wind (Xerophytic adaptation).
  • In conifers (e.g. Pinus), the needle-like leaves reduce the surface area and their thick cuticle and sunken stomata also help to reduce water loss.

3. Anatomical character :

  • Stem bears eustelic condition.
  • Vessels are absent in the xylem of gymnosperms except Gnetales (e.g. Welwitschia, Ephedra, Gnetum).
  • Phloem has sieve cells and albuminous cells and sieve tubes and companion cells are absent.
  • Xylem has tracheids but vessels are absent.
  • Secondary growth occurs in stem and root.
  • Bordered pits are present.
  • Wood of gymnosperms is homoxylous, nonporous and soft.
  • Transfusion tissue is found in gymnosperm leaves around the vascular bundle. It includes tracheid (dead cell), Parenchyma and albuminous cell.

Life cycle of gymnosperms
 

life-cycle


(1) The gymnosperms are heterosporous as they produce haploid microspores and megaspores.

(2) The two kinds of spores are produced within sporangia that are borne on sporophylls which are arranged spirally along an axis to form lax (loosely packed) or compact strobili or cones.

(3) The male or female cones or strobili may be borne on the same tree (Pinus- Monoecious) and in Cycas and Ginkgo (Dioecious) male cones and megasporophylls are borne on different trees.

(4) Male cone -

  • The strobili bearing microsporophylls and microsporangia are called microsporangiate or male strobili.
  • The microspores develop into a male gametophytic generation which is highly reduced and is confined to only a limited number of cells (Cycas - 5 cells and Pinus - 6 cells).
  • This reduced gametophyte is called a pollen grain.
  • The development of pollen grains takes place within the microsporangia.

(5) Female cone -

  • The cones bearing megasporophylls with ovules or megasporangia are called megasporangiate or female strobili.
  • The ovules are borne on megasporophylls which may be clustered to form the female cones.
  • In Cycas, female cones are absent.
  • The nucellus (2n) is protected by envelopes / integument (unitegmic) and the composite structure is called an ovule or integumented megasporangium.
  • Usually the megasporangium in gymnosperms is orthotropous (straight ovule), unitegmic and sessile.
  • The megaspore mother cell (2n) gets differentiated from one of the cells of the nucellus.
  • The megaspore mother cell divides meiotically to form four megaspores (n) out of which 3 megaspore degenerates.
  • One of the megaspores (functional) enclosed within the megasporangium develops into a multicellular female gametophyte (also known as endosperm) that bears two or more archegonia or female sex organs.
  • Archegonia lacks neck canal cells.
  • As the endosperm is formed before fertilization hence it is a haploid structure.
  • The multicellular female gametophyte is also retained within megasporangium / ovule.

Note:

i. Unlike bryophytes and pteridophytes, in gymnosperms the male and the female gametophytes do not have an independent free-living existence. They remain within the sporangia retained on the sporophytes.
ii. The development of male gametophyte and female gametophyte take place within microsporangia and megasporangia respectively.

(6) Pollination and Fertilization -

  • The pollen grain is released from the microsporangium.
  • They are carried in air currents (Anemophily) and come in contact with the opening of the ovules borne on megasporophylls.
  • Pollen grain is released in 3 celled stage in Cycas and 4 celled in Pinus.
  • The pollen tube carrying the male gametes (two) grows towards archegonia in the ovules and discharges their contents (two male gametes) near the mouth of the archegonia.
  • Pollen chamber refers to the cavity in the nucellus for resting pollen grains after pollination.
  • One male gamete fuses with a female gamete and another male gamete degenerates. The fertilization is performed which results in zygote and then embryo formation within the ovule.
  • After fertilization, zygote develops into an embryo and the ovules into seeds.
  • These seeds are not covered by ovary (naked seed).


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Note:

i. Cycas, Ginkgo and Sequoia are considered as ‘living fossils’. Ginkgo is extinct in the wild.
ii. Ovule, male gamete, egg and male cone of Cycas are largest in the plant kingdom.
 

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Gymnosperms differs from the pteridophytes in a number of ways which are summarized as below-
 

Pteridophytes Gymnosperms
1. They are found in moist and shady places. 1. They are xerophytic
2. Secondary growth is absent 2. Secondary growth is quite common.
3. Ovules absent 3. Ovules present
4. Pollen tube is not formed 4. Pollen tube is formed.
5. Neck canal cell is found in the archegonium. 5. It is absent in archegonium.
6. Seed formation doesnot take place. 6. Seed formation takes place.


Classification of Gymnosperms
 

Introduction:

  • Gymnosperms are the spermatophytes that lack flowers and fruits.
  • They are heterosporous and possess naked seeds.
  • They form the smallest group of the plant kingdom.
  • They are placed between pteridophytes and angiosperms.
  • The most common and accepted scheme of classification of gymnosperms is into four groups-
    1. Cycads
    2. Conifers
    3. Ginkgophytes
    4. Gnetophytes

Detailed Explanation:
 

1. Cycads:

  • They are generally found in warm climates and are xerophytic, having a palm-like trunk and fern-like leafy appearance.
  • They possess pinnately compound leaves which exhibit circinate vernation when young.
  • They are heterosporous with motile male gametes born in male cones and female gametes formed in female cones.
  • In the genus Cycas, female cones are absent, rather female gametes are formed in seed bearing megasporophylls.
  • They have less xylem in their stems.
  • Due to their beautiful shape, they are frequently used as ornamental plants in the gardens of tropics and subtropics areas.


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2. Conifers:

  • The members are cone-shaped trees and thus, are known as conifers.
  • Conifers form the dominant gymnosperm group, with the highest species diversity.
  • Conifers are tall trees with needle-like leaves. The narrow shape and thick cuticle of leaves reduce water loss through transpiration.
  • Snow easily slips off the needle-shaped leaves, maintaining the snow load light and thus reducing breaking of the branches.
  • These adaptations to tolerate the cold and dry weather help to explain why conifers predominate at high altitudes and in cold areas.
  • Conifers include the well-known evergreen trees pines, firs, cedars, spruces, sequoias, etc.
  • Very few species are deciduous, meaning they lose their leaves in the fall.
  • Deciduous conifers include the Bald cypress, European larch, Dawn redwood, and Tamarack.
  • Coniferous trees are harvested in high quantities for gaining paper pulp and timber.
  • Conifer wood is more ancient than angiosperm wood. It bears tracheids but no vessel elements and is thus referred to as "softwood."

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3. Ginkgophytes :

  • Most of the members have become extinct,Ginkgo biloba is the only surviving species.
  • It is also called maidenhair tree because of the resemblance of its bilobed leaves to those of the maidenhair fern. It is regarded as the oldest living seed plant, and as one of the wonders of the world.
  • It belongs to higher gymnosperms but its male gametes are motile.
  • Its leaves exhibit dichotomous venation and are fan shaped. They change their color and turn yellow in autumn and fall from the tree.
  • Ginkgo biloba was cultivated in monasteries for centuries by Chinese Buddhist monks, ensuring its preservation.
  • It is cultivated in public places due to its unusually high resistance to pollution.
  • Male and female plants are different as only one type of sex organ is produced on one plant.
  • Gardeners typically grow only male trees since the seeds formed by female plants have an unpleasant odor similar to that of rancid butter.


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4. Gnetophytes:

  • The phylogenetic status of gnetophytes is currently unknown.
  • Because they have vessel elements, they are closely related to the present angiosperms.
  • Molecular analysis, however, puts them relatively close to the conifers.
  • The three living genera are very different: Gnetum, Ephedra, and Welwitschia.
  • They have broad leaves similar to that of angiosperms.
  • Archegonia is generally absent in them with the exception of Ephedra.
  • Ephedra can be found in dry areas and its scale-like small leaves produce the chemical compound ephedrine, which is used in medicine to treat blocked nose.
  • Gnetum species include trees, shrubs, and vines and can be found in parts of South America, Africa, and Southeast Asia.
  • Welwitschia is reported in the Namib desert and is supposed to be the group's oldest member. It has only two leaves that grow continuously all through the plant's life.
  • Welwitschia, like Ginkgo, forms male and female sex organs on different plants.


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Economic Importance of Gymnosperms
 

Introduction:

  • Gymnosperms are plants in which ovules are exposed and not covered with the ovary. Thus, the seeds they produced are naked and not covered with a fruit wall.
  • These are seeded plants without flowers and fruits.
  • Gymnosperms are economically more important than bryophytes and pteridophytes.
  • Gymnosperms are an excellent food source. Their seeds are broadly used to make edible spices in the production of a variety of food products. These plant species involve Ginkgo, Pinus, and Cycas, among others.
  • Apart from food, gymnosperms are broadly used in the pharmaceutical manufacturing of various medicines that are helpful in the treatment of infectious diseases and other allergies such as cough, cold, asthma, colds, bronchitis, etc.
  • Oil derived from the woody parts of gymnosperms is used in making cosmetics like perfumes, deodorants, and room fresheners.
  • Aside from food and medicine, some of the gymnosperm trees are grown extensively in parks, gardens, and other places due to their ornamental leaves.

Detailed Explanation:
 

Economic importance of Gymnosperms:

(1) Source of Wood - e.g. Cedrus deodara (deodar, strongest of all soft wood), Sequoia (red wood tree), Pseudotsuga (Douglas fir), Taxodium, Taxus (Yew).
(2) Food - Sago is a kind of starch obtained from cortex and pith of stem and seeds of Cycas. The Roasted seeds of Pinus gerardiana(Chilgoza) are used as dry fruit. Seeds of Ginkgo biloba are eaten in China and japan.
(3) Cedar wood oil is obtained from the stem of Juniperus virginiana (Red cedar) and used as immersion oil in oil immersion lenses.
(4) Medicinal use - Ephedrine is obtained from stem branches of Ephedra and used to cure cough, cold, bronchitis, asthma and fever. Taxolis extracted from Taxus baccata (yew) and used in the treatment of cancer.
(5) Canada balsam is a turpentine extracted from Abies balsamea used in mounting of permanent slides.
(6) Many Gymnosperms are grown in the gardens as ornamental plants e.g. Cycas, Taxus, Thuja (Morpankh), Araucaria excelsa (x-mas/christmas tree), Ephedra, Cupressus, Ginkgo (Pagoda tree / Maidenhair tree), Araucaria imbricata (Monkey’s puzzle).

Frequently asked questions (FAQ)

Q1. What is taxol and from where it is obtained?
Ans.

  • Taxol is an anticancer drug derived from the Taxus tree bark and is used to cure various cancers.
  • Taxus, a coniferous tree genus, is well-known for the extraction of the drug taxol.

Q2. What are the exceptional features of Cycas ?
Ans.

Cycas also known as sago palm has the largest ovule, male gamete, egg and male cone in the plant kingdom. Its male gametes are top shaped. In It female cone is not found. The embryo consists of 2 cotyledons.

Q3. What is the name of the tallest gymnosperm tree species?
Ans.

The tallest conifer species, Sequoiadendron giganteum belonging to the family Sequoioideae, is commonly known as Redwood or Giant Sequoia and can grow to heights of more than 100 meters.

Q4. Which gymnosperm species have mycorrhizal and coralloid roots? What is the meaning of these terms?
Ans.

  • Mycorrhiza is a symbiotic association between a fungus and the roots of a vascular plant. Mycorrhiza is found in some genus of gymnosperms; like Pinus.
  • Coralloid Roots: These roots show a symbiotic association but in this case, the association is with nitrogen-fixing cyanobacteria. Many nodules are formed in the roots giving them coralloid appearance. Examples are Cycas and plants of Leguminosae.

Q5. What are the features of gymnosperm leaves that make them distinct from other types of plants?
Ans.

  • The gymnosperms have needle-like leaves in general.
  • The leaves can be either simple or compound.
  • Their leaves are well-adapted to tolerate temperature, humidity, and wind extremes.
  • The needle-like leaves of conifers reduce surface area.
  • The thick cuticle and sunken stomata of the leaves also aid in water conservation as they reduce the loss of water.

Q6. What is a pollen chamber?
Ans.

Pollen chamber refers to the cavity in the nucellus for resting pollen grains after pollination.

Q7. Why Cycas is also called a ‘relic of the past’?
Ans.

Cycas is among the few surviving members of the gymnosperm subclass Cycadeae and is known as a "relic of the past" because it exhibits many characteristics similar to pteridophytes, such as flagellated antherozoids, leaf-like megasporophyll, the presence of archegonia, and so on.

Q8. What is the ploidy of the endosperm in gymnosperms?
Ans.

The megaspore mother cell divides meiotically to form four megaspores (n) out of which 3 megaspore degenerates. One of the megaspores (functional) enclosed within the megasporangium develops into a multicellular female gametophyte (also known as endosperm) that bears two or more archegonia or female sex organs. As the endosperm is formed before fertilization hence it is a haploid structure.

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