Ch. 3 Kingdom Plantae - Phanerogams

 

3.3 Salient features of major plant groups under Phanerogams -
Gymnosperms (Gymnos : naked, sperma : seed)

•        About 70 genera and 1000 living species of Gymnosperms in world.

•        In India, it is represented by 16 genera and 53 species.

•        Most of the Gymnosperms are evergreen, shrubs or woody trees.

•        These are primitive group of flowering plants producing naked seeds.

•        Seeds are not covered by fruit.

•        They are vascular plants having

–      Xylem with tracheids and

–      Phloem with sieve cells.

•        The plant body is sporophyte.

•        It is differentiated into root, stem and leaves.

•        The root system is tap root type.

•        In some, roots form symbiotic association with other life forms.

•        Coralloid roots of Cycas show association with blue green algae and

•        Roots of Pinus show association with endophytic fungi called mycorrhizae.

•        Stem is mostly erect, aerial, solid and cylindrical.

–      Secondary growth is seen in Gymnosperms due to presence of cambium.

–      In Cycas it is usually unbranched, while in conifers it is branched.

•        The leaves are dimorphic.

–      The foliage leaves are green, simple needle like or pinnately  compound, where as

–      Scale leaves are small, membranous and brown. 

•        Spores are produced by microsporophyll (Male) and megasporophyll (Female).

 

Economic importance –

•        Cycas is grown as ornamental plant.

•        Pinus is used as source of

–      pine wood,

–      turpentine oil and

–      pine resin.

Do you know?

•        Gymnosperms like Ginkgo biloba is called living fossil.

•        It is because the plant is found in living as well as fossil form and the number of fossil forms is much more than the living forms.

•        Gymnosperms vary in their size.

•        e.g.

–      Sequoia sempervirens is the tallest living plant in the world.

•        It is commonly called coast red wood of California.

•        The height of the plant is about 366 feet.

–      Taxodium mucronatum has a girth of about 125 feet.

–      Zamia pygmaea is the smallest Gymnosperms and is about 25 cm only.

Try this

•        the leaves of Hibiscus, Peepal, Canna, Grass and Tulsi.

•        Classify them as Monocot and Dicot.

Can you recall?

•        1. What are the salient features of Angiosperms?

•        2. What is double fertilization ?

•        3. Explain in brief two classes of Angiosperms. Draw and label one example of each class.

B. Angiospermae (Angios: enclosed: vessel, Sperma: seed)

•        Angiosperms are the most advanced group of flowering plants.

•        In these plants the seeds are enclosed within the fruit.

•        Angiosperms is a group of highly evolved plants, primarily adapted to terrestrial habitat.

•        They vary in size.

•        Angiosperms show heteromorphic alternation of generation.

–      The sporophyte is diploid, dominant, autotrophic and independent.

–      The gametophytes (male or female) are recessive, haploid and dependent on the sporophyte.

•        Angiosperms are heterosporous.

–      Microspores (commonly called pollens) are formed in microsporangia (or anthers).

•        They develop in highly specialized microsporophyll or stamens while

–      Megaspores are formed in megasporangia (or ovules)

•        They are borne on highly specialized megasporophyll called carpel.

•        Besides the essential whorls of microsporophylls (Androecium) and megasporophylls (Gynoecium) there are accessory whorls namely calyx (sepals) and corolla (petals) arranged together to form flowers.

Do you know?

•        Wolffia is the smallest Angiosperm, 1mm in size and

•        Eucalyptus grows to over 100 meters.

a. Dicotyledonae:

•        These plants have two cotyledons in their embryo.

•        They have a tap root system.

•        The stem is branched.

•        Leaves show reticulate venation.

•        The flowers show tetra or pentamerous symmetry.

•        Vascular bundles are conjoint, collateral and open type.

•        Cambium is present between Xylem and Phloem for secondary growth.

•        In Dicots secondary growth is commonly found.

e.g.

–      Helianthus annuus (Sunflower),

–      Hibiscus rosa-sinensis (China rose).

 

a. Monocotyledonae:

•        These plants have single cotyledon in their embryo.

•        They have adventitious root system.

•        Stem is rarely branched.

•        Leaves generally have sheathing leaf base and parallel venation.

•        The flowers are generally trimerous.

•        The vascular bundles are conjoint, collateral and closed type.

•        In Monocots, except few plants secondary growth is absent.

•        e.g.

–      Zea mays (Maize),

–      Sorghum vulgare (Jowar).

Can you tell?

•        . Give general characters of Gymnosperms and Angiosperms.

•        2. Distinguish between Dicotyledonae and Monocotyledonae.

•        3. Why do Dicots show secondary growth while Monocots don't?

3.4 Plant life cycle and alternation of generations:

•        Life cycle of a plant includes two phases or distinct generations namely

–      sporophyte (diploid : 2n) and

–      gametophyte (haploid : n).

•        Some special diploid cells of sporophyte divide by meiosis to produce haploid cells.

•        These haploid cells divide mitotically to give rise to gametophyte.

•        The gametophyte produces male and female gametes which fuse during fertilization to produce diploid zygote.

•        It divides by mitosis to form diploid sporophyte.

•        The sporophytic and gametophytic generations generally occur alternately in the life cycle of a plant.

•        This phenomenon is called alternation of generations.

•        In Bryophytes and Pteridophytes

–      Distinct alternation of these two generations is observed.

•        In Gymnosperms and Angiosperms,

–      gametophyte is much reduced and exists within sporophyte.

•        In algae, based upon the nature of dominant phase in life cycle,

–      It is called haplontic, diplontic or haplo-diplontic life cycle.

•        In Bryophytes

–      Haploid gametophyte is dominant.

–      It is photosynthetic, independent thalloid or erect phase.

–      Sporophyte is short lived, multicellular and depends totally or partially on gametophyte for nutrition and anchorage.

•        Whereas in Pteridophytes,

–      Sporophyte is dominant, independent and vascular plant body.

–      Haploid multicellular gametophyte is generally autotrophic and short lived.

–      It alternates with Sporophyte.

Diplontic:

•        Here mitotic divisions occur only in diploid cells.

•        Gametes formed through meiosis are haploid in nature.

•        The diploid zygote divides mitotically.

•        In this process production of multicellular diploid organism or in the production of many diploid single cells takes place.

–      e.g., Animals.

Haplontic:

•        Here mitosis occurs in haploid cells.

•        It results in the formation of

–      single haploid cells or

–      a multicellular haploid organism.

•        These forms produce the gametes through mitosis.

•        Zygote is formed after fertilization.

•        This cell is the only diploid cell in the entire life cycle of the organism.

•        Thus, the same zygotic cell later undergoes meiosis.

–      e.g., Some Algae and Fungi.

Haplo-diplontic:

•        Here mitosis occur in both diploid and haploid cells.

•        These organisms undergo through a phase in which they are multicellular and haploid (the gametophyte), and a phase in which they are multicellular and diploid (the sporophyte).

–      e.g., Land plants and in many algae.

Can you tell?

•        1. What is alternation of generations?

•        2. Which phase is dominant in the life cycle of Bryophyta and Pteridophyta?

🙏Thank You🙏