Biology - Salient features of major plant groups under Phanerogams

Salient features of major plant groups under Phanerogams :

A. Gymnospermae (Gymnos : naked, sperma : seed) :-

There are about 70 genera and 1000 living species of Gymnosperms in world. In India it is represented by 16 gener

a and 53 species.

Cycas plant details

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 i.e. ovary. 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. In Gymnosperms, 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 diamorphic. 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).

Pinus tree with cones

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 i.e. ovary. Angiosperms is a group of highly evolved plants, primarily adapted to terrestrial habitat. They vary in size. Angiosperms show heteromorphic alternation of generation in which 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) 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.

Angiosperms are subdivided into two classes:

a. Dicotyledonae : 

These plants have two cotyledons in their embryo. They have a tap root system and the stem is branched. Leaves show reticulate venation while 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. H elianthus annus (sunflower), H ibiscus rosa- sinensis (China rose).

Helianthus annus (Sunflower)

b. Monocotyledonae : 

These plants have single cotyledon in their embryo. They have adventitious root system and stem is rarely branched. Leaves generally have sheathing leaf base and parallel venation while 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).

Zea mays (Maize)

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.

Distinct alternation of these two generations is observed in Bryophytes and Pteridophytes. 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.

Alternation of generation

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.

Types of cycle

Diplontic : 

Here mitotic divisions occurs only in diploid cells. Gametes formed through meiosis are haploid in nature. The diploid zygote divide 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.