Plant Kingdom

According to five kingdom classification, living organisms are classified into five Kingdoms namely, kingdom Monera, Kingdom Protista, Kingdom Fungi, Kingdom Animalia and Kingdom Plantae.

Kingdom Plantae includes algae, bryophytes, pteridophytes, gymnosperms and angiosperms.
Algae are chlorophyll-bearing, simple, thalloid and autotrophic plants. They are largely aquatic;both fresh and marine, but they are found in a variety of habitats like moist stones, soils and wood. The algae reproduce by vegetative, asexual and sexual methods. The sexual reproduction can be: isogamous, anisogamous and oogamous.

Depending upon the types of pigments, they can be classified as Chlorophyceae, Phaeophyceae and Rhodophyceae.

Bryophytes are found in soil but they are dependent on water for sexual reproduction. The plant body is thallus-like, differentiatedand prostrate or erect. The plant body is attached to the substratum by rhizoids. The main plant body of a bryophyte that produces gametes is called gametophyte. It bears the male sex organs called antheridia and female sex organs called archegonia.During fertilisation, the male and female gametes fuse to form zygote that produces a multicellular body called a sporophyte. It produces haploid spores. This spore germinates and form sporophyte.

The main plant body of pteridophytesis sporophyte and differentiated into true root,stem and leaves.Theyhave well-differentiated vascular tissues. The sporophytes bear sporangia. The sporangia are subtended by leaf-like appendages called sporophylls. The sporangia produce spores by the process of meiosis in spore mother cells.The spores germinate in cool and damp places and form gametophytes. The gametophytes bear male and female sex organs called antheridia and archegonia.  Water plays an important role in the process of fertilisation. The zygote formed after fertilisation, produces a sporophyte.

The pteridophytes are classified into four classes: Psilopsida, Lycopsida, Sphenopsida andPteropsida.

In gymnosperm plants the ovules are not enclosed by any ovary wall. In these plants the seeds remain exposed after fertilisation and therefore these plants are called naked-seeded plants. These plants produce microspores and megaspores. These spores are produced in microsporangia and megasporangia borne on the sporophylls. The sporophylls are of two types  microsporophylls and megasporophylls. These sporophylls are arranged spirally on axis and form male and female cones, respectively.  After fertilisation, the zygote develops into an embryo and the ovules develop into seeds.

In angiosperms, the male sex organs called stamen and the female sex organs called pistil are borne in a flower. Each stamen consists of a filament and an anther. The anther produces pollen grains. The pistil consists of an ovary. An ovary encloses one to many ovules. Within the ovule is the female gametophyte calledthe embryo sac contains the egg cell. During pollen germination, the pollen tube enters the embryo-sac where two male gametes are discharged. During fertilisation, one male gamete fuses with egg cell (syngamy) and other fuses with diploid secondary nucleus (triple fusion). This phenomenon of two fusions is called double fertilisation. Depending upon the number of cotyledons, angiosperms are divided into two classes – the dicotyledons and the monocotyledons.

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