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Plantae are popularly referred to as the 'Plant Kingdom' in the classification of the five kingdoms. The plant kingdom includes algae, bryophytes, pteridophytes, gymnosperms, and angiosperms. Algae are simple, thalloid, autotrophic and largely aquatic organisms containing chlorophyll. Algae are classified into the following classes on the basis of pigment type and type of food stored:

1. Chlorophyceae,

2. Phaeophyceae,

3. Rhodophyceae.

Algae are chlorophyll-containing, simple, thalloid, autotrophic and largely aquatic (both freshwater and marine) organisms. Algae are found in many other habitats: moist rock, soil, and wood. Some algae are also found with animals and fungi. They usually reproduce vegetatively by fission, asexually by producing various types of spores, and sexually by producing gametes that may show isogamy, anisogamy or oogamy.

Divisions of Algae and their Main Characteristics

Bryophytes are plants that can live in soil but depend on water for sexual reproduction. Their plant body is more differentiated than that of algae.

It is thallus-like and prostrate and attached to the base by rhizoids.

They have root-like, leaf-like and stem-like structures. Bryophytes are divided into liverworts and mosses. The plant body of liverworts is thalloid and dorsiventral whereas mosses have straight, slender axes with spirally arranged leaves.

The main plant body of the bryophyte is the gamete-producer and is called the gametophyte. It consists of male sex organs called antheridia and female sex organs called archegonia.

Male and female gametes fuse to form the zygote which produces a multicellular body called a sporophyte. It produces haploid spores. The spores germinate to form the gametophyte.

They were mainly based on botanical traits or the androecium structure (the system given by Linnaeus).

Such systems were artificial, separating closely related species because they were based on certain characteristics. Artificial systems that gave equal importance to vegetative and sexual characteristics is not acceptable because we know that often vegetative traits are more easily influenced by the environment.

In contrast, natural classification systems evolved, which were based on natural similarities between organisms and consider.

In the case of pteridophytes, the main plant is a sporophyte which is differentiated into true root, stem and leaves. These organs contain well-differentiated vascular tissue. Sporophytes bear sporangia which produce spores. spores Germs germinate to form gametophytes that require cool, moist places to grow. Gametophytes have male and female sex organs called antheridia and archegonia, respectively. Water is required for the transfer of male gametes to the archegonium, where the zygote is formed after fertilization. The zygote produces a sporophyte.

Gymnosperms are plants in which ovules are not surrounded by any ovary wall. The seeds remain open after fertilization and hence these plants are called naked-seeded plants. Gymnosperms produce microspores and megaspores that form into microsporangia and megasporangia that form on sporophylls. Sporophils – Microsporophylls and megasporophylls are arranged spirally on the spindle to form male and female cones, respectively. The pollen grains germinate and the pollen tube releases the male gamete into the ovule, where it merges with the egg cell in the archegonia. After fertilization, the zygote develops into the embryo and the ovule into the seed.

In the case of angiosperms, male sex organs (stamens) and female sex organs (pistils) are borne in a flower. Each stamen has a filament and an anther. The anther produces the pollen grain (male gametophyte) after meiosis. The pistil has one ovary which contains one to many ovules. Within the ovum is the female gametophyte or embryo sac that contains the egg cell. The pollen tube enters the embryo sac where two male gametes are released. One male gamete fuses with the egg cell (parallel) and the other with the diploid secondary nucleus (triple fusion). This phenomenon of fusion of two is called double fertilization and is unique to angiosperms. Angiosperms are divided into two classes :

1. dicots and

2. monocotyledons.

During the life cycle of any sexually reproducing plant, there is an alternation of generations between the haploid gametophyte producing gametes and the diploid sporophyte producing spores. However, different plant groups as well as individuals may show different patterns of life cycle:

1. Haplontic,

2. Diplontic or

3. Intermediate.