SUMMARY
Algae, bryophytes, pteridophytes, gymnosperms, and angiosperms are all members of the plant kingdom. Algae are mostly aquatic, simple, thalloid, autotrophic organisms that contain chlorophyll. Algae are divided into three classes, Chlorophyceae, Phaeophyceae, and Rhodophyceae, based on the type of pigment they possess and the type of food they store. Algae typically reproduce sexually by creating gametes that may exhibit isogamy, anisogamy, or oogamy, vegetatively by fragmentation, and asexually by creating various types of spores.
Plants known as bryophytes may survive in soil but require water for sexual reproduction. They have a more varied plant structure than algae. It is thallus-like, upright or protruding, and rhizoids hold it to the substrate. They have structures that resemble stems, leaves, and roots. Liverworts and mosses are two types of bryophytes. While mosses have erect, thin axes containing spirally arranged leaves, liverworts have a thallold and dorsiventral plant body. A gametophyte is the term for the primary plant body of a bryophyte, which produces gametes. It contains both the female archegonia and the male antheridia sex organs. The resulting male and female gametes unite to create a zygote, which gives rise to a multicellular organism known as a sporophyte. It creates spores that are haploid. Gametophytes are created when the spores germinate.
In pteridophytes the main plant is a sporophyte which is differentiated into true root, stem and leaves. These organs possess well-differentiated vascular tissues. The sporophytes bear sporangia which produce spores. The spores germinate to form gametophytes which require cool. damp places to grow. The gametophytes bear male and female sex organs called antheridia and archegonia. respectively. Water is required for transfer of male gametes to archegonlum where zygote is formed after fertilisation. The zygote produces a sporophyte. The gymnosperms are the plants in which ovules are not enclosed by any Ovary wall. After fertilisation the seeds remain exposed and therefore these plants are called naked-seeded plants. The gymnosperms produce microspores and megaspores which are produced in microsporangia and megasporangia borne on the sporophylls. The sporophylls - microsporophylls and megasporophylls - are arranged spirally on axis to form male and female cones, respectively. The pollen grain germinates and pollen tube releases the male gamete into the ovule, where it fuses with the egg cell in archegonia Following fertilisation, the zygote develops into embryo and the ovules into seeds.
The male sex organs (stamen) and female sex organs (pistil) are carried in a flower in angiosperms. Each stamen has a filament and an anthers. After meiosis, the anther produces pollen grains (male gametophyte). An ovary housing one to several ovules makes up the pistil. The female gametophyte, or embryo sac, which houses the egg cell, is found inside the ovule. Two male gametes are released from the embryo sac when the pollen tube penetrates it. Both male gametes merge with a diploid secondary mucleus after one fuses with an egg cell (syngamy) (triple fusion). Angiosperms are the only organisms that experience this phenomena of two fusions, known as double fertilisation. The dicotyledons and the monocotyledons are the two classes that make up the angiosperms.
Any sexually reproducing plant has a cycle in which generations of gametophytes, which produce gametes, and sporophytes, which produce spores alternate. Ahaplontic, diplontic, or intermediate life cycle patterns can be seen in various plant groups as well as individuals.
