Key Points

The Plant Kingdom is classified into five major groups: Algae, Bryophytes, Pteridophytes, Gymnosperms, and Angiosperms.

Classification systems progressed from artificial (e.g., Linnaeus) to natural (e.g., Bentham and Hooker) and finally to phylogenetic systems based on evolutionary relationships.

Algae are simple, thalloid, autotrophic, and largely aquatic organisms. Sexual reproduction can be isogamous, anisogamous, or oogamous.

Green algae contain chlorophyll $a$ and $b$, store food as starch, and have a cellulose cell wall. Examples include Volvox and Spirogyra.

Brown algae contain chlorophyll $a, c$, and fucoxanthin, storing food as mannitol and laminarin. The cell wall contains cellulose and algin.

Red algae contain chlorophyll $a, d$, and r-phycoerythrin, storing food as floridean starch. They lack flagellated stages.

Bryophytes are called amphibians of the plant kingdom because they live in soil but require water for sexual reproduction. The dominant plant body is a haploid gametophyte.

The gametophyte produces gametes. After fertilization, the zygote develops into a sporophyte that is dependent on the gametophyte for nourishment.

Bryophytes are divided into liverworts (e.g., Marchantia), which have a thalloid body, and mosses (e.g., Funaria), which have a life cycle with a protonema stage.

Pteridophytes are the first land plants to possess vascular tissues (xylem and phloem). The main plant body is a diploid sporophyte, differentiated into true root, stem, and leaves.

Most pteridophytes are homosporous (one type of spore), but genera like Selaginella and Salvinia are heterosporous (two types of spores), which is a precursor to the seed habit.

Gymnosperms are plants where the ovules are not enclosed by any ovary wall and remain exposed, both before and after fertilization. The seeds are therefore naked.

They are heterosporous, with microspores and megaspores produced in strobili or cones. They do not require water for fertilization as pollen is transferred by wind.

Angiosperms are flowering plants in which pollen grains and ovules are developed in flowers. The seeds are enclosed within a fruit, which develops from the ovary.

The dominant phase in bryophytes is the haploid gametophyte, while in pteridophytes, gymnosperms, and angiosperms, the dominant phase is the diploid sporophyte.