Biological Classification - Kingdom Plantae

Kingdom Plantae includes all eukaryotic, multicellular, chlorophyll-containing organisms characterized by a cellulosic cell wall. They follow an 'Alternation of Generations' between the haploid gametophytic ($n$) and diploid sporophytic ($2n$) phases.

Algae are simple, thalloid, autotrophic organisms classified into three main classes based on pigments: Chlorophyceae (Green Algae - $Chlorophyll\ a, b$), Phaeophyceae (Brown Algae - $Chlorophyll\ a, c$ and Fucoxanthin), and Rhodophyceae (Red Algae - $Chlorophyll\ a, d$ and r-phycoerythrin).

Bryophytes are known as the 'Amphibians of the Plant Kingdom' because they live in soil but depend on water for sexual reproduction. The dominant phase is the free-living haploid gametophyte ($n$).

Pteridophytes are the first terrestrial plants to possess vascular tissues ($Xylem$ and $Phloem$). The main plant body is a sporophyte ($2n$), and they exhibit evolutionary traits like heterospory (production of two types of spores: microspores and megaspores).

Gymnosperms are plants in which the ovules are not enclosed by an ovary wall and remain exposed both before and after fertilization. They produce 'naked' seeds ($Gymnos$ = naked, $Sperma$ = seeds).

Angiosperms (Flowering Plants) are characterized by double fertilization, where one male gamete fuses with the egg (Syngamy) to form a zygote ($2n$), and the second male gamete fuses with the two polar nuclei (Triple Fusion) to form the Primary Endosperm Nucleus ($3n$).