Plant Kingdom

The Plant Kingdom encompasses a diverse array of photosynthetic eukaryotes ranging from simple aquatic algae to complex flowering angiosperms. Classification is based on the presence of vascular tissue, seed formation, and the dominant phase in the life cycle (alternation of generations).

Algae are thalloid (no true roots, stems, or leaves), primarily aquatic, and classified by their pigment composition. Chlorophyceae (green algae) possess chlorophyll a and b, store starch, and have cellulose cell walls. Examples include Chlamydomonas (unicellular, motile), Volvox (colonial, hollow sphere), Ulothrix (filamentous, unbranched), Spirogyra (filamentous, ribbon-shaped chloroplast, reproduces by conjugation), and Chara (advanced, calcified). Phaeophyceae (brown algae) contain chlorophyll a and c plus the accessory pigment fucoxanthin (giving the brown colour), store laminarin, and have cell walls of cellulose and algin. Examples include Ectocarpus, Dictyota, Laminaria (kelp), Sargassum, and Fucus. Rhodophyceae (red algae) contain chlorophyll a and d plus the red pigment phycoerythrin, store floridean starch, and have cellulose cell walls. Examples include Polysiphonia, Porphyra, Gelidium, and Gracilaria — notably, Gelidium and Gracilaria are the commercial sources of agar (not brown algae).

Bryophytes are called the "amphibians of the plant kingdom" because they depend on water for fertilization (flagellated antherozoids must swim to the archegonium). They lack vascular tissue (no xylem or phloem) and the dominant phase is the gametophyte. Liverworts (Marchantia) have a flat, thalloid body with dorsiventral symmetry and reproduce asexually through gemma cups containing gemmae. Mosses (Funaria, Sphagnum) have a more complex gametophyte with a distinct protonema stage (filamentous, develops from spore) that gives rise to an upright leafy gametophore. The sporophyte is dependent on the gametophyte and consists of a foot, seta, and capsule.

Pteridophytes are the first vascular plants, possessing true xylem and phloem, allowing them to grow larger than bryophytes. The dominant phase shifts to the sporophyte, which is the familiar fern plant. The gametophyte (prothallus) is small, independent, and photosynthetic. Most pteridophytes are homosporous (producing spores of one kind — e.g., Dryopteris, Equisetum, Adiantum). Some are heterosporous, producing large megaspores and small microspores — Selaginella and Salvinia are the key heterosporous examples, considered evolutionary precursors to seed habit.

Gymnosperms ("naked seeds") produce seeds not enclosed within a fruit. Ovules are exposed on megasporophylls, often organised into cones. Pollination occurs by wind, and fertilization occurs via pollen tubes (no water needed). Examples include Pinus (most commonly tested, with needle-like leaves), Cycas (palm-like, dioecious, produces the largest ovules in the plant kingdom), Ginkgo biloba (living fossil, fan-shaped deciduous leaves), and Sequoia sempervirens (tallest living tree).

Angiosperms (flowering plants) have seeds enclosed within fruits (developed from the ovary wall). They are divided into Monocotyledons and Dicotyledons: monocots have one cotyledon, parallel venation, fibrous root system, scattered vascular bundles, trimerous flowers, and lack secondary growth; dicots have two cotyledons, reticulate venation, tap root system, vascular bundles arranged in a ring, pentamerous flowers, and undergo secondary growth.

Alternation of generations is a universal feature: the haploid gametophyte (produces gametes by mitosis) alternates with the diploid sporophyte (produces spores by meiosis). In bryophytes, the gametophyte is dominant and independent while the sporophyte is dependent. In pteridophytes, gymnosperms, and angiosperms, the sporophyte becomes increasingly dominant, and the gametophyte becomes progressively reduced — in angiosperms, the male gametophyte is just the pollen grain and the female gametophyte is the 7-celled embryo sac.

The key testable concept is the progressive shift from gametophyte dominance (bryophytes) to sporophyte dominance (pteridophytes onward), the distinction between homosporous and heterosporous pteridophytes, and the source of agar from red algae (Gelidium, Gracilaria).