Plant Kingdom

The plant kingdom (Plantae) is divided into major groups based on plant body organisation, presence of vascular tissue, seed habit, and enclosure of ovules.

Algae are simple, chlorophyll-bearing, aquatic (or moist terrestrial) autotrophs. They lack roots, stems, and leaves — their plant body is a thallus.

• Chlorophyceae (green algae) — chlorophyll a and b; food stored as starch; e.g., Chlamydomonas, Volvox, Spirogyra, Ulva.
• Phaeophyceae (brown algae) — chlorophyll a, c and fucoxanthin; food stored as laminarin; e.g., Ectocarpus, Laminaria, Fucus.
• Rhodophyceae (red algae) — phycoerythrin (red pigment); food stored as floridean starch; e.g., Polysiphonia, Porphyra, Gracilaria.

Bryophytes are called the amphibians of the plant kingdom because they need water for sexual reproduction (for fertilisation) but can live on land. They lack true roots (have rhizoids instead) and vascular tissue.

• Liverworts — flat, ribbon-like thallus; asexual reproduction by fragmentation or gemmae; e.g., Marchantia.
• Mosses — protonema stage after spore germination; gametophyte more elaborate; e.g., Funaria, Sphagnum.

In bryophytes, the gametophyte is the dominant phase; the sporophyte is dependent on the gametophyte for nutrition.

Pteridophytes are the first truly vascular plants — they have xylem and phloem — but reproduce by spores (cryptogams). They are the first plants to show a dominant sporophyte generation.

• True roots, stems, and leaves are present.
• Examples: Selaginella, Equisetum, Pteris, Dryopteris.
• Homosporous — produce one type of spore (most pteridophytes).
• Heterosporous — produce microspores and megaspores; e.g., Selaginella, Salvinia. Heterospory is significant as it is a precursor to the seed habit.

Gymnosperms — seed plants with naked ovules (not enclosed in an ovary). The word means 'naked seed'.

• Bear male cones (microsporophylls) and female cones (megasporophylls).
• Cycas has separate male and female plants (dioecious). Pinus has both on the same plant (monoecious).
• Polyembryony is common in gymnosperms.
• Examples: Cycas, Pinus, Cedrus, Gnetum, Sequoia (tallest living tree).

Angiosperms — flowering plants with enclosed ovules inside the ovary; form seeds enclosed in fruit after double fertilisation.

• Double fertilisation is unique to angiosperms — one sperm fertilises the egg (forms zygote); the other fuses with the polar nuclei (forms triploid endosperm).
• Most diverse plant group; range from Wolffia (smallest) to tall trees.