Plant Kingdom

Kingdom Plantae includes all eukaryotic, multicellular, chlorophyll-containing organisms that make their own food (photosynthetic autotrophs). The plant kingdom is divided into algae, bryophytes, pteridophytes, gymnosperms and angiosperms.

Algae are simple, chlorophyll-bearing, mostly aquatic plants (also found in moist soil, on stones and on other plants). Their body is a simple thallus (not differentiated into root, stem and leaf). They reproduce by vegetative, asexual (spores) and sexual methods. Based on their pigments, stored food and cell-wall composition, algae are grouped into three classes:

• Chlorophyceae (green algae) — green because of chlorophyll a and b; store food as starch; cell wall of cellulose. Examples: Chlamydomonas, Spirogyra, Ulothrix, Volvox.
• Phaeophyceae (brown algae) — brown due to the pigment fucoxanthin (plus chlorophyll a and c); store food as laminarin/mannitol; mostly marine. Examples: Laminaria, Fucus, Sargassum, Ectocarpus.
• Rhodophyceae (red algae) — red due to the pigment r-phycoerythrin; store food as floridean starch; mostly marine, found even in deep water. Examples: Polysiphonia, Gracilaria, Gelidium.

Algae are economically important: they perform about half of the total photosynthesis (fixing CO2 and adding oxygen); many (Porphyra, Laminaria, Sargassum) are eaten; agar (from Gelidium, Gracilaria) is used in lab cultures and foods.

Bryophytes are the 'amphibians of the plant kingdom' — they live on land but need water for sexual reproduction. The plant body is more differentiated than algae but lacks true roots, stem and leaves (it has root-like rhizoids). They have no vascular tissue (no xylem/phloem). The main plant body is the gametophyte (haploid). Examples include mosses (e.g. Funaria, Sphagnum) and liverworts (e.g. Marchantia). Bryophytes help in soil formation and prevent soil erosion; Sphagnum (peat moss) is used as fuel and packing material.

Pteridophytes are the first true land plants to have vascular tissue (xylem and phloem) and a body differentiated into true root, stem and leaves. They still need water for reproduction. Examples: ferns (Pteris, Adiantum), Selaginella, Equisetum (horsetail), Lycopodium. In pteridophytes the dominant plant body is the sporophyte (diploid), which bears spore-producing structures (sporangia) on leaf-like sporophylls; spores germinate into a small, free-living gametophyte (called the prothallus).

Most pteridophytes are homosporous (produce one kind of spore), but some (e.g. Selaginella, Salvinia) are heterosporous — they produce two kinds of spores (large megaspores and small microspores). This heterospory, where the female gametophyte is retained on the parent plant, is considered an important step towards the seed habit seen in higher plants.

Gymnosperms (Greek gymnos = naked, sperma = seed) are plants that bear naked seeds — the seeds (ovules) are not enclosed inside a fruit. They are mostly evergreen, woody trees and shrubs. They have well-developed vascular tissue, but the xylem lacks vessels and the phloem lacks companion cells. Examples: Pinus, Cycas, Cedrus, Ginkgo. The dominant body is the sporophyte, bearing male and female cones; pollination is mostly by wind.

Angiosperms (angio = covered) are the flowering plants, in which seeds are enclosed inside a fruit (formed from the ovary). They are the most advanced and largest group of plants. They show double fertilisation (a unique feature) and have flowers as their reproductive organs. Angiosperms are of two classes: dicotyledons (two seed leaves, reticulate leaf venation, e.g. mango, pea) and monocotyledons (one seed leaf, parallel venation, e.g. wheat, grass).

In all plants, the life cycle shows an alternation of generations: a haploid (n) gametophyte phase alternates with a diploid (2n) sporophyte phase. There are three patterns:

• Haplontic — the dominant phase is the haploid gametophyte; the diploid phase is just the zygote (e.g. many algae like Spirogyra).
• Diplontic — the dominant phase is the diploid sporophyte; the gametophyte is greatly reduced (e.g. seed plants — gymnosperms and angiosperms).
• Haplo-diplontic — both phases are multicellular and prominent (e.g. bryophytes and pteridophytes).