Plant Kingdom

Algae are chlorophyll-bearing, simple, thalloid, largely aquatic autotrophs. Their body shows little differentiation into root, stem and leaf, ranging from microscopic single cells (Chlamydomonas) to colonies (Volvox) and large filaments (Spirogyra, Ulothrix). They reproduce vegetatively (fragmentation), asexually (commonly by zoospores) and sexually, where the fusion of gametes may be isogamous (similar gametes), anisogamous (dissimilar size) or oogamous (large non-motile egg + small motile sperm, as in Volvox, Fucus).

Algae are classified into three classes, and NEET reliably tests the table of pigments, stored food and cell wall for each. Chlorophyceae (the green algae) contain chlorophyll a and b giving a grass-green colour; they store food as starch and have a cellulose cell wall. Common members are Chlamydomonas, Volvox, Ulothrix, Spirogyra and Chara.

Phaeophyceae (the brown algae) possess chlorophyll a and c plus the brown carotenoid fucoxanthin, which masks the green and gives them their colour. Their stored food is laminarin or mannitol, and the cellulose wall is covered by a gelatinous coating of algin. They are mostly marine and include large 'seaweeds' such as Laminaria, Sargassum, Fucus, Ectocarpus and Dictyota.

Rhodophyceae (the red algae) are predominantly marine and red because of the dominant pigment phycoerythrin (with chlorophyll a and d); they store food as floridean starch. Examples are Polysiphonia, Porphyra, Gracilaria and Gelidium. Economically, algae are vital: agar is obtained from Gelidium and Gracilaria, algin from brown algae and carrageen from red algae, while many (e.g. Porphyra, Sargassum) are eaten as food.

Bryophytes — the liverworts, hornworts and mosses — are the simplest land plants and are aptly called the 'amphibians of the plant kingdom': they live in damp, shaded places on land but still need water for sexual reproduction. The plant body is more differentiated than algae but lacks true roots, stem and leaves; instead it has root-like rhizoids for anchorage and leaf-like and stem-like structures.

A defining feature, frequently tested, is that the dominant, photosynthetic, free-living plant body is the haploid gametophyte. It bears the sex organs: the male antheridium produces flagellated antherozoids (sperms), and the female archegonium is flask-shaped and produces a single egg. The antherozoids are released into water and swim to the archegonium — which is exactly why a film of water is essential.

After fertilisation, the zygote develops into a multicellular sporophyte. Unlike in algae, this sporophyte is not free-living: it remains attached to and partially dependent on the gametophyte for nutrition. The sporophyte produces haploid spores by meiosis, and these germinate to form the next gametophyte generation — so bryophytes show a clear alternation of generations with the gametophyte dominant.

The two main groups are easy to separate. Liverworts (e.g. Marchantia) have a flat, dorsiventral thallus and reproduce asexually by special structures called gemmae borne in gemma cups. Mosses (e.g. Funaria, Sphagnum, Polytrichum) have a life cycle with two stages — a creeping, green, branched protonema stage followed by the upright leafy stage that bears the sex organs. Ecologically bryophytes are important pioneers that reduce soil erosion, and Sphagnum (peat moss) is used as packing material and a fuel source.

Pteridophytes (ferns and their allies — Selaginella, Equisetum, Pteris, Adiantum, Lycopodium) are the first true vascular plants, the first to possess specialised conducting tissues, xylem and phloem. Here the dominant, conspicuous plant body is the diploid sporophyte, differentiated into true root, stem and leaves. The leaves bearing sporangia are called sporophylls, and meiosis in the sporangia produces haploid spores.

The spores germinate into a small, inconspicuous, free-living, photosynthetic gametophyte called the prothallus, which needs cool, damp, shady conditions to grow. The prothallus bears antheridia and archegonia, and — as in bryophytes — water is still required for fertilisation because the antherozoids must swim to the egg. This dependence on water and the need for two specific habitats restrict the spread of pteridophytes. Most are homosporous (one kind of spore), but Selaginella and Salvinia are heterosporous, producing small microspores and large megaspores — a condition regarded as a precursor to the seed habit.

Gymnosperms (e.g. Cycas, Pinus, Ginkgo, Ephedra) are seed plants in which the ovules are not enclosed in an ovary, so after fertilisation the seeds remain naked (uncovered) — hence the name (Greek gymnos, naked). They are mostly tall, woody, perennial trees and shrubs; the giant redwood Sequoia is among the tallest of all plants. The dominant body is again the sporophyte.

Gymnosperms are heterosporous, bearing microspores and megaspores in male and female cones (strobili). A key advance over pteridophytes is that water is no longer needed for fertilisation: pollen grains (the male gametophyte) are carried by wind and develop a pollen tube that delivers the male gametes directly to the ovule. The male and female gametophytes are tiny and never leave the parent sporophyte — they remain enclosed within the spores. These features make gymnosperms much better adapted to dry land than pteridophytes.

Angiosperms (the flowering plants) are the most advanced and dominant land plants. Their defining feature is that the ovules are enclosed within an ovary, which after fertilisation develops into a fruit, so the seeds are enclosed (Greek angio, covered). They bear flowers, and the male sex organs (stamens, producing pollen in microsporangia) and female organs (carpels, with ovules in the ovary) are housed within them. They range from the tiny Wolffia to the towering Eucalyptus.

Angiosperms are split into two classes — the dicotyledons (two cotyledons, reticulate leaf venation, tap roots) and the monocotyledons (one cotyledon, parallel venation, fibrous roots). A hallmark of all angiosperms, examined often, is double fertilisation: one male gamete fuses with the egg to form the diploid zygote (syngamy) and the second fuses with two polar nuclei to form the triploid primary endosperm nucleus (triple fusion), which gives rise to the nutritive endosperm.

All plants alternate between a diploid spore-producing sporophyte and a haploid gamete-producing gametophyte — this is alternation of generations. The relative size and independence of these two phases differ across groups, and NEET expects you to recognise the three patterns. In the haplontic life cycle the gametophyte is the dominant, free-living phase and the only diploid stage is the zygote, which immediately undergoes meiosis — seen in many algae such as Spirogyra, Volvox and Chlamydomonas.

In the diplontic life cycle the diploid sporophyte is the dominant, photosynthetic phase and the gametophyte is reduced to a few cells — this is the pattern in gymnosperms and angiosperms (and the alga Fucus). The intermediate haplo-diplontic pattern has both a multicellular sporophyte and a multicellular gametophyte: bryophytes follow it with the gametophyte dominant, while pteridophytes follow it with the sporophyte dominant. Reading off which phase is dominant for a given group is a classic one-mark question.