Plant Kingdom

In this chapter: algae (three classes), bryophytes (liverworts and mosses), pteridophytes, gymnosperms, angiosperms. (The section on plant life cycles has been removed from the rationalised NCERT; a short note is kept below.)

Algae

Algae are chlorophyll-bearing, simple, thalloid, autotrophic and largely aquatic (fresh and marine) organisms. They also occur on moist stones, soil and wood, and in association with fungi (lichens) and animals (on the sloth bear). Form ranges from colonial (Volvox) and filamentous (Ulothrix, Spirogyra) to massive marine kelps.

Reproduction

• Vegetative: fragmentation.
• Asexual: spores, most commonly flagellated zoospores.
• Sexual: isogamous (similar gametes: flagellated in Chlamydomonas, non-flagellated in Spirogyra), anisogamous (dissimilar in size, as in species of Eudorina) or oogamous (large non-motile female, small motile male: Volvox, Fucus).

Why algae matter

They carry out about half of the total carbon dioxide fixation on Earth by photosynthesis and are primary producers in water. Porphyra, Laminaria and Sargassum are eaten. Algin (brown algae) and carrageen (red algae) are used commercially. Agar, used to grow microbes and in ice creams and jellies, comes from Gelidium and Gracilaria. Chlorella, a unicellular protein-rich alga, is used as a food supplement, even by space travellers.

Brown algae have a body differentiated into a holdfast (attachment), stipe (stalk) and leaf-like frond. Red algae get their colour from r-phycoerythrin; they reproduce vegetatively by fragmentation, asexually by non-motile spores and sexually by non-motile gametes (oogamous, with complex post-fertilisation development).

Bryophytes

Called the amphibians of the plant kingdom: they live on land but need water for sexual reproduction. They grow in damp, shaded places.

• The plant body is thallus-like, prostrate or erect, attached by unicellular or multicellular rhizoids. No true roots, stems or leaves.
• The main plant body is haploid: it is the gametophyte.
• Male sex organ: antheridium, producing biflagellate antherozoids. Female: flask-shaped archegonium with a single egg.
• Antherozoids reach the archegonium through water. The zygote forms a sporophyte that is not free-living: it stays attached to the gametophyte and depends on it. Some cells undergo meiosis to form haploid spores, which grow into new gametophytes.
• Importance: some mosses feed herbivores; Sphagnum provides peat (fuel) and packing material for living material (it holds water). Mosses and lichens are among the first organisms to colonise rocks, and mosses form dense mats that reduce soil erosion.

Pteridophytes

Pteridophytes include horsetails and ferns, and are the first terrestrial plants with vascular tissues (xylem and phloem). Found in cool, damp, shady places; some in sandy soil.

• The main plant body is the sporophyte, with true roots, stems and leaves.
• Leaves are small (microphylls, as in Selaginella) or large (macrophylls, as in ferns).
• Sporangia are borne on leaf-like sporophylls; in some, sporophylls form compact strobili or cones (Selaginella, Equisetum).
• Spores germinate into a small, multicellular, free-living, mostly photosynthetic gametophyte called the prothallus, which needs cool, damp, shady places. Water is still needed for fertilisation.
• Most are homosporous (one kind of spore). Selaginella and Salvinia are heterosporous: large megaspores and small microspores, giving female and male gametophytes. The female gametophyte is retained on the parent sporophyte for variable periods. This development is a precursor to the seed habit, an important step in evolution.
• Classes: Psilopsida (Psilotum), Lycopsida (Selaginella, Lycopodium), Sphenopsida (Equisetum), Pteropsida (Dryopteris, Pteris, Adiantum).
• Used for medicine, as soil binders and as ornamentals.

Gymnosperms

The name means "naked seeds", because the ovules are not enclosed by an ovary wall, before or after fertilisation. Gymnosperms are medium or tall trees and shrubs; the giant redwood Sequoia is one of the tallest tree species.

• Roots are generally tap roots. Some genera have fungal associations (mycorrhiza, as in Pinus); Cycas has small specialised coralloid roots associated with nitrogen-fixing cyanobacteria.
• Stems unbranched (Cycas) or branched (Pinus, Cedrus). Leaves simple or compound; in Cycas the pinnate leaves persist for a few years. In conifers, needle-like leaves with a thick cuticle and sunken stomata reduce water loss.
• All are heterosporous. Microsporophylls with microsporangia form male cones; the microspore develops into a highly reduced male gametophyte, the pollen grain. Megasporophylls with ovules form female cones. Male and female cones may be on the same tree (Pinus) or different trees (Cycas).
• The megaspore mother cell in the nucellus divides meiotically; one megaspore develops into a multicellular female gametophyte with two or more archegonia. The female gametophyte is retained within the megasporangium.
• Pollen is carried by air to the ovule opening; a pollen tube carries male gametes to the archegonia. The zygote forms an embryo and the ovule becomes a seed, which is not covered.

Angiosperms

Flowering plants, in which pollen grains and ovules develop in flowers and the seeds are enclosed in fruits. They are the largest group, found in a wide range of habitats, from the tiny Wolffia to tall Eucalyptus trees (over 100 m). They are divided into dicotyledons and monocotyledons. Their reproduction is covered in the Class 12 chapter Sexual Reproduction in Flowering Plants.

Plant life cycles and alternation of generations

Not in the current syllabus. This section was removed from the rationalised NCERT (2023 onwards). We have kept it only because the idea helps you understand bryophytes and pteridophytes, so do not spend revision time on it.

Plants alternate between a haploid gametophyte (makes gametes by mitosis) and a diploid sporophyte (makes spores by meiosis). Which phase dominates differs:

Common trap: in bryophytes the dominant, free-living phase is the gametophyte; in pteridophytes it is the sporophyte. Both are haplo-diplontic. Fucus is an alga with a diplontic cycle, an exception students forget.

NEET focus

• The algae comparison table: pigments, stored food, flagella and examples.
• Commercial products: agar, algin, carrageen, Chlorella.
• Bryophyte vs pteridophyte: dominant generation, vascular tissue, dependence of sporophyte.
• Heterospory in Selaginella and Salvinia as a precursor to seeds.
• Coralloid roots of Cycas, mycorrhiza in Pinus, retention of female gametophyte in gymnosperms.

Practice questions