NCERT groups algae into Chlorophyceae, Phaeophyceae and Rhodophyceae using characters such as major pigments, reserve food material, nature of cell wall, number and position of flagella and habitat.
Algae are classified mainly on the basis of pigments, stored food, cell-wall composition, flagella and habitat.
In liverworts and mosses, cells of the sporophyte capsule undergo meiosis to form haploid spores. In ferns, spore mother cells in sporangia undergo meiosis. In gymnosperms and angiosperms, microspore mother cells in microsporangia and megaspore mother cells in megasporangia/ovules undergo meiosis to produce haploid spores.
Reduction division occurs during spore formation in liverworts, mosses and ferns; during microspore and megaspore formation in gymnosperms and angiosperms.
In bryophytes, the dominant plant body is the haploid gametophyte. It bears antheridia and archegonia. Water carries biflagellate antherozoids to the archegonium, where fertilisation forms a zygote. The zygote develops into a multicellular sporophyte attached to and nourished by the gametophyte. Meiosis in the sporophyte forms haploid spores, which germinate into new gametophytes.
Bryophytes, pteridophytes and gymnosperms bear archegonia.
Gametophytic structures and gametes are haploid; sporophytic vegetative tissues and zygotes are diploid. In angiosperms, the primary endosperm nucleus is formed by fusion of one male gamete with two polar nuclei, so it is triploid.
Protonemal cell of moss: haploid. Primary endosperm nucleus in dicot: triploid. Leaf cell of moss: haploid. Prothallus cell of fern: haploid. Gemma cell in Marchantia: haploid. Meristem cell of monocot: diploid. Ovum of liverwort: haploid. Zygote of fern: diploid.
Algae fix a large fraction of carbon dioxide, release oxygen and support aquatic food chains. Porphyra, Laminaria and Sargassum are used as food; algin, carrageen and agar are commercially important; Chlorella is protein-rich. Gymnosperms such as Pinus, Cedrus and Cycas are used for timber, resin, turpentine, paper pulp, ornamentals and some medicinal products.
Algae are important as primary producers, food, hydrocolloid sources, agar sources and protein supplements. Gymnosperms provide timber, resins, turpentine, medicines and ornamentals.
In gymnosperms, ovules are not enclosed by an ovary wall and remain exposed before and after fertilisation. In angiosperms, ovules develop inside the ovary and seeds are enclosed in fruits; pollen grains and ovules are associated with flowers.
They are separated because gymnosperm seeds are naked, while angiosperm seeds are enclosed within fruits.
Microspores develop into male gametophytes and megaspores into female gametophytes. Retention and development of the female gametophyte on the parent sporophyte protects the developing embryo. Selaginella and Salvinia are examples.
Heterospory is the production of two kinds of spores: microspores and megaspores. It is significant because it is considered a precursor of the seed habit.
(i) Protonema is the creeping, green, branched stage that develops from a moss spore, e.g. Funaria. (ii) Antheridium is the male sex organ that produces antherozoids. (iii) Archegonium is the flask-shaped female sex organ that produces an egg. (iv) Diplontic life cycle has dominant diploid sporophyte and highly reduced gametophyte, as in seed plants. (v) Sporophyll is a leaf-like structure bearing sporangia, e.g. sporophylls in Selaginella. (vi) Isogamy is fusion of similar gametes, as in Ulothrix or Spirogyra.
The terms refer to plant life-cycle stages, sex organs, life-cycle type, spore-bearing leaves and gamete similarity.
(i) Red algae contain chlorophyll a, d and phycoerythrin, store floridean starch and lack flagella; brown algae contain chlorophyll a, c and fucoxanthin, store laminarin/mannitol and usually have unequal lateral flagella. (ii) Liverworts are usually thalloid and dorsiventral, reproduce asexually by gemmae and have a simpler sporophyte; mosses have protonema and leafy gametophyte stages and a more elaborate sporophyte. (iii) Homosporous pteridophytes produce one kind of spore; heterosporous pteridophytes produce microspores and megaspores, as in Selaginella and Salvinia.
The pairs differ in pigments/storage products, plant body/life cycle details, and spore type respectively.
Chlamydomonas is a green alga. Cycas is a gymnosperm. Selaginella is a heterosporous pteridophyte. Sphagnum is a moss.
(a) Chlamydomonas - (iii) Algae; (b) Cycas - (iv) Gymnosperm; (c) Selaginella - (ii) Pteridophyte; (d) Sphagnum - (i) Moss.
They are mostly trees or shrubs with tap roots, often with mycorrhiza in Pinus or coralloid roots in Cycas. Leaves are adapted to reduce water loss, such as needle-like leaves, thick cuticle and sunken stomata in conifers. They are heterosporous, producing microspores and megaspores in male and female strobili or cones. Male and female gametophytes are not free living. Pollen reaches the ovule, fertilisation forms a zygote, and the ovule develops into an uncovered seed.
Gymnosperms are naked-seeded vascular plants with exposed ovules, dominant sporophytes and reduced gametophytes.