Chapter 3: Plant Kingdom (Diversity in the Living World)

Algae are classified mainly on the kind of pigments and the type of stored food, together with the cell wall composition and the number and position of flagella (Table 3.1): green algae (chlorophyll a, b; starch; cellulose; 2–8 equal apical flagella), brown algae (chlorophyll a, c, fucoxanthin; laminarin or mannitol; cellulose + algin; 2 unequal lateral), red algae (chlorophyll a, d, phycoerythrin; floridean starch; cellulose, pectin, polysulphate esters; no flagella).

In all, meiosis occurs when spores are formed by the sporophyte.

Bryophytes, pteridophytes and gymnosperms. Life cycle of a fern (pteridophyte): the main plant is a diploid sporophyte with root, stem and leaves. Sporangia on sporophylls form haploid spores by meiosis. A spore grows into a small, free-living, green, heart-shaped prothallus (gametophyte) bearing antheridia and archegonia. Antherozoids swim in a film of water to the archegonium and fertilise the egg; the diploid zygote grows into a new sporophyte, at first nourished by the prothallus, which then dies. The sporophyte is dominant; both generations are free-living.

Protonemal cell: n (gametophyte). Primary endosperm nucleus: 3n (triple fusion). Moss leaf cell: n. Prothallus cell: n. Gemma cell: n. Monocot meristem cell: 2n (sporophyte). Liverwort ovum: n. Fern zygote: 2n.

Algae: fix at least half of the earth's CO₂ and add oxygen to water; primary producers of aquatic food chains; about 70 marine species (Porphyra, Laminaria, Sargassum) are eaten; brown algae give algin, red algae carrageen; agar from Gelidium and Gracilaria is used to culture microbes and in ice-creams and jellies; Chlorella is a protein-rich food supplement (also for space travellers). Gymnosperms: timber and pulp for paper (pine, deodar Cedrus); resins and turpentine from Pinus; edible seeds (chilgoza from Pinus gerardiana); sago from Cycas; ornamentals (Cycas, Araucaria, Thuja); medicines (ephedrine from Ephedra, taxol from Taxus).

In gymnosperms the ovules are not enclosed in an ovary; they lie exposed on megasporophylls and the seeds are naked; there are no flowers or fruits; xylem lacks vessels and phloem lacks companion cells; the female gametophyte bears archegonia; endosperm is haploid and formed before fertilisation. In angiosperms the ovules are inside an ovary, which becomes the fruit enclosing the seeds; they have flowers, double fertilisation and a triploid endosperm; archegonia are absent. These basic differences put them in separate groups.

Heterospory is the production of two kinds of spores, large megaspores and small microspores, which give rise to female and male gametophytes respectively. Significance: the megaspore and the female gametophyte are retained on the parent sporophyte, and the zygote develops into an embryo within the female gametophyte; this is a precursor to the seed habit, an important step in evolution. Examples: Selaginella and Salvinia (also all gymnosperms and angiosperms).

(i) Protonema: the first, creeping, green, branched, filamentous stage of the moss gametophyte, formed directly from a spore (Funaria). (ii) Antheridium: the multicellular male sex organ of bryophytes and pteridophytes producing antherozoids (Marchantia, ferns). (iii) Archegonium: the flask-shaped multicellular female sex organ with a single egg (bryophytes, pteridophytes, gymnosperms). (iv) Diplontic: a life cycle in which the diploid sporophyte is the dominant, independent, photosynthetic phase and the gametophyte is reduced to a few cells (all seed plants, e.g. angiosperms; among algae, Fucus). (v) Sporophyll: a leaf-like appendage bearing sporangia (fern sporophylls; microsporophylls and megasporophylls of Cycas). (vi) Isogamy: fusion of two gametes similar in size and form, flagellated (Ulothrix) or non-flagellated (Spirogyra).

(i) Red algae: chlorophyll a, d and phycoerythrin; floridean starch; no flagella; most marine, at all depths; e.g. Polysiphonia. Brown algae: chlorophyll a, c, fucoxanthin; laminarin or mannitol; algin wall; 2 unequal lateral flagella; holdfast, stipe, frond; e.g. Fucus. (ii) Liverworts: thalloid, dorsiventral (Marchantia); unicellular rhizoids; gemmae; no protonema; simple sporophyte. Mosses: protonema and leafy stage with spirally arranged leaves; multicellular rhizoids; elaborate sporophyte and spore dispersal (Funaria). (iii) Homosporous: one kind of spore, bisexual gametophyte (most ferns, Lycopodium). Heterosporous: megaspores and microspores, separate female and male gametophytes, embryo develops in the retained female gametophyte (Selaginella, Salvinia).

(a)–(iii), (b)–(iv), (c)–(ii), (d)–(i).

(i) Ovules not enclosed in an ovary; seeds naked. (ii) Medium to tall trees and shrubs (Sequoia among the tallest); tap roots, with mycorrhiza (Pinus) or coralloid roots with N₂-fixing cyanobacteria (Cycas). (iii) Stems unbranched (Cycas) or branched (Pinus, Cedrus); leaves simple or pinnate; needle leaves with thick cuticle and sunken stomata resist extremes of temperature, humidity and wind. (iv) Heterosporous; sporophylls in male and female strobili (cones). (v) Male gametophyte reduced to the pollen grain; female gametophyte multicellular with archegonia, retained in the ovule; gametophytes not free-living. (vi) Wind pollination; a pollen tube carries male gametes to the archegonia. (vii) Zygote → embryo, ovule → naked seed.