PLANT KINGDOM

Kingdom Plantae includes algae, bryophytes, pteridophytes, gymnosperms and angiosperms. Fungi and cell-wall-bearing members of Monera and Protista are no longer included in Plantae; cyanobacteria are not algae.

Numerical taxonomy uses coded observable characters and computers. Cytotaxonomy uses chromosome information; chemotaxonomy uses chemical constituents.

Algae are chlorophyll-bearing, simple, thalloid, autotrophic and largely aquatic organisms. They occur in fresh water, marine habitats, moist stones, soils and wood, and may associate with fungi or animals.

Reproduction

Vegetative reproduction occurs by fragmentation. Asexual reproduction occurs through flagellated zoospores. Sexual reproduction may be isogamous (similar gametes: Ulothrix, Spirogyra), anisogamous (unequal gametes: Eudorina) or oogamous (large non-motile egg and small motile male gamete: Volvox, Fucus).

Major classes

Bryophytes are called the amphibians of the plant kingdom because they live on soil but depend on water for sexual reproduction. The main plant body is haploid and produces gametes; the sporophyte remains attached to and dependent on the gametophyte.

Liverworts

Liverworts grow in moist, shady habitats. Their thalloid body is dorsiventral and closely appressed to the substrate. Asexual reproduction may occur by fragmentation or gemmae in gemma cups. Examples: Marchantia, Riccia.

Mosses

Mosses have a protonema stage and a leafy stage. The leafy plant bears sex organs; after fertilisation the zygote develops into a sporophyte with foot, seta and capsule. Examples: Funaria, Polytrichum, Sphagnum. Peat moss (Sphagnum) has water-holding capacity and is used as packing material.

Pteridophytes are the first terrestrial plants with vascular tissues (xylem and phloem). Their dominant independent plant body is the sporophyte, differentiated into true roots, stem and leaves. They occur in cool, damp, shady places; some are aquatic.

Sporangia are borne on sporophylls, often arranged in strobili or cones. Spores germinate into a small, free-living, photosynthetic gametophyte called a prothallus. Water is required for male gametes to reach the egg.

Examples include Psilotum, Lycopodium, Selaginella, Equisetum, Pteris, Adiantum and Dryopteris.

Gymnosperms are plants with naked seeds: ovules are not enclosed by an ovary wall, so fruits are not formed. They are usually perennial, evergreen and woody; many have tap roots, while Cycas has coralloid roots associated with nitrogen-fixing cyanobacteria.

The sporophyte bears male and female cones. Microspores develop into pollen grains (male gametophytes); megaspores develop into female gametophytes bearing archegonia. Pollination is generally by wind. Examples: Cycas, Pinus, Gnetum.

Angiosperms are flowering plants. Their ovules are enclosed in the ovary; after fertilisation, ovules become seeds and the ovary becomes fruit. They range from tiny Wolffia to tall Eucalyptus.

Male and female gametophytes

The microspore develops into a pollen grain; the male gametophyte has two male gametes. The megaspore develops into the embryo sac, the female gametophyte. Double fertilisation is unique to angiosperms: one male gamete fuses with the egg (syngamy), while the other fuses with two polar nuclei to form triploid endosperm.

Dicots and monocots

Dicotyledons have two cotyledons, reticulate venation and generally tap roots. Monocotyledons have one cotyledon, parallel venation and fibrous roots.

All plants alternate between a haploid gametophytic phase and a diploid sporophytic phase. In a haplontic life cycle, the dominant plant body is haploid and the zygote is the only diploid stage; many algae show this. In a diplontic life cycle, the sporophyte is dominant and the gametophyte is highly reduced; seed plants show this.

In a haplo-diplontic life cycle, both multicellular phases occur. Bryophytes have dominant gametophytes and dependent sporophytes; pteridophytes have dominant sporophytes and small independent gametophytes. Some algae show isomorphic alternation, while others have different-looking phases.

1. Compare artificial, natural and phylogenetic classification systems.
2. Describe pigments, reserve food and examples of green, brown and red algae.
3. Why are bryophytes called amphibians of the plant kingdom?
4. Explain the role of prothallus in pteridophytes.
5. What is heterospory and how does it relate to seed habit?
6. State the defining reproductive features of gymnosperms and angiosperms.
7. Differentiate haplontic, diplontic and haplo-diplontic life cycles.

Algal class details

Green algae generally have cellulose-rich walls, equal flagella and chloroplasts that may be cup-, spiral- or star-shaped. Brown algae range from simple branched filaments to large kelps; their body often has holdfast, stipe and frond, and their motile cells bear two unequal laterally attached flagella. Red algae are mainly marine, often at greater depths because phycoerythrin absorbs blue-green light; they have no flagellated stages.

Further reproductive and structural points

In bryophytes, meiosis in the capsule produces haploid spores. Pteridophyte leaves may be small microphylls or large megaphylls; spores form in sporangia on sporophylls. Gymnosperm xylem generally lacks vessels and phloem lacks companion cells; Gnetum is an exception with vessels. In angiosperms, endosperm develops after triple fusion and provides nourishment to the developing embryo.