How plants are classified — the systems and the names

• ›Five kingdoms — R.H. Whittaker, 1969: Monera, Protista, Fungi, Plantae, Animalia
• ›Three domains — Carl Woese, 1990: Bacteria, Archaea, Eukarya

Binomial nomenclature. Linnaeus's Species Plantarum (1753) is the starting point of botanical names. The genus begins with a capital, the species with a small letter, both in italics — or underlined when handwritten — often followed by the author: Mangifera indica L.

The trap: Bentham and Hooker's system is natural, not phylogenetic — it was built before the idea of evolution shaped classification, even though it remains the most practical for identifying a flowering plant.

Algae — three classes, told apart by pigment and food

Algae in use

• ›agar — for jellies and culture media — from Gelidium and Gracilaria
• ›algin from brown algae; carrageen from red algae — both hydrocolloids
• ›Porphyra, Laminaria and Sargassum are among the marine algae eaten as food
• ›Chlorella, a single-celled alga rich in protein, has been used as a food supplement even by space travellers
• ›algae fix about half of all the CO₂ fixed on Earth by photosynthesis

The trap: agar comes from red algae, not brown; algin is the brown one. The two hydrocolloids are set side by side precisely because their sources are swapped so easily.

Bryophytes and pteridophytes — the move onto land

Bryophytes — the amphibians of the plant kingdom: they live on land but need water for fertilisation

• ›the plant body is the gametophyte and is dominant; the sporophyte grows on it and depends on it
• ›no xylem or phloem
• ›liverworts — Marchantia, which also multiplies by gemmae in gemma cups
• ›mosses — Funaria; Sphagnum, whose peat is fuel and a packing material that holds water

Pteridophytes — the first vascular plants, with xylem and phloem

• ›the sporophyte is dominant — the fern plant itself
• ›four classes: Psilopsida (Psilotum), Lycopsida (Selaginella, Lycopodium), Sphenopsida (Equisetum), Pteropsida (Dryopteris, Pteris, Adiantum)
• ›most are homosporous, but Selaginella and Salvinia are heterosporous — they make small male and large female spores, a step towards the seed habit

The trap: in a moss, the green plant you see is the gametophyte; in a fern, it is the sporophyte. The dominant generation changes sides between the two groups.

Gymnosperms, angiosperms — and the three life cycles

Gymnosperms — naked seeds: the ovules are not enclosed in an ovary, so there is no fruit

• ›Cycas has coralloid roots that house nitrogen-fixing cyanobacteria; Pinus roots live with fungi as mycorrhiza
• ›Sequoia, the giant redwood, is among the tallest trees on Earth; Ginkgo is a living fossil
• ›all are heterosporous

Angiosperms — enclosed seeds, inside a fruit

• ›double fertilisation, and a triploid endosperm
• ›they range from Wolffia, the smallest, to Eucalyptus, over 100 metres tall

Life cycles

The trap: Fucus is an alga but its life cycle is diplontic — like a seed plant's, not like most algae's.

Points to Remember

• ›Artificial Linnaeus · natural Bentham and Hooker · phylogenetic Engler and Prantl, Hutchinson
• ›Whittaker 1969 — five kingdoms · Woese 1990 — three domains · Species Plantarum 1753
• ›Green algae: starch · brown: fucoxanthin, laminarin, mannitol · red: phycoerythrin, floridean starch
• ›Agar from red Gelidium, Gracilaria · algin from brown · carrageen from red · Chlorella — food for space travellers
• ›Bryophytes — amphibians of the plant kingdom, gametophyte dominant, no vascular tissue · Sphagnum peat
• ›Pteridophytes — first vascular plants, sporophyte dominant · heterospory in Selaginella, Salvinia
• ›Gymnosperms — naked seeds · Cycas coralloid roots · Pinus mycorrhiza · Sequoia
• ›Angiosperms — double fertilisation, triploid endosperm · Wolffia smallest, Eucalyptus tallest
• ›Haplontic most algae · diplontic seed plants and Fucus · haplo-diplontic bryophytes, pteridophytes, Ectocarpus
• ›Exam tip: in a life-cycle question, ask which generation you are looking at — the green plant is a gametophyte in a moss and a sporophyte in a fern