How plants are classified
Early systems used only easy-to-see features like habit (tree, shrub, herb) and were called artificial. Later natural systems used many features including inner structure. Today phylogenetic systems try to show evolutionary relationships. Scientists also use numerical taxonomy (computers compare hundreds of characters), cytotaxonomy (chromosome number and shape) and chemotaxonomy (chemicals in the plant).
Algae
Algae are simple, thalloid (body not divided into root, stem and leaf), chlorophyll-bearing plants that live mostly in water. Some are single cells (Chlamydomonas), some form colonies (Volvox) and some are threads (Spirogyra, Ulothrix). Giant kelps can be very long.
Reproduction: vegetative by fragmentation; asexual by zoospores (spores with flagella); sexual by isogamy (similar gametes), anisogamy (different sizes) or oogamy (large egg, small moving sperm).
Uses: algae do about half of the Earth's photosynthesis; agar (from Gelidium, Gracilaria), algin (brown algae) and carrageen (red algae) are used in food; Chlorella is a protein supplement.
Three classes of algae
| Class | Pigment | Stored food | Wall | Examples |
|---|---|---|---|---|
| Chlorophyceae (green) | Chlorophyll a, b | Starch | Cellulose | Chlamydomonas, Volvox, Ulothrix, Spirogyra, Chara |
| Phaeophyceae (brown) | Chlorophyll a, c + fucoxanthin | Laminarin, mannitol | Cellulose + algin | Ectocarpus, Dictyota, Laminaria, Sargassum, Fucus |
| Rhodophyceae (red) | Chlorophyll a, d + r-phycoerythrin | Floridean starch | Cellulose, pectin | Polysiphonia, Porphyra, Gracilaria, Gelidium |
Bryophytes: amphibians of the plant kingdom
Bryophytes live on land but need water for sexual reproduction, so they are called the amphibians of the plant kingdom. They grow in damp, shady places. The body has rhizoids (root-like threads) but no true roots, stems, leaves or vascular tissue.
The main plant body is the haploid gametophyte. It makes antheridia (male, give swimming sperm) and archegonia (female, flask-shaped, hold the egg). After fertilisation the zygote grows into a small sporophyte (foot, seta, capsule) that stays attached to the gametophyte for food. Spores from the capsule grow into new gametophytes.
Liverworts
Flat, thalloid body, e.g. Marchantia. Asexual reproduction by gemmae, small green buds in cups.
Mosses
Two stages: a green thread-like protonema (from the spore), then an upright leafy stage, e.g. Funaria, Polytrichum, Sphagnum.
Uses: mosses and lichens are among the first plants on bare rock; Sphagnum (peat) is a fuel and holds water in packing.
Pteridophytes: first plants with pipes
Pteridophytes are the first land plants with vascular tissue: xylem carries water, phloem carries food. Examples: Selaginella, Equisetum (horsetail), ferns like Pteris, Dryopteris and Adiantum.
The main plant is the diploid sporophyte with true root, stem and leaves. Leaves may be small (microphylls, as in Selaginella) or large (macrophylls, as in ferns). Sporangia on leaf-like sporophylls make spores. A spore grows into a small, free-living gametophyte called a prothallus, which needs cool, damp, shady places and water for fertilisation.
Most make one kind of spore (homosporous). Selaginella and Salvinia make large and small spores (heterosporous). This is the first step towards the seed habit.
Four classes: Psilopsida (Psilotum), Lycopsida (Selaginella, Lycopodium), Sphenopsida (Equisetum) and Pteropsida (Dryopteris, Pteris, Adiantum).
Gymnosperms: naked seeds
In gymnosperms (gymno = naked, sperma = seed) the ovules are not enclosed in an ovary; seeds sit open on cone scales. They include medium to tall trees; the giant redwood Sequoia is among the tallest trees.
- Roots are often tap roots; Pinus roots have fungal partners (mycorrhiza); Cycas has coralloid roots with nitrogen-fixing cyanobacteria.
- Leaves are needle-like in conifers, with thick cuticle and sunken stomata to save water.
- They are heterosporous: male cones make microspores (pollen grains), female cones carry megaspores inside ovules.
- Pollen is carried by wind; a pollen tube takes the male gamete to the egg. The gametophytes never live free.
Examples: Cycas, Pinus, Ginkgo, Cedrus (deodar).
Angiosperms: flowering plants
In angiosperms the ovules are inside an ovary; after fertilisation the ovary becomes a fruit that covers the seeds. The flower holds stamens (male) and pistils (female). They range from tiny Wolffia to tall Eucalyptus.
- Dicots: two seed leaves (cotyledons), e.g. pea, mango, mustard.
- Monocots: one cotyledon, e.g. wheat, rice, maize, banana.
Double fertilisation
The female gametophyte is the embryo sac with 7 cells (8 nuclei). One male gamete fuses with the egg to form the diploid zygote (syngamy). The other fuses with the two polar nuclei to form the triploid primary endosperm nucleus (triple fusion). Two fusions happen, so it is called double fertilisation, found only in angiosperms. The endosperm feeds the growing embryo.
Plant life cycles and alternation of generations
Plants alternate between a haploid (n) gametophyte that makes gametes and a diploid (2n) sporophyte that makes spores by meiosis. Which stage is bigger depends on the group:
- Haplontic: the gametophyte is the main plant; the only diploid cell is the zygote. Many algae: Volvox, Spirogyra, some Chlamydomonas.
- Diplontic: the sporophyte is the main plant; the gametophyte is tiny. All seed plants (gymnosperms and angiosperms); Fucus among algae.
- Haplo-diplontic: both stages are many-celled. Bryophytes (gametophyte dominant) and pteridophytes (sporophyte dominant); Ectocarpus, Polysiphonia, kelps among algae.
Try it: find the groups around you
In the monsoon, look at a damp wall: the green velvet is moss (bryophyte). Turn over a fern leaf in a garden: the brown dots are sori, groups of sporangia. Break open a pine cone: the seeds sit on open scales. Cut a pea pod: the seeds are covered by a fruit. Tick off all five groups in one week! In the 3D free-play step, switch features on and off to check your guesses.
Key formulas and definitions
- Algae → Bryophytes (land) → Pteridophytes (xylem + phloem) → Gymnosperms (naked seeds) → Angiosperms (seeds in fruit)
- Gametophyte = n, makes gametes; Sporophyte = 2n, makes spores by meiosis
- Double fertilisation: syngamy (egg + male gamete = 2n zygote) + triple fusion (2 polar nuclei + male gamete = 3n endosperm)
- Green algae: chl a, b, starch · Brown: chl a, c, fucoxanthin, laminarin · Red: chl a, d, r-phycoerythrin, floridean starch
- Haplontic (algae) · Diplontic (seed plants) · Haplo-diplontic (bryophytes, pteridophytes)
Worked examples
1. A seaweed is red, stores floridean starch and gives agar. Name its class and one genus.
Rhodophyceae (red algae). Genus: Gelidium or Gracilaria.
2. Why are bryophytes called amphibians of the plant kingdom?
They live on land but their sperm must swim through water to reach the egg, so they depend on water to complete sexual reproduction.
3. A plant has true roots, stem and leaves, xylem and phloem, but no seeds. Which group?
Pteridophytes, for example a fern like Dryopteris.
4. Why are pine seeds called naked?
The ovules sit on open cone scales with no ovary around them, so the seeds are not covered by a fruit.
5. In an angiosperm, the egg cell has 12 chromosomes. How many chromosomes are in the zygote and in the endosperm?
Egg is n = 12. Zygote = 2n = 24. Endosperm = 3n = 36 (two polar nuclei + one male gamete).
6. Classify these life cycles: Spirogyra, Pinus, Funaria.
Spirogyra: haplontic. Pinus: diplontic. Funaria (moss): haplo-diplontic with a dominant gametophyte.
Common mistakes
- Thinking the moss plant we see is the sporophyte. In bryophytes the leafy plant is the gametophyte.
- Saying ferns have seeds. Pteridophytes spread by spores; seeds begin with gymnosperms.
- Calling the endosperm diploid. In angiosperms it is triploid (3n).
- Mixing up the red and brown algae pigments: fucoxanthin is brown, r-phycoerythrin is red.