Plant systematics is the science that studies the diversity of plants and sorts them into groups by relatedness. The plant kingdom is divided into lower plants (algae) and higher plants (mosses, fern allies and ferns, gymnosperms, angiosperms). Studying the divisions in order shows plant evolution: the move from water onto land, the appearance of tissues and organs, the change from spore to seed and from seed to flower and fruit. Exams often ask about the features of the divisions, the life cycle (gametophyte and sporophyte), and the families of flowering plants with their floral formulas.
§1. Taxonomic ranks and the life cycle
In plant systematics the ranks run as follows:
Kingdom → division → class → order → family → genus → species.
Animals use “phylum”; plants use division in its place. A species’ scientific name has two words (C. Linnaeus, binomial nomenclature): genus name + species epithet.
The life cycle of higher plants alternates two generations:
the gametophyte — the sexual generation, haploid cells (n), makes gametes;
the sporophyte — the asexual generation, diploid cells (2n), makes spores by meiosis.
A spore (n) grows into a gametophyte, gametes fuse into a zygote (2n), and the zygote grows into a sporophyte. During evolution the sporophyte grew more important and the gametophyte shrank: in mosses the gametophyte dominates, from the ferns onwards the sporophyte dominates, and in seed plants the gametophyte is reduced to a few cells.
§2. Algae
Algae are lower plants: the body is a thallus, not divided into tissues and organs (root, stem, leaf). They live mostly in water and absorb water and salts over their whole surface. The chloroplast is often called a chromatophore.
Unicellular: Chlamydomonas (two flagella, red eyespot, motile), Chlorella (non-motile, tested as an oxygen source on spacecraft).
Filamentous (multicellular): Spirogyra — its chromatophore is a spiral ribbon; Ulothrix.
Large multicellular: kelp (Laminaria, “sea cabbage”, a brown alga rich in iodine), red algae (live at great depths).
By colour there are green, brown and red algae. Importance: the main source of organic matter and oxygen in water, food for aquatic animals, human food and the raw material for agar.
§3. Mosses
Mosses are the simplest higher plants and grow in damp places.
No roots; instead there are rhizoids (thread-like outgrowths); they have a stem and leaves; vascular tissue is weak.
The gametophyte (n) dominates the life cycle: the green plant we see is the gametophyte. The sporophyte — a capsule on a stalk — grows on the gametophyte and feeds from it.
Fertilisation needs water: sperm swim to the egg in a drop of water.
Examples: Funaria, haircap moss (Polytrichum, dioecious — separate male and female plants), Sphagnum (bog moss). Sphagnum has dead water-holding cells and absorbs several times its own mass of water; in bogs its remains form peat.
§4. Ferns and their allies
This group includes clubmosses, horsetails and ferns.
They have true roots, stems, leaves and well-developed vascular tissue, so they can grow tall.
The sporophyte (2n) dominates: the fern plant we see is the sporophyte. Sporangia with spores ripen in sori on the underside of the leaves.
A spore grows into a small heart-shaped green plate — the prothallus; this is the gametophyte, on which gametes form.
Fertilisation still needs water.
The stem of the field horsetail is jointed and rich in silica; in spring it produces spore-bearing shoots. Clubmosses are creeping evergreen plants. Coal formed from the remains of ancient (Carboniferous) tree-sized clubmosses, horsetails and ferns.
The seed has appeared, but it lies naked on the scales of a cone; there are no flowers or fruits.
Pollination is by wind; fertilisation does not need water — the pollen tube delivers the sperm.
A pine has male cones (pollen is made) and female cones (ovules); the seed ripens about 1.5–2 years after pollination.
Leaves are usually needles covered with cuticle and wax; most are evergreen (the larch sheds its needles for winter).
A seed is better than a spore: it contains a ready embryo and a food store, and the coat protects it.
Importance: timber, paper, resin, turpentine; forests clean the air; ephedra gives a medicinal substance.
§6. Angiosperms (flowering plants)
The most advanced and widespread division. Features: the flower, the fruit (the seed is protected inside it), double fertilisation, strongly reduced gametophytes, true vessels in the xylem. The division has two classes.
Dicots: 2 cotyledons in the embryo, taproot, net venation, flower parts in fours or fives.
Monocots: 1 cotyledon in the embryo, fibrous roots, parallel or arcuate venation, flower parts in threes.
In a floral formula: K — calyx (sepals), C — corolla (petals), P — simple perianth, A — stamens, G — pistil, ∞ — many, brackets — parts fused.
Dicot families:
Rose family (apple, pear, apricot, cherry, dog rose, strawberry): K₅ C₅ A∞ G₁–∞; fruits — pome, drupe, aggregate of achenes.
Cabbage family (crucifers) (cabbage, radish, turnip, shepherd’s purse): K₄ C₄ A₄₊₂ G₁; cross-shaped corolla, 6 stamens (4 long, 2 short), fruit a siliqua or silicle.
Pea family (legumes) (pea, bean, mung bean, lucerne, soya): butterfly-like corolla (standard, wings, keel), fruit a legume (pod), root-nodule bacteria on the roots.
Nightshade family (potato, tomato, aubergine, pepper, tobacco): K₍₅₎ C₍₅₎ A₅ G₁; fruit a berry or capsule.
Daisy family (composites) (sunflower, chamomile, dandelion): inflorescence a head (capitulum), fruit an achene.
Mallow family (cotton, mallow): fruit a capsule; the hairs on cotton seeds are cotton fibre.
Carrot family (umbellifers) (carrot, dill, parsley, cumin): inflorescence a compound umbel.
Monocot families:
Grass family (cereals) (wheat, barley, rice, maize, sorghum): stem a hollow jointed culm, fruit a grain (caryopsis), wind-pollinated.
Lily family (tulip, lily): P₃₊₃ A₃₊₃ G₁; many have bulbs.
Onion family (onion, garlic): bulbs, inflorescence an umbel.
Worked example
A floral formula is K₄ C₄ A₄₊₂ G₁. Which family is it?
Four sepals and four petals — a cross-shaped corolla.
Six stamens: 4 long and 2 short.
Flower parts in fours — class of dicots.
Answer: the cabbage family (crucifers).
Worked example
A fern spore has 16 chromosomes. How many chromosomes are in the prothallus, a leaf cell and a sperm cell?
The spore is made by meiosis, so it is haploid: n = 16.
The prothallus (gametophyte) grows from the spore by mitosis: n = 16.
Sperm form on the gametophyte by mitosis: n = 16.
The leaf belongs to the sporophyte: 2n = 32.
Answer: prothallus 16, sperm 16, leaf cell 32.
Worked example
Use its features to prove which class wheat belongs to.
The embryo in a wheat grain has one cotyledon.
The root system is fibrous and the leaves have parallel veins.
Answer: wheat is a monocot, in the grass family.
Check yourself
Which generation dominates the moss life cycle?
Why does a fern need water for fertilisation?
Do gymnosperms have fruits?
If a moss sporophyte has 2n = 20, how many chromosomes are in its spore?
Which family does cotton belong to?
Which moss forms peat?
Which plants have a thallus body and no tissues?
What is the fruit of a pea called?
Common mistakes
Calling a pine cone a fruit — gymnosperms have no fruits.
Writing that the sporophyte dominates in mosses — the gametophyte does.
Thinking algae have roots and leaves.
Mixing up monocot and dicot features (cotyledons, roots, venation, number of flower parts).
Writing that gymnosperms need water for fertilisation — they have a pollen tube.
Using “phylum” in plant systematics — plants use “division”.