A flowering plant is a complex multicellular organism. Its body is built of groups of cells doing the same job — tissues — and tissues form organs. Organs fall into two groups: vegetative (root, stem, leaf — nutrition and growth) and generative (flower, fruit, seed — reproduction). Knowing plant structure is the basis for understanding photosynthesis, water transport, reproduction and agriculture. Exams often ask about tissue functions, root and stem zones, modified organs, flower structure and double fertilisation.
§1. Plant tissues
A tissue is a group of cells similar in origin, structure and function. Plants have five main tissue types.
Meristem (formative tissue) — small, thin-walled, constantly dividing cells from which all other tissues arise. Apical meristem at the tips of root and shoot gives growth in length; lateral meristem — the cambium — gives growth in thickness; intercalary meristem lies at the base of internodes (in grasses).
Dermal tissue protects organs: the epidermis (one layer of living cells with stomata and hairs), cork (on perennial stems and roots — dead cells, airtight), bark.
Ground tissue (parenchyma): photosynthetic (chlorenchyma), storage (accumulates starch and other substances — potato tuber, seeds), air-filled (in water plants).
Mechanical tissue for support: collenchyma (living, in young organs) and sclerenchyma (thick-walled dead fibres — flax fibre).
Vascular tissue: xylem (wood) — water and mineral salts move upwards from the root through dead vessels and tracheids; phloem (bast) — organic substances made in the leaves move down and to other organs through living sieve tubes.
Xylem: water + salts, upwards, dead vessels. Phloem: organic substances, from leaves to other organs, living sieve tubes.
§2. The root
The root anchors the plant, absorbs water and mineral salts and sometimes stores food.
Kinds of roots: the main (primary) root (from the embryo), lateral roots (from the main root), adventitious roots (from stems and leaves). Root systems:
taproot system — the main root is well developed (dicots: pea, cotton, beet);
fibrous system — the main root dies early and adventitious roots are equally developed (monocots: wheat, maize, onion).
Root zones (from the tip upwards):
root cap — protects the growing point;
zone of division — meristem cells divide;
zone of elongation (growth) — cells lengthen and push the root into the soil;
zone of absorption — root hairs absorb water and salts;
zone of conduction — substances pass to the stem; lateral roots arise here.
Water rises because of root pressure (pushing from below) and transpiration in the leaves (pulling from above).
Modified roots: storage roots (carrot, beet, turnip — the main root is swollen), root tubers (dahlia — swollen adventitious roots), aerial roots (orchids), haustoria (dodder — a parasite), prop roots (maize).
§3. The stem and shoot
A shoot is a stem bearing leaves and buds. Where a leaf joins is a node; the part between nodes is an internode. A bud is an undeveloped shoot: a vegetative bud gives leaves and stem, a flower bud gives a flower.
The stem holds leaves up to the light, links roots and leaves and transports substances.
Cross-section of a tree trunk (outside to inside):
cork — protection;
bast — phloem and bast fibres, organic substances downwards;
cambium — divides to lay down wood inwards and bast outwards;
wood — xylem, water upwards; one annual ring is added each year;
pith — soft storage cells.
An annual ring consists of spring wood with large cells and autumn wood with small cells; counting rings gives the age of the tree.
Modified shoots (underground shoots carry buds and scale leaves — this is how they differ from roots):
rhizome — a horizontal underground shoot (couch grass, reed, water lily);
tuber — the swollen tip of an underground shoot (potato; the eyes are buds);
bulb — a short flat stem (basal plate) with fleshy leaves (onion, tulip, garlic);
The leaf is the organ of photosynthesis, gas exchange and transpiration. Parts: the leaf blade, the petiole (some leaves have none), sometimes stipules.
Simple leaf — one blade (apple, poplar); compound leaf — several leaflets on one petiole (rose, pea, walnut).
Venation: net (in dicots), parallel and arcuate (in monocots).
Internal structure: upper and lower epidermis (covered with cuticle), palisade mesophyll (richest in chloroplasts; most photosynthesis happens here), spongy mesophyll (air spaces for gas exchange), veins (xylem and phloem).
Stomata — a pore between two guard cells, mostly on the lower surface. CO₂ enters, O₂ and water vapour leave through them.
Transpiration — evaporation of water from leaves; it pulls water up from the root and stops the leaf overheating.
Leaf fall — an adaptation to reduce water loss in autumn and winter and to get rid of unwanted substances.
§5. Flower, pollination and double fertilisation
A flower is a modified short shoot, the organ of seed reproduction. Parts: pedicel, receptacle, calyx (sepals), corolla (petals) — together the perianth; stamens (anther + filament) and the pistil (stigma, style, ovary; inside the ovary are the ovules).
Stamens and pistil in one flower — a bisexual flower; in separate flowers — unisexual (maize, cucumber).
Inflorescences: spike (wheat), head (sunflower, chamomile), umbel (carrot, dill), panicle (rice), raceme (cabbage, lily of the valley), spadix-like cob (maize).
Pollination is the transfer of pollen to the stigma: self-pollination (wheat, pea) and cross-pollination — by insects (bright, scented flowers with nectar), wind (small flowers with a simple perianth and lots of pollen), water.
Double fertilisation (S. G. Navashin, 1898): sperm 1 + egg cell → zygote → embryo (2n); sperm 2 + central cell (2n) → endosperm (3n).
The pollen tube brings two sperm cells into the ovule, so fertilisation is called double. It occurs only in flowering plants (angiosperms).
§6. Fruit and seed
After fertilisation: ovary → fruit, ovule → seed; the embryo and endosperm stay inside the seed. Seed parts: seed coat, embryo (radicle, hypocotyl, plumule, cotyledons), food store (in the endosperm or the cotyledons).
In monocots (wheat) the store is in the endosperm; in dicots (bean, pea) usually in the cotyledons.
Fruits: fleshy — berry (grape, tomato), drupe (apricot, cherry), pome (apple, pear); dry — legume (pea), siliqua (cabbage), capsule (cotton, poppy), grain (wheat), achene (sunflower). Fruits and seeds are spread by wind, water, animals, people, or are shot out by the plant itself (acacia, bean).
Worked example
Prove that a potato tuber is a stem.
The tuber has “eyes” — these are buds; roots never have buds.
In the light the tuber turns green (chlorophyll forms).
The tuber develops not on a root but at the tip of an underground shoot (stolon).
Answer: the tuber is a modified underground shoot, that is, a stem.
Worked example
A tree stump shows 24 annual rings. How old is the tree and which tissue made the rings?
Each year the cambium lays down one layer of wood inwards — an annual ring.
The number of rings equals the number of years: 24 rings.
Answer: the tree is about 24 years old; the rings were made by the cambium.
Worked example
A somatic cell of a flowering plant has 2n = 14 chromosomes. How many chromosomes are in the embryo, the endosperm and a sperm cell?