The plant's organs and their jobs
- Root: holds the plant in the soil and takes in water and minerals.
- Stem: holds leaves up to the light and carries sap between root and leaves.
- Leaf: makes food by photosynthesis (light + water + carbon dioxide → sugar + oxygen).
- Flower: makes seeds for the next generation; seeds sit inside fruits.
A plant is fixed to one place. It cannot run from heat or hunters, so its whole body is shaped to collect what it needs where it stands, and to protect itself.
Large exchange surfaces, roots and fungi
An exchange surface is where materials pass in or out. A plant has huge ones:
- Each root hair is one long thin cell. A single rye plant can have billions, giving hundreds of square metres of surface underground.
- Thin, wide leaves with tiny pores called stomata let carbon dioxide in and water vapour out. Inside, spongy cells with air spaces add even more surface.
Mycorrhiza is a partnership (symbiosis) between roots and soil fungi. The fungal threads spread far into the soil and pass water and phosphate to the root; the plant gives the fungus sugar. Both win. Most land plants have it.
Two saps: xylem and phloem
Raw sap (xylem sap): water + mineral ions, moving up from roots to leaves in xylem vessels, which are dead, hollow tubes with walls made strong by lignin.
- Transpiration pull: water evaporates from leaves through stomata. Water molecules stick together (cohesion) and to the tube walls (adhesion), so the whole water column is pulled up like a drink through a straw.
- Root pressure: roots pump ions in, water follows by osmosis and pushes a little from below.
Elaborated sap (phloem sap): water + sugars (mainly sucrose) and amino acids, moving from a source (a leaf making sugar) to a sink (root, fruit, bud, storage organ like a potato). Phloem is living sieve tubes with companion cells. Sugar loaded at the source draws in water, building pressure that pushes the sap towards the sink (pressure flow). It can go up or down.
Growth, hormones, defences and adaptations
Growth happens in meristems (shoot and root tips) where cells divide, then stretch. Hormones control it: auxin makes stems bend towards light (phototropism) and roots grow down (gravitropism); gibberellins lengthen stems and wake seeds; cytokinins drive cell division; abscisic acid closes stomata in drought; ethylene ripens fruit.
Defences: physical (bark, thick wax, thorns, hairs) and chemical (bitter or toxic substances like nicotine or tannins, latex that glues insect mouths). Some plants send chemical signals that call wasps to attack caterpillars.
Adaptations: xerophytes (dry places) have thick waxy skin, few sunken stomata, water-storing stems and spines (cactus). Hydrophytes (water) have floating leaves with stomata on top, air spaces for buoyancy and small roots (water lily). Plants in shade grow big thin leaves; plants in cold places stay low and hairy.
Humans domesticated wild plants by choosing the best seeds year after year: wheat whose grains do not fall off, bigger fruit, less bitter taste.
Try it
Put a white flower or celery stalk in a glass of water with a few drops of food colour or ink. Check after 2, 6 and 24 hours. Then tie a clear plastic bag around a leafy branch in the sun for an hour and look for water drops: that is transpiration you can see.
Key formulas and definitions
- Raw (xylem) sap: water + minerals, root → leaves (up only)
- Elaborated (phloem) sap: sugars + water, source → sink (up or down)
- Transpiration pull = evaporation + cohesion + adhesion
- Photosynthesis: carbon dioxide + water —light→ glucose + oxygen
- Mycorrhiza: fungus gives water/phosphate ↔ plant gives sugar
- Xerophyte = dry-land plant; hydrophyte = water plant
Worked examples
1. On a hot, dry, windy day, does water rise faster or slower in the xylem? Why?
Faster. More evaporation from leaves means a stronger transpiration pull on the water column.
2. A potato tuber: is it a source or a sink for sugar in summer?
A sink. Sugar from leaves is carried in phloem and stored in it as starch.
3. A ring of bark (with phloem) is removed from a tree trunk. What happens above and below the ring?
Sugar coming down from the leaves piles up above the ring, so the bark there swells. Roots below get no sugar and slowly starve. Xylem (deeper wood) still carries water up for a while.
4. Why are stomata on the upper side of a water lily leaf?
The lower side touches water. Air and carbon dioxide are only available on top.
Common mistakes
- Saying phloem only carries food downward. It carries sugar from source to sink, which can be up (to fruit) or down (to root).
- Thinking xylem cells are alive and pump water. Mature xylem vessels are dead tubes; the pull comes from evaporation.
- Mixing up raw sap (water + minerals) and elaborated sap (sugars).
- Thinking a cactus has no leaves at all. Its leaves became spines; the green stem does photosynthesis.