Plant Anatomy

The organs of a plant

Flowering plants have four main organs. Roots anchor the plant and absorb water and minerals. The stem supports the plant and transports materials. Leaves make food by photosynthesis. Flowers produce seeds for reproduction. Organs are made of tissues — groups of similar cells working together — such as epidermis, mesophyll, xylem and phloem.

Inside a leaf

• A waxy cuticle reduces water loss.
• The upper epidermis is transparent, letting light through.
• Palisade mesophyll cells are tall and packed with chloroplasts for maximum photosynthesis.
• Spongy mesophyll has air spaces for gas exchange.
• Stomata, mostly in the lower epidermis, let CO₂ in and O₂ and water vapour out; guard cells open and close them.
• Veins contain xylem (bringing water) and phloem (taking sugars away).

Inside a stem

In the stem of a typical dicot (a plant like a bean or sunflower), vascular bundles are arranged in a ring. Each bundle has xylem toward the centre and phloem toward the outside, with a layer of dividing cambium between them that lets the stem grow thicker. The cortexand central pith store food and provide support.

Roots and root hairs

A root grows from its tip. The root cap protects the tip and senses gravity. Just behind it, cells in the zone of cell division multiply; in the zone of elongation they lengthen, pushing the root forward; and further back, cells mature and grow root hairs. Root hairs provide an enormous surface area for absorbing water by osmosis and minerals by active transport.

Parts of a flower

• Sepals protect the flower bud; petals attract pollinators.
• The male stamen is made of a filament supporting an anther, which produces pollen.
• The female carpel is made of the stigma (catches pollen), style and ovary, which contains ovules.
• After pollination and fertilization, ovules become seeds and the ovary becomes the fruit.

Transport: xylem and phloem

Interesting facts

• The tallest living tree, a coast redwood in California, is about 115 m tall. Water reaches its top leaves through xylem, pulled up by evaporation from the leaves.
• A classic 1937 study of a single rye plant found more than 600 km of roots, not counting the root hairs, which multiply the surface area many times more.
• Most of a leaf’s stomata are on its lower surface, where they are shaded and lose less water.
• The rings in a tree trunk are layers of xylem: a wide, pale ring grows in spring and a narrow, dark one later in the season, so counting them gives the tree’s age.

Key terms

Tissue

A group of similar cells that work together to carry out a function.

Vascular bundle

A strand of xylem and phloem running through stems, leaves and roots.

Mesophyll

The inner tissue of a leaf where photosynthesis takes place.

Translocation

The movement of sugars through the phloem.

Pollination

The transfer of pollen from an anther to a stigma.

Why it matters

• Agriculture: understanding roots and leaves guides watering, fertilizing and pruning.
• Food: we eat many plant organs — roots (carrots), stems (potatoes are swollen stems), leaves (lettuce), flowers (broccoli) and fruits (tomatoes).
• Materials: wood is mostly old xylem, and cotton comes from seed hairs.
• Pollinators: flower structure explains why bees, birds and bats are essential to many crops.

Common misconceptions

• “Plants take in food through their roots.” Roots absorb water and minerals; plants make their own food in the leaves.
• “Xylem and phloem both carry water up.” Phloem carries sugars in both directions.
• “Potatoes are roots.” Potatoes are tubers — swollen underground stems.

Key takeaways

• Roots, stems, leaves and flowers each have structures suited to their functions.
• Leaves are layered to capture light and exchange gases efficiently.
• Xylem carries water up; phloem carries sugars where they are needed.
• Flowers contain male (stamen) and female (carpel) parts for producing seeds.

Model note: diagrams are simplified and not to scale; real tissues contain many more cell types.