Stems and Transport

Stems support the plant and contain xylem and phloem vessels that transport water, minerals, and sugars throughout the plant.

What Does a Stem Do?

The stem is the main structural axis of a plant. It connects the roots below to the leaves and flowers above. Stems have three key roles:

Support — Stems hold leaves up to the light and flowers and fruit where they can be seen or reached by pollinators and seed dispersers.
Transport — Stems contain bundles of vessels that carry water, minerals, and sugars between all parts of the plant.
Storage — Some stems (e.g., potato tubers, cacti) store food and water.

The Two Transport Systems

Inside every stem are two types of tube-like tissue bundled together in vascular bundles:

Xylem — Carries water and dissolved mineral salts upward from the roots to the leaves and other parts. Xylem vessels are dead cells with thick, strengthened walls, forming hollow tubes. The movement of water through xylem is driven mainly by transpiration — the evaporation of water from leaves creates a pull that drags water up through the plant (the transpiration stream).

Phloem — Carries dissolved sugars (mainly sucrose) made by photosynthesis in leaves to the rest of the plant, including roots, flowers, and fruits. This movement in both directions up and down is called translocation. Phloem is made of living cells called sieve tube elements connected end-to-end.

Note

In cross-section, a stem shows vascular bundles arranged in a ring (in dicot plants like sunflowers) or scattered throughout (in monocot plants like grasses and maize).

Transpiration Stream

Water moves through a plant following this path:

Soil → Root hair cells (osmosis) → Root tissues → Xylem in stem → Leaves → Evaporates through stomata

This continuous flow is called the transpiration stream. Because xylem vessels form an unbroken column of water from root to leaf, the pulling force created by evaporation at the top is transmitted all the way down to the roots.

Tip

You can demonstrate the transpiration stream by placing a white carnation in coloured water. Within a day or so, the petals show streaks of colour as coloured water travels up through the xylem.

Types of Stems

Not all stems look the same:

Herbaceous stems — Soft and green, like in tomatoes or daisies.
Woody stems — Hard and rigid, reinforced with lignin in older xylem cells (wood). Examples: trees and shrubs.
Modified stems — Potato tubers (underground storage stems), cacti (thick, water-storing stems), strawberry runners (horizontal stems that produce new plants).

Stem Adaptations

In cacti, the stem is thick and swollen to store water, and it is green (containing chloroplasts) because leaves are reduced to spines. In climbing plants like ivy, flexible stems and tendrils wrap around supports. These variations show how the basic stem structure is modified to suit different environments.

Real world: Cut flowers in a vase

When you place cut roses in a vase of water, the xylem in the stems continues to draw water upward by the transpiration stream — even without roots. Florists sometimes cut stems underwater at an angle to maximise the cross-section of xylem exposed and prevent air bubbles from blocking the tubes. Adding sugar to the water gives phloem cells nutrients to keep the flower alive longer.

Real world: Tree rings and xylem growth

Each year, a tree adds a new ring of xylem around its trunk during the growing season. In a good year (warm, wet), the ring is wide; in a drought year, it is narrow. Counting tree rings reveals a tree's age, and studying ring widths allows scientists to reconstruct hundreds of years of climate history. The hard "wood" you see in a log is almost entirely dead xylem cells reinforced with lignin.

Real world: Aphids tapping phloem

Greenfly (aphids) pierce plant stems with a needle-like mouthpart that penetrates exactly into phloem sieve tubes. The phloem is under pressure from sugary sap moving via translocation, so sap flows out of the plant and directly into the aphid — it barely needs to suck. This is why aphid infestations cause plants to wilt and leaves to curl; the sugar supply to growing tips is literally being drained away.

Tip Where do you see this in real life? Split the stem of a fresh celery stalk and place it in coloured water overnight. By morning, the coloured streaks running up the stalk clearly show the xylem bundles that transported the dye upward. This is also why white carnations can be turned blue or red in floral arrangements.

Practice Problems

What is the difference between xylem and phloem?

Show answer

Xylem carries water and minerals upward from roots to leaves through dead, hollow vessels. Phloem carries dissolved sugars (sucrose) in both directions — from leaves to roots and other parts — through living sieve tube cells. This is called translocation.

Explain what drives water up through the xylem of a tall tree.

Show answer

Water evaporates from leaves through stomata (transpiration). This creates a pulling force at the top of the xylem column. Because water molecules stick together (cohesion), the entire column of water is pulled upward from the roots.