MAVO Year 4 · Plants · Section 9.2

Stems and roots

Transport through vessels

Stems support a plant and contain vascular bundles. These include xylem and phloem.

Xylem vessels carry water and mineral nutrients from the roots towards leaves, flowers and growing parts. They are formed from cells that are dead at maturity, with strong walls containing cellulose and lignin.

Phloem transports dissolved organic substances, especially sucrose, from sources such as mature leaves to growing or storage tissues. Its conducting tubes are made of living cells with perforated end walls, supported by companion cells. Transport can occur in different directions in different tubes according to the plant's needs.

In a typical stem bundle, xylem lies towards the inside and phloem towards the outside. In many leaf veins, xylem is towards the upper surface and phloem towards the lower surface. In woody trunks, wood is largely xylem, with phloem nearer the outside in the inner bark.

Support from cells and fibres

Soft, herbaceous plants rely substantially on turgor pressure in living cells and may wilt when water is scarce. Woody plants gain lasting support from lignified walls in wood and fibres. Strong fibres often reinforce vascular bundles. This structural support does not rely directly on maintaining turgor, although the living plant still needs water.

Functions of roots

Roots anchor the plant, take up water and minerals and may store food reserves. Root hairs are extensions of root epidermal cells, increasing the surface area for uptake. Water crosses cell membranes into root tissues and reaches the xylem; mineral uptake also involves transport processes.

Water movement

Transpiration, the loss of water vapour from leaves, creates tension pulling a continuous water column upwards through xylem. Cohesion between water molecules helps maintain this column. Root pressure can also push water upwards under some conditions, but transpiration pull is the main driving force in many plants.

Most water taken up eventually evaporates. A smaller amount is used in photosynthesis or retained in tissues. Mineral nutrients are distributed and used in metabolism and growth rather than evaporating with the water.