Plant Tissues, Organs & Systems

Plant tissues

A leaf is a plant organ, and it contains several tissues, each with a role:

• Epidermal tissue — covers the leaf surfaces. The upper epidermis is often covered by a waxy cuticle that reduces water loss.
• Palisade mesophyll — packed with chloroplasts, and positioned near the top of the leaf to absorb the most light for photosynthesis.
• Spongy mesophyll — contains large air spaces that allow gases to diffuse to and from the cells.
• Xylem and phloem — transport water, mineral ions and dissolved sugars.
• Meristem tissue — found at growing tips of shoots and roots, able to differentiate into any plant cell type.

Plant organ system

The roots, stem and leaves form an organ system that transports substances around the plant.

Stomata are tiny openings, mostly on the lower surface of the leaf, that control gas exchange and water loss. Each is surrounded by two guard cells, which change shape to open or close the pore. When water is plentiful the guard cells swell and the stomata open; when the plant is short of water they lose water, become flaccid and the stomata close, limiting further loss.

Being on the underside reduces water loss, because that surface is cooler and shaded.

Transpiration and translocation

Transpiration is the loss of water vapour from the leaves. Water evaporates from the surfaces of cells inside the leaf and diffuses out through the stomata. This creates a pull that draws water up through the xylem from the roots — the transpiration stream. Xylem consists of hollow tubes strengthened by lignin, carrying water and mineral ions in one direction only: upwards.

Translocation is the movement of dissolved sugars made in the leaves to the rest of the plant, for immediate use or storage. This happens in the phloem, which is made of columns of elongated living cells with small pores in their end walls, and can move substances in both directions.

A clean way to keep them apart: xylem carries water up, phloem carries food both ways.

Factors affecting transpiration

The rate of transpiration increases with:

• higher temperature — water molecules evaporate and diffuse faster,
• lower humidity — a steeper concentration gradient of water vapour between leaf and air,
• more air movement — wind removes water vapour from around the stomata, maintaining the gradient,
• higher light intensity — stomata open wider for photosynthesis, so more water escapes.

Transpiration rate can be measured with a potometer, which tracks the movement of an air bubble along a capillary tube to show the rate of water uptake.