Plant Tissues, Organs and Systems¶

Part of Organisation.

Plant organisation uses the same cell-to-tissue-to-organ pattern as animals, but the transport problems are different. Plants must move water, minerals and sugars without a pump like a heart.

What You Need to Learn¶

A plant’s tissues form a connected system for obtaining resources and distributing them. Link leaf and root structures to their roles, distinguish water movement from sugar transport, and explain how weather and stomata affect water loss. Practise reading and calculating transport rates from experimental data.

Plant Tissues¶

Epidermal tissue covers the outside of the plant and helps reduce water loss through a waxy cuticle. The lower epidermis contains guard cells that control stomata.
Palisade mesophyll is packed with chloroplasts and arranged in columns for photosynthesis, maximizing light absorption.
Spongy mesophyll contains air spaces for gas exchange and is less densely packed, allowing CO₂ diffusion.
Xylem carries water and mineral ions from the roots to the leaves, and consists of dead, hollow cells that form continuous tubes (vessels).
Phloem carries dissolved sugars (from photosynthesis) around the plant in both directions depending on demand, using living cells with sieve tubes and companion cells.
Meristem tissue contains unspecialised cells that keep dividing and allow the plant to keep growing throughout life (located at root tips, shoot tips and within stems).

Explore a Leaf Cross-Section¶

Select a tissue in the leaf model, then connect its position to its job. Follow light towards the palisade cells, gases through the air spaces and stoma, and water into the leaf through xylem. This is a simplified section of a typical leaf; the spacing and colours make the tissues easier to distinguish.

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Try and explain: why do the spongy mesophyll's air spaces help photosynthesis even though the palisade layer receives most of the light? Include the route that carbon dioxide takes from the outside air to a photosynthesising cell.

Plant Organs and Transport¶

Roots absorb water and mineral ions through root hair cells (which have large surface area), stems support the plant and connect transport tissues, and leaves are broad and thin to maximise light capture and gas exchange.
Root hair cells use active transport to absorb mineral ions against the concentration gradient, and osmosis to absorb water.
Transpiration is the loss of water vapour from leaves through the stomata. Water evaporates from moist mesophyll cell surfaces into leaf air spaces, then water vapour diffuses through stomata. This water loss draws a continuing supply up the xylem from the roots: the transpiration stream.
Factors affecting transpiration: temperature (higher = faster), humidity (lower = faster), light intensity (higher = faster, more stomata open), and wind (faster, removes humid air).
Translocation is the movement of dissolved sugars (and other solutes) in the phloem from sources (like photosynthetic leaves) to sinks (like growing tissues or storage areas). Energy is needed to load sugars into the transport system; the movement along the phloem tube should not itself be called active transport.
Leaves are adapted for photosynthesis and gas exchange: large surface area, thin (short diffusion distance), have vascular tissues, stomata for gas exchange, and palisade cells for photosynthesis.

Xylem vessel walls contain lignin, which strengthens the tubes against collapse. Phloem consists of elongated cells with pores in their end walls so sugar-rich sap can move between them. Guard cells alter the width of stomatal pores: opening allows gas exchange but also increases water loss.

To compare transpiration, estimate water uptake using a potometer and divide the volume taken up by elapsed time. For example, 0.6 cm³ in 10 min gives 0.06 cm³/min. Uptake is an estimate of water loss because some water is used within the plant. Change one condition at a time and average repeated readings. Put the changed condition on a graph’s horizontal axis and uptake rate on its vertical axis, using labelled units and an even scale. Higher temperature increases evaporation, lower humidity steepens the water-vapour gradient, and moving air removes moist air around the leaf. Light usually promotes stomatal opening, allowing faster loss.

Explore Water Uptake With a Potometer¶

Follow water from a root hair to the leaf, then investigate a prepared cut shoot. A potometer measures water uptake rather than water loss directly; it is an optional practical technique for studying transpiration. The cut shoot has no roots: the apparatus supplies its water. Prepare real apparatus underwater, seal its joints and use one air bubble as a position marker. A leak makes bubble movement unreliable.

Choose one environmental factor, keep the others fixed and compare repeated readings. Calculate bubble displacement divided by elapsed time in minutes to obtain a distance rate in mm/min. With the same capillary, distance rates can compare uptake; calculating volume requires the capillary's cross-sectional area. The simulation uses synthetic teaching data with small trial variation. Its trends describe moderate conditions, not extreme drought, heat or every real plant's response.

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Try and explain: predict how higher humidity will affect uptake, then compare repeated measurements. Why should a second shoot with more leaves not be used to test humidity alone? Explain why an uptake rate cannot be assumed to equal the transpiration rate exactly.

Common Confusions¶

Xylem vs. phloem: Xylem transports water and minerals upward only (from roots); phloem transports sugars in both directions. Xylem has dead cells forming tubes; phloem has living sieve tube elements.
Transpiration vs. translocation: Transpiration is water loss (physical evaporation); translocation is sugar movement through phloem. They are different processes.
Guard cells and stomata: Guard cells are the cells that control the stomata (pores). When turgid, they open the pore; when flaccid, they close it.
Palisade vs. spongy mesophyll: Palisade cells are columnar and densely packed (photosynthesis); spongy cells are irregular and loosely arranged (gas exchange). Both are mesophyll.

Key Terms¶

Xylem: plant tissue that transports water and mineral ions from the roots.
Phloem: plant tissue that transports dissolved sugars around the plant.
Transpiration: the loss of water vapour from plant leaves.
Translocation: the movement of dissolved sugars through the phloem.
Transpiration stream: the continuous upward movement of water from roots through xylem to leaves, driven by transpiration.
Meristem: plant tissue containing cells that can divide and differentiate.
Stoma: a pore in the leaf surface through which gases and water vapour move.
Guard cell: specialized cell pair controlling the opening and closing of stomata.
Palisade mesophyll: columnar photosynthetic tissue in the upper leaf.
Spongy mesophyll: loosely arranged tissue in the lower leaf for gas exchange.
Root hair cell: epidermal cell with large surface area for water and ion absorption.
Vascular bundle: group of xylem and phloem tissues in roots, stems and leaves.