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Unlock This CoursePlants need to transport water and minerals from their roots to every part of their body, including leaves that might be hundreds of metres above ground in tall trees. This amazing feat is achieved by a specialised transport system called xylem. Unlike animals, plants don't have a heart to pump fluids around their body, so they've evolved clever mechanisms to move water upwards against gravity.
Key Definitions:
Xylem is made up of several types of cells, but the most important for water transport are xylem vessels. These are long, hollow tubes formed from dead cells that have lost their contents, creating an uninterrupted pathway for water flow. The cell walls are strengthened with lignin, making them strong enough to withstand the negative pressure created during water transport.
Water transport in plants follows a continuous pathway from soil to atmosphere. This process relies on the physical properties of water and doesn't require any energy input from the plant - it's entirely passive!
This theory explains how water moves up through plants, even in the tallest trees. It works like a biological drinking straw that extends from roots to leaves.
Water evaporates from leaf surfaces through tiny pores called stomata. This creates a 'pull' that draws more water up from below, like sucking on a straw.
Water molecules stick together due to hydrogen bonding. This creates an unbroken column of water from roots to leaves that can be 'pulled' upwards as one unit.
The pulling force creates tension (negative pressure) in the xylem vessels. This tension is transmitted down through the entire water column to the roots.
Giant redwoods can grow over 100 metres tall, yet they successfully transport water from their roots to their highest leaves using only the transpiration-cohesion-tension mechanism. These trees can move up to 600 litres of water per day during peak growing season, demonstrating the incredible efficiency of the xylem transport system.
Several environmental and plant factors influence how efficiently water moves through xylem vessels. Understanding these factors helps explain why plants behave differently in various conditions.
The rate of water transport is primarily controlled by transpiration rate, which varies with environmental conditions.
Bright light and warm temperatures increase transpiration rates. Light causes stomata to open for photosynthesis, whilst heat provides energy for water evaporation. This is why plants often wilt on hot, sunny days if they can't absorb water fast enough.
Low humidity and windy conditions speed up transpiration by removing water vapour from around leaves. High humidity slows transpiration because the air is already saturated with water vapour.
Plants have evolved numerous adaptations to make their xylem transport system as efficient as possible, especially those living in challenging environments.
The design of xylem vessels is perfectly suited for their function, with several key features that maximise water flow.
Wider vessels allow faster water flow, but they're more likely to form air bubbles (cavitation) that block transport. Plants balance vessel width with reliability.
Lignin strengthens vessel walls, preventing them from collapsing under the negative pressure created during water transport. It also makes vessels waterproof.
Small holes in vessel walls allow water to move between adjacent vessels if one becomes blocked, maintaining continuous water flow.
Cacti and other desert plants have adapted their xylem to cope with water scarcity. They have narrower vessels that are less prone to air bubble formation and they can store water in specialised tissues. Some desert plants also have waxy coatings and modified leaves (spines) to reduce water loss through transpiration.
Whilst transpiration provides the main driving force for water transport, roots also play an active role in water uptake, especially when transpiration rates are low.
Roots absorb water through both passive and active processes, ensuring plants can access water even when soil conditions are challenging.
Root cells contain dissolved minerals that create a lower water potential than soil water. This causes water to move into roots by osmosis, following the concentration gradient.
Active transport of minerals into root xylem creates positive pressure that can push water upwards. This is most noticeable at night when transpiration is low and explains why some plants 'weep' water droplets from their leaves.
The xylem transport system faces several challenges that could disrupt water flow. Plants have evolved solutions to overcome these problems.
Air bubbles can form in xylem vessels under high tension, blocking water flow. This is called cavitation and the resulting air-filled vessel is an embolism.
Plants can isolate blocked vessels and redirect water flow through alternative pathways. Some plants can also dissolve small air bubbles by increasing root pressure at night. In extreme cases, plants may shed leaves to reduce water demand and prevent further vessel damage.
Deciduous trees face a major challenge each spring when they need to restart water transport after winter. During cold weather, air bubbles often form in xylem vessels. Trees solve this by growing new vessels each spring and using root pressure to clear existing blockages. This is why tree sap flows strongly in early spring - it's the tree's transport system getting back to work!
Scientists can measure the rate of water transport in plants using various techniques, helping us understand how different factors affect this vital process.
A potometer measures the rate of water uptake by a cut shoot. By changing environmental conditions like light, temperature, or humidity, we can see how these factors affect transpiration and water transport rates.
Coloured dyes can track water movement through plants. By cutting stems and placing them in coloured water, we can see exactly which vessels transport water and how quickly it moves through different parts of the plant.