Plants have two different types of transport tissue. Xylem transports water and solutes from the roots to the leaves, phloem transports food from the leaves to the rest of the plant. Transpiration is the process by which water evaporates from the leaves, which results in more water being drawn up from the roots. Plants have adaptations to reduce excessive water loss.
Xylem and phloem
Plants have transport systems to move food, water and minerals around. These systems use continuous tubes called xylem and phloem.
Xylem vessels are involved in the movement of water through a plant from its roots to its leaves. Water:
- Is absorbed from the soil through root hair cells
- Is transported through the xylem vessels up the stem to the leaves
- Evaporates from the leaves (transpiration)
Phloem vessels are involved in translocation. This is the movement of food substances from the stems to growing tissues and storage tissues.
Root - xylem and phloem in the centre of the root to withstand stretching forces.
Xylem and phloem - Higher tier
Xylem vessels consist of dead cells. They have a thick, strengthened cellulose cell wall with a hollow lumen. On the other hand, phloem consists of columns of living cells.
Diagram showing the cross sections of a xylem and a phloem.
Transpiration explains how water moves up the plant against gravity in tubes made of dead xylem cells without the use of a pump.
Water on the surface of spongy and palisade cells (inside the leaf) evaporates and then diffuses out of the leaf. This is called transpiration. More water is drawn out of the xylem cells inside the leaf to replace what's lost. As the xylem cells make a continuous tube from the leaf, down the stem to the roots, this acts like a drinking straw, producing a flow of water and dissolved minerals from roots to leaves.
Factors that speed up transpiration will also increase the rate of water uptake from the soil. When water is scarce, or the roots are damaged, it increases a plant's chance of survival if the transpiration rate can be slowed down. Plants can do this themselves by wilting, or it can be done artificially, like removing some of the leaves from cuttings before they have chance to grow new roots.
Environmental factors that affect the rate of transpiration
Plants transpire more rapidly in the light than in the dark. This is largely because light stimulates the opening of the stomata (mechanism). Light also speeds up transpiration by warming the leaf.
Plants transpire more rapidly at higher temperatures because water evaporates more rapidly as the temperature rises. At 30°C, a leaf may transpire three times as fast as it does at 20°C.
The rate of diffusion of any substance increases as the difference in concentration of the substances in the two regions increases.When the surrounding air is dry, diffusion of water out of the leaf goes on more rapidly.
When there is no breeze, the air surrounding a leaf becomes increasingly humid thus reducing the rate of transpiration. When a breeze is present, the humid air is carried away and replaced by drier air.
- Soil water
A plant cannot continue to transpire rapidly if its water loss is not made up by replacement from the soil. When absorption of water by the roots fails to keep up with the rate of transpiration, loss of turgor occurs, and the stomata close. This immediately reduces the rate of transpiration (as well as of photosynthesis). If the loss of turgor extends to the rest of the leaf and stem, the plant wilts.
The volume of water lost in transpiration can be very high. It has been estimated that over the growing season, one acre of corn (maize) plants may transpire 400,000 gallons (1.5 million liters) of water. As liquid water, this would cover the field with a lake 15 inches (38 cm) deep. An acre of forest probably does even better.
A healthy plant must balance its water loss from the leaves with its water uptake through the roots. Transpiration provides plants with water for:
· Movement of minerals
However, the plant's cells will become flaccid and the plant will wilt if it loses too much water. The root hair cells provide a large surface area for efficient uptake of water by osmosis.
The structure of a leaf is adapted to reduce excessive water loss. Its adaptations include:
A waxy cuticle
Only having a small number of stomata on the upper surface
Now try a Test Bite.
Adaptions - Higher tier
Transpiration and water loss from leaves happen because of the way that leaves are adapted for efficient photosynthesis. The flat, thin shape of a leaf, its spongy mesophyll layer and stomata are adaptations that also allow water loss from the leaf. Features involving the guard cells around the stomata provide a way to reduce excessive water loss.
The size of the stomatal opening can be altered by the plant in response to the availability of water and the light intensity. For example, in conditions where there is plenty of water and bright light:
- Chloroplasts make sugars at a high rate
- Water enters the guard cells from other cells by osmosis
- The guard cells become turgid The stomatal opening gets bigger
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