Transport in Plants
Source of Water for the Plant
The soil is the source of water and also the plant’s source of mineral nutrients. The roots are well adapted to absorb water and mineral nutrients from the soil. After absorption, water is transported up the plant to the leaves in a specialised tissue called xylem.
The mineral nutrients are water-soluble and they are carried in solution up through the plant with the water.
Plant Transport Tissue
Xylem transports water and mineral nutrients up the plant, ending at the leaves. Xylem is composed of long empty but water-filled dead cells. The dead xylem cells, with their missing end walls, form long open microscopic pipes from the roots to the margins of the leaves. The water, with the dissolved mineral nutrients, travels through these pipes being pulled by the evaporation of water from the leaves – the plant does not have to work to transport water. Because xylem is so hard and strong it also plays an important role in the support of the plant. In woody plants the wood is mostly composed of xylem.
Phloem transports food in the form of a sugar solution. Food is transported from the leaves to the other parts in summer. In early spring food is transported from the roots to all the growing points and especially for the formation of new leaves. Phloem is composed of living cells – the plant has to work to transport food.
Phloem is a soft tissue and in woody stems it is in the bark.
Transpiration is the evaporation of water from the surface of a plant in the form of water vapour.
Transpiration mostly occurs from the leaves. The leaves are the major evaporation sites because of the huge number of microscopic holes in their surface through which water vapour can escape.
These holes are called ‘stomata’ and are open in light to allow the entry of carbon dioxide for photosynthesis. Beyond the stomata the leaves and stems have a water-proof covering to reduce evaporation and so conserve water.
Each plant is ‘full of water’ from the tips of the roots to the tips of the leaves. The roots are in direct contact with the water in soil. Evaporation of water from the leaves ‘pulls’ on the column of water all the way through the plant into the soil. As water is lost from the leaves it pulls water up the plant and pulls water from the soil into the roots. The upward flow of water in the plant is called the transpiration stream.
The faster the evaporation of water i.e. transpiration, the faster the transpiration stream and the faster the absorption of water from the soil.
Investigation: To Show the Path of Water Through Plant Tissue
Cut the base, non-leafy end, of a fresh stalk of celery under water.
Place the cut end into a beaker of water stained with red food dye.
Leave the celery stalk in bright light at room temperature in a breeze.
Note the red lines moving up the stalk and then along the veins of the leafy parts.
Cut across the stalk and note the curve of red dots close to the outer edge.
If a thin section is examined under the microscope it is the xylem which has been stained red in colour.
Investigation: To Show that Water Evaporates from the Surface of a Leaf by Transpiration
Place a clear plastic bag around the leafy portion of a potted plant.
Remove the leaves of a second potted plant and place a clear plastic bag over the leafless shoot system.
The plants are left in bright light at room temperature (20˚C) for 6 hours.
A colourless liquid condenses on the inside of both bags.
Much more liquid condenses in the bag surrounding the leafy shoot.
The liquid is water because when tested it turned dry white copper sulphate blue; it also turns dry blue cobalt chloride pink.
The water is the condensed water vapour evaporated from the plants.
The leaves are the major sites of evaporation because with leaves much more water is lost from the plant.
Factors Affecting Transpiration
Transpiration is a special example of evaporation so any factor that affect evaporation will also affect transpiration.
1. Temperature: the higher the temperature the faster the rate of transpiration
2. Humidity: the lower the humidity the faster the rate of transpiration
3. Wind Speed: transpiration increases as wind speed increases.
4. Light: stomata open in light for photosynthesis and so transpiration will be high; they close when it is dark and so transpiration is greatly reduced.
5. Water Supply: if plant is short of water, stomata close reducing transpiration and so conserve water.