The rate of transpiration can be investigated by measuring the decrease in mass due to water loss, or by measuring the volume of water absorbed.
Decrease in mass
Cut the leaves from the plant, and one or both surfaces may be coated with grease to prevent transpiration. The table shows some typical results:
|Leaf 1||Leaf 2||Leaf 3||Leaf 4|
|Surface coated with grease||None||Upper only||Lower only||Both|
|% decrease in mass||40||36||4||2|
The results show that most transpiration happens from the lower surface of the leaf:
- coating the upper surface caused water loss similar to coating no surface at all (leaf 2 vs leaf 1)
- coating the lower surface caused water loss similar to coating both surfaces (leaf 3 vs leaf 4)
Volume of water absorbed
A potometer can be used to measure the volume of water absorbed. Usually a leafy shoot is used instead of a single leaf.
A simple potometer comprises:
- a container of water
- a capillary tube
- rubber tubing to connect the capillary tube to the stem of the leafy shoot
- a scale to measure the distance travelled by a bubble in the capillary tube
The faster the bubble moves, the greater the rate of water uptake - and so the greater the rate of transpiration. You can vary the conditions to investigate the effect of changing temperature, humidity, wind speed (for example, with a hair dryer) or light intensity.
Transport in plants
Small unicellular organisms such as amoeba and paramecium, and small multicellular organisms like flat worms and sea anemones have a large surface area to volume ratio. Such properties allow them to transport food and gases around their cells by the process of diffusion. Their large surface areas allow gases to travel into their cells by diffusion, and once inside the cell diffusion is sufficient to cover the small distances from one part of the cell to the other.
Multicellular organisms like plants and animals have a small surface area to volume ratio. They are made up of tissues, organs and organ systems. In addition, the more complex organisms may be covered by shells, scales, feathers, skin or hairs which do not allow diffusion. Such organisms therefore need a transport or circulatory system because diffusion alone is not capable of meeting the transport requirements for survival.
The transport system in plants is made up of a network of thin tubes which carry liquids all around the plant. Collectively these tubes are bundled together and form the vascular bundles. The vascular bundles reach all the tissues and organs of the plant.
There are two different types of thin tubes or transport tissues which make up the vascular bundles in plants.
The xylem vessels transport water and minerals from the roots to the shoots and leaves.
This movement of water and minerals through the plant is called transpiration.
Xylem vessels are hollow tubes or lumen with a thick strengthened cellulose cell wall.
The hollow tubes act like pipes allowing water and dissolved minerals to flow through them. They develop from cylindrical cells arranged end to end, in which the cytoplasm dies and the cell walls between adjoining cells breaks down leaving a dead empty tube.
The cell walls in xylem vessels contain a substance called lignin which strengthens the cells and gives structural support.
Phloem tubes carry food substances like sugar and amino acids produced in leaves during photosynthesis to every part of the plant.
The movement of food substances through the plant is called translocation.
Phloem tubes are made up of columns of living cylindrical cells. The cell walls between adjoining cells develop holes like a sieve allowing transport through the tube.
The image below shows the structure of the xylem and phloem
The table below summarise the main points about the xylem and phloem
|Transport||Water and mineral from the roots to the shoots and leaves||Sugar and amino acids produced in leaves during photosynthesis to every part of the plant.|
|Structure||Cylindrical cells arranged end to end, in which the cytoplasm dies and the cell walls between adjoining cells breaks down leaving a dead empty tube with strengthened cell walls.||Phloem tubes are made up of columns of living cylindrical cells. The cell walls between adjoining cells develop holes like a sieve allowing transport through the tube.|