Objective:

• to explore the mechanisms by which water is transported throughout the tree.

Lesson Information:

	Grade Level	12
	Subject	Biology 30
	Curriculum Correlation	Cell Structure and Function
	CEL's	Technological Literacy, Communication
	Duration	1 to 3 hours
	Group Size	any
	Setting	indoors, lab situation
	Vocabulary	xylem, vessels, tracheids, capillary action, transpiration. photosynthesis phloem.

Activities:

• Design a Tree Model of Water Transport

• Water Transport Through Plant Stems

Advance Preparation

• Plan for at least two class periods to conduct Activity # 1. The first part of the activity requires one class period to hold the planning, discussion, and brainstorming session (see Activity # 1 for ideas). The second part requires at least one more class period to build the model trees.

• If you plan to conduct the demonstration in Activity #2, please note that parts of the experiment require more time than one class period. You may modify the experiment to fit your schedule.

• If you have more than one group conducting the experiment in Activity #2, you will have to provide a complete set of the materials for each of the groups.

• Read through the activities. Decide which ones you wish to conduct and obtain the appropriate materials.

• If you conduct Activity # 1, students will be creating a list of materials to be obtained for their designs. Remind students that materials should be relatively inexpensive and readily available.

• If you wish to have students conduct the experiment in Activity #2, make copies of the experiment sheet (one per student or group).

Background Information

An individual tree can be thought of as molecules, cells, and tissues organized into leaves, branches, trunk, and roots. But a tree is more than the sum of these parts - it is a living, functioning organism. Trees, in fact, are among the most widespread, the largest, the longest living, and the most successful of all living things on the planet. This lesson examines how a tree is designed and the functions played by each of its different parts. It explores how they all work together to keep the tree alive and growing.

A key to understanding tree design is the fact that the bulk of a mature tree is made up of dead tissue, whose sole function for the tree is to provide protection and structural support. Only about one per cent of a treeÌs volume consists of living cells that need constant maintenance. Nourishment for these cells comes from both ends of the tree - from sugars manufactured in the leaves at the top and from water and minerals drawn up by the roots at the bottom.

The roots of a tree have several functions.

• They anchor the tree in the soil;

• they absorb and conduct water and mineral salts;

• and, in some species, the roots may sprout to produce new seedlings.

Water and dissolved minerals enter the tree from the soil through fine root hairs, which are only 0.01mm in diameter and live for just a few days. Most of the root is inactive, but growing tissue at the root tips produces new root hairs that push the roots deeper into the soil.

The water and minerals absorbed by the roots pass up the trunk to the branches and leaves. Water is conducted through the tree in tissue composed of xylem cells . In deciduous trees, the elongated xylem cells join end to end to form long, open, microscopic pipes called vessels. A single vessel may be several metres long. In coniferous species, water conduction is carried out by needle shaped cells called tracheids, which are 3 to 5 mm long and about 0.03 mm wide. Tracheids have small openings or pits at their ends. These openings overlap with those of other tracheids, forming open gateways for water to pass through from one cell to another. The special design of these water conducting cells allows them to carry water to a considerable height against the force of gravity, to supply the needs of the tissues at the top of the tree.

The exact process by which water is taken up by plants against the force of gravity is not fully known. The most commonly accepted theory to explain this process is the cohesion-pull theory, which involves principles of osmosis, molecular cohesion, capillary action, and transpiration. Essentially, water is drawn up the long, fine cells by capillary action, a term describing the tendency of water to move in fine spaces because of the molecular attraction between the water molecules and the vessel walls. (Capillary action is also responsible for the movement of water up through soil.) In the leaves, water is expelled to the atmosphere by evaporation in the process of transpiration. This loss of water creates an osmotic pressure, exerting a pull through the water-conducting tissues that, together with capillary action, allows the plant to raise water up 100m or more to the top of the highest trees. This is well above the limit of tom, the maximum height to which water can be raised against the force of gravity by human-made suction pumps.

Water has two main functions in the tree.

• The pressure of the columns of water moving through the plant helps to give the tissues strength and rigidity. You can see this clearly in smaller plants that do not have any woody tissue for support. They depend on their water columns to keep their tissues firm and upright; in hot, dry weather, such plants droop and bend over because they are unable to keep enough water moving through their stems. Water must be constantly drawn up by the roots to replace the water lost through transpiration. A large leafy tree may draw up and transpire as much as 900L of water every day - enough to fill a bathtub four times.
• Water is required in the process of photosynthesis, in which sugars are manufactured in the leaves from water and carbon dioxide using the energy of the sun. The sugars provide energy for active and growing cells. Sugars are transported from the leaves to the rest of the tree through phloem cells located in the inner bark.

Resources

1. The Tree, an 18 minute film showing how an oak tree lives, grows, and interacts with a variety of organisms (NFB: No.106C 0177 331), and

2. A Tree Is a Living Thing, an 11-minute film for younger students that shows through microphotography of the leaf cells how a tree feeds, exchanges gases, and grows (NFB: No. 106C 0164 020).

Related Internet Links:

• Saskatchewan Education Evergreen Curriculum

• Canadian Forests