The Did You Know Show 7

DYK7--water circulation in the tallest trees...visit www.dykshow.blogspot.com

Circulation of Tall Trees
by Fred Haase

Tallest trees:

How do the tallest trees in the world transport water over 300ft vertically from their roots to their tops without any moving parts? The world's tallest tree is a redwood that is 379ft tall. There are other species that occasionally exceed 350 ft. For examples; Eucalyptus regnans, douglas fir, and sitka spruce sometimes exceed 350 ft. Indeed! There is evidence that there were individual Eucalyptus regnans in Australia were even taller than the coastal redwoods before being cut down. However, no one measured their height while the trees were standing.

Things to understand before explaining water transport in tall trees.

Osmosis:

Consider a membrane that partitions a container. The same membrane is made of polymers are spaced far enough apart to allow small molecules such as water to pass through. However, the gaps between the polymers of this membrane will not permit large molecules such as sugar or proteins to pass through. If this membrane separates two fluids, such that on one side of the membrane is pure water; while on the other side is a solution of water and sugar. If the two sides of the membrane are initially at the same temperature and pressure, more water molecules will pass through the membrane from the side with the pure water to the side with the sugar solution than from the side with the sugar solution to the pure water side. Why? Since the sugar molecules occupy volume, there are fewer water molecules per unit volume on the solution side of the container than on the pure water side. Since heat is motion, and both sides have the same temperature, the percentage of water molecules that reach the membrane will pass through the membrane from both sides will be the same. However, since there are fewer water molecules per unit volume on sugar solution side of the membrane than there are on the pure water side, more water molecules will pass through the membrane from the pure water side than will pass through the membrane from the solution side. Thus, pressures will rise on the solution side and fall on the pure water side causing the membrane to bow towards the pure water side. This pressure difference that develops is called osmotic pressure. This is step one on explaining how water can be transported in a living plant without any moving parts.

Step 2. Tensile Strength of Water.

One of the amazing properties of liquids is that their molecules attract each other. This is the source of surface tension. Water molecules attraction for each other is greater than most other liquids. Experiments have shown that it takes a negative pressure of at least 24 atmospheres to pull apart a column of water. One may ask: how come a vacuum pump can not lift a column of water more than about 32ft? The reason is that a column of water that is being stretched will increase its length. The water molecules will be pulled farther apart. At time point, the column will break because the distance between the molecules will become so great that they can no longer attract each other. If the column has a surface exposed to air or a vacuum then as the column is stretched, the molecules at the exposed surface will also become farther apart. The random motion of molecules cause by heat will cause some of the interior water molecules to become inserted into the newly formed spaces between the molecules on the exposed surface allowing the surface to grow. That is, if there are any bubbles of air within the column, it will break at tensions far less than one atm.

That is the key. If a column of water between the roots and the leaves is bubble free, it can hold together even with a negative pressure exceeding 24 atm. If such negative pressures can be generated, then it would be possible to pull a column of water through a tube from the roots of a tall tree to the leaves much like pulling piano wire through a duct.

This brings us back to osmosis. The cells of the leaves of trees contain a solution of water, sugars, and proteins. The water in a tube extending from the roots to the leaves is a rather dilute solution. The cell walls between the tube and the leave cells will allow water molecules to diffuse through but block the movement of sugar and protein molecules. Thus, a strong negative pressure will develop between tubes and the cells in the leaves as the water diffuses into the leaves. It turns out just from the energy of molecular motion due to the fact that temperatures are well above absolute zero are more that enough to generate negative pressures in the plant tubes of at least 12 atm. This has been actually measured.

Thus, the mechanism of lifting water from the roots to the leaves has been accomplished. But there are complications.

Step 3.

It seems that the mechanism for the movement of water upward from the roots to the leaves has been explained. It has been explained. However, the products of photosynthesis must be transported from the leaves to the roots or else the roots will starve. It would seem that if negative osmotic pressure can lift water from the roots to the leaves, it would prevent the downward transport of photosynthetic products. Before proceeding, some definitions must be presented.

1. Phloem: Cells that transport photosynthetic products from the leaves to the roots. Some of the cells are hollow tubes for the transport. Phloem is the fibrous tissue found in the inner bark.
2. Tracheids: Hollow tubular dead cells that are found in wood and water is transported from the roots to the leaves through these cells. The tracheids have very small diameter and make up the spring wood in conifers.
3. Vessels: Hollow tubular dead cells with a much greater diameter than tracheids. Vessels also transport water from the roots to the leaves and they are much more efficient than the tracheids. Vessels are easily observed in some hard woods. They are the pores arranged in a ring very noticeable in such hard woods as oaks. In other hard woods, the pores are more uniformly distributed in the wood.

The advantage of vessels is that they are much more efficient at moving water to the leaves than the tracheids because the viscosity effects of a fluid moving through a large diameter tube is less than in a small diameter tube. However, the advantage of tracheids is that bubbles that form when ice forms is much smaller in the tracheids than in the vessels. The larger bubbles in vessels are not as easily reabsorbed by melting ice as the small bubbles in tracheids. Thus, the column of water is much more likely to break in vessels. That is why most conifers are better cold climate trees than most hard woods.

Step 4. The transport of photosynthetic products down the phloems.

Cell membranes between the leaf cells and the phloem will allow the photosynthetic products to pass from the leaf cells into the phloem. Thus, the osmotic pressure that is created in the leaf cells by water diffusing from the tracheids or vessels will also increase pressures in the phloem. This pressure will shove (assisted by gravity) the solution from the leaf to the root. Thus, the circulation system of trees is complete. This fluid movement is driver entirely by osmosis with no moving parts. Much is the system is not even alive. (tracheids and vessels)