A tree’s body is every bit as intricate as your own: many types of cells, formed into many tissues, energy produced in one zone, water and nutrients extracted in another. How to orchestrate all this activity of trillions of cells that have constant needs? How to make enough energy to survive the winter and keep everybody happy? Life, a highly ordered set of procedures, has all this figured out for all of us, from single-celled bacteria on up in size to redwoods and whales. Quite the planet you got there!
From the tree’s perspective there are two main operations: first how to get the water and minerals out of the ground and up into the most remote leaves where they are needed and, secondly, how to get the abundant sugars produced by the leaves all the way back down to the farthest root tip, where they are needed. Two different processes, two different tissues. The first, groundwater and minerals moving up, is called the ascent of sap, or the cohesion/tension (CT theory) model of the ascent of sap. It is accomplished by the tissue called xylem. The second, the distribution of sugar to all hungry plant parts, is called the pressure flow hypothesis, and uses a tissue called the phloem. Traffic in both ways is busy, so each system has its separate series of lanes (pipes).
First, the ascent of sap. I mentioned earlier when speaking of plant evolution that early plants seemed to have been given little to work with. This couldn’t be more true when it comes to the major problem of how to lift a lot of water a long way up. The long time of evolution, many failed experiments, back to the blackboard again must have been the struggle of early plants as they attempted to rise above the mud. The solution was twofold; first, we needed powerful cell walls that could stand up to high pressure, hold up the trunk, and yet the cells had to be hollow and, therefore dead, to allow water movement. They needed to be in a range of internal sizes that worked with the inherent natural properties of water. The first problem, cell wall strength was solved by the creation of two miracle molecules: cellulose and lignin. When the two are combined in interior cell wall fortifications, the strength issue is solved. The second problem, dealing with the natural properties of water, may have taken some time. To say water is the miracle molecule of our home planet is the biggest understatement possible, simply water is life, and learning how to use it is some of the mystery.
A triangular shaped molecule with two hydrogen atoms and a single oxygen molecule, H2O has some amazing properties, considering its simplicity. Water molecules love each other, love to stay together. Apart from being very “sticky” with each other (tension), they also adhere to surfaces with great strength (cohesion). It is these two seemingly simple properties that early plants utilized to get water up to their tops. So if you had pipes small enough to take advantage of these properties and could find a way to move the water up, you would have conquered gravity, with no expenditure of resources, apart from the building of the structure. The final puzzle piece is the evaporation of water from the leaves, again dealing with the natural properties of water. A warm, hot wind has an amazing pull (convection), and literally sucks water vapor away from the surface and open stomata of the leaves. So great is this ability to literally suck water away that the leaves together act as the “mouth” sucking at the end of a great, long, but very thin straw.
So the warm wind blows, drying out the leaf, the leaf asks for water from the twig, the twig asks the branch, the branch asks the trunk, and the trunk asks the roots. The roots ask the water present in the soil. In your garden, that won’t be a problem, because you love your trees and water them. We feed what we love. In dry conditions, the water’s bond to the soil particles is stronger than the root’s ability to try to draw the water away and the root gets nothing or very little. In extreme drought, water can be drawn from the roots into the much drier soil. On its long journey to the upper crown, the water is asked to do many things and go many places. Living radial parenchyma, arranged horizontally in the trunk, lie wrapped up with the vertically aligned dead vessel elements. Water, if needed, easily passes from vessels to rays, through tiny holes in the cells walls, and is then moved laterally where needed. This is how the phloem gets all the water it needs.
