- Written by Kevin R. Lee Kevin R. Lee
- Published: 17 November 2019 17 November 2019
Liquid Transport Downward:
When it comes to feeding the roots and all other non-energy-producing parts of the tree, we rely on another miracle tissue called the phloem. The phloem is responsible for getting the sugar produced in leaves distributed wherever it is needed. The phloem is not in the trunk with the growth rings, the xylem, but in the bark. Remove all of the bark from a branch or trunk and you have removed the phloem, the cortex, the cork and other tissues.
If we consider the whole of the bark from the sapwood, xylem outward, there are seven distinct tissues. Starting closest to the sapwood we have the vascular cambium; this thin layer of mother cells covers the entire tree body under the bark and is responsible for enlarging branches and trunks through a process called secondary growth. The next layer outward is the phloem, followed by the cortex. The last four layers make up the outer bark, phelloderm, cork cambium, cork, and epidermis or the rough outer layer we call the bark.
Our concern right now is the phloem and the feeding of non-energy-producing parts of the tree. The downward movement of sugars takes place in the phloem.
Sugars are always moved from source, the leaves, to sink, the roots. The concentration of sugars produced in the leaves, is moved in the phloem tissue called sieve tubes. And it is the change in sugar concentration that draws them downward, again osmotic pressure. On their way they feed all the tissues of the living tree that are without the ability to feed themselves, tissues that perform essential work but are not green and exposed to the sunlight.
This is a rough working model of the complex system called liquid transport, meant to give you a glimmer of wonder about the amazing intricacy and efficiency of the tree body. I am not a botanist, but use sound botanical principles to explain this, and also in my work. The more botany I read, the better arborist I become. I strongly advise anyone truly interested to follow up with a reading in such classics as " Biology of Plants " by Peter Raven, Ray Evert and Susan Eichorn. I have the sixth edition. Currently in its 8th edition, it sells new for close to $200, but all of the editions, even the first, which can now be picked up for pennies, abound in the amazing science of the life of plants.