In my Jan. 31 column, I wrote about the incredible complexity of trees’ hydraulic system.
Now, I’m going to focus on another amazing way trees have evolved to survive: their hormonal system. Yes, that’s right — their hormonal system!
The dictionary defines hormones as “regulatory substances produced in organisms to stimulate specific cells or tissues into action.” Like humans and other animals, plants make hormones to start, stop and control all sorts of functions. For instance, trees use hormones to stimulate leaf, stem and root growth, along with seed production and flowering.
Trees, in fact, produce quite a variety of hormones, with names like auxins, gibberellins, cytokinins, ethylene and abscisic acid.
Auxins generally promote growth and bud development, and help create other hormones. Cytokinins affect cell division and shoot formation.
Gibberellins develop the flowers and induce seeds to sprout.
Ethylene makes fruit ripen but also increases trunk and branch strength.
Abscisic acid causes the leaves to break off and fall in autumn, and also puts seeds into dormancy (which helps them survive the long winter). And there are many, many more.
Here is how they work: In the spring, light shines on the buds of new growth in the trees’ branches stimulating hormones to go down to the roots to encourage growth. These new roots then make a hormone that is sent back up to the buds to get them to open and grow.
Growth hormones are very potent and can cause damage to cells, so almost as soon as they are formed, the tree produces other hormones to counteract or lessen their effects.
Next, the buds at the ends of branches make a special hormone to restrain the growth of other buds. This is especially evident in coniferous (cone-bearing) trees. If you look at the tops of evergreens, you’ll notice right away how long the top or terminal shoot is. The topmost bud causes this to happen by making a specialized hormone that completely controls the growth of the rest of tree. The result is the pyramidal shape that is characteristic of these trees.
If this topmost “leader” shoot dies or breaks, the blocking hormones stop and growth hormones flood the trees, which causes side branches to explode with growth and race to become dominant. The tree’s shape then radically changes, often to its detriment or demise.
Meanwhile, the tree is using hormones to affect how it grows. Some cause the tree to grow toward the light by causing cells on the dark side of a branch to stretch, bending the branch toward the sun. (If you’ve grown house plants on windowsills, you’ve probably seen this happening.)
Other hormones are triggered by gravity to help direct the roots down deeper into the earth. Still others react to insect and rodent damage by manufacturing bitter chemicals or opening new buds for replacement growth. And yet more hormones do the opposite by closing pores and purposely killing off branches to fight infections.
As the season progresses, trees produce new hormones to initiate flowering and then fruit production. And later, in the fall, still different hormones cause the leaves to turn color and break off, and help the tree to harden off cells, protect new buds from the coming winter and to store nutrients for the following year.
Over the course of one year, any given large tree will manufacture and use hundreds of different hormones. Like with their hydraulic systems, there’s nothing passive or simple here. Trees actively sense and respond to environmental conditions like temperature, moisture, light, day length and herbivore damage, using hormones to adapt to changes as they happen.
These complex processes are difficult to study because they involve tiny amounts of a substance manufactured in one part of the tree and affecting different faraway parts — from shoots to roots and the opposite, from one bud to others, etc. Slowly scientists are parsing this mystery. But trees will always have a magical effect on us humans. No matter how much we learn, the ways in which trees emerge, grow and survive are miracles of evolution, a reason for respect and celebration.
Henry Lappen is an environmental educator who performs the show “A Passion for Birds.” His website is www.HenryLappen.com. He is also the chair of the Amherst Public Shade Tree Committee.
Earth Matters, written by staff and associates of the Hitchcock Center for the Environment at 525 South Pleasant St., Amherst, appears every other week. For more information, call 413-256-6006, or write to us.Click here to return to full list of Earth Matters articles.