The Effect of Gravity on Tree Growth
While a branch’s origin on the trunk doesn’t change—one thing I remember answering correctly on my biology SAT test 400 years back—the branch itself may bend and reorient.
Trunks and branches are subject to many forces that change their shape. Some are environmental. And some are of the tree’s own making.
One curious force that the tree initiates is called gravitropism. This is when a trunk has fallen over, as it might from a stiff gale, and the trunk bends upwards to compensate. It does this by growing reaction wood.
What’s fascinating is that angiosperms (flowering plants) and gymnosperms (cone plants) have evolved opposite reaction wood methods.
The angiosperms create tension wood, where the cells on the inside (upper) side of a prostrate trunk contract. Gymnosperms on the other hand create compression wood, where cells on the lower side of a trunk elongate. Both move the trunk into a more vertical position, one by pulling and the other by pushing.
A gymnosperm (a Hemlock) trunk. The arrows indicate the effect of compression wood. Cells on the lower side of the trunk elongate to create growth asymmetry, curving the trunk up.
Let’s imagine this same tree as if it were an angiosperm. The cells on the top of a prostrate trunk have contracted to create tension wood, pulling the trunk upright.
Trunks are not the only places we find such buttressing. Old, heavy branches appear to create reaction wood to prevent breakage.
If all that isn’t wild enough, where reaction wood is built is controlled by graviperception, found in epidermal cells that have an apparatus that can tell up from down. Roughly this is a specialized cell with independently moveable starchy things—imagine an orb half-filled with marbles—and when the orb moves, the marbles reorient. Depending on where they reorient, a physical response is initiated, changing plant shape. Roots and shoots have opposing reactions to this process so the roots don’t grow up and the shoots don’t seek the dark.
So, your tree’s cells can perceive gravity. Which is more than I can do some days.
6 Comments
Great post! We often talk about how environmental factors shape a tree – I mean the obvious ones like snow load, the action of the wind and a nearby space that gets more sunlight. Gravity is unseen though and so are the varied processes happening within the tree to hold the shape, so that’s a couple of new layers to consider. Thanks!
Thanks Darren! Glad you enjoyed that one-
I remember learning about this way back in high school biology…. I’m curious, though; does the formation of reaction wood still happen if the branch has been artificially bent, through wires or other means? i.e. if there isn’t actually a need to change the direction of growth at that spot, because the artist has already taken care of that for the tree–will reaction wood still grow, strengthening the tree at the bend? Or does the tree just ‘set’ the turn by filling in cells of normal size on both sides of the bend? And if the latter, then when we wire in upward or downward bends, are we leaving the tree weaker at those points than it would have been if it had executed the turn on its own?
What a wonderful set of questions…thanks Adiv!
I am not qualified to answer these. My understanding has been that wiring sets up a non-gravitational reaction, meaning, I’ve assumed that the answer to your first question is a yes, with the “set” being a reaction that surrounds the area, not just part of it. Given what I see on a bent branch, the biting in does not seem to be on one side only. BUT, I would love it if others more knowledgeable about this would chime in! I’d like to know.
Love…..LOVE….this kind of info!!!!!!
Like always you provide excellent information. i was recently introduced to Dr Suzanne Simard author of Finding the Mother Tree. Anyone who loves trees should read what she has discovered about trees communicating among themselves. Yes they do talk to each other and they know their kids and they share nutrients through the mushroom medium with their kids and sometimes with other types of trees. It is really amazing and solid proven science. Our trees probably all have a ‘sense of self’. Do good things everyone!