How Do Plants Organize The Orientation of Their Branches
While it is clear that gravity is the main force that keeps trees growing vertically, it is less clear how the angles of the branches are determined by plants. If a tree gets blown over in the wind the trunk and branches all re-orientate so that the new shoots grow vertically despite the tree toppling over.
But how do plants set and keep their branches growing out at an angle of 20-30 degrees towards the horizontal. Most species of plants appear to have their own angles that often differ from other species. How are these angles and growing patterns organised?
Understanding this puzzle is complicated by the observation that the angle of shoots or roots is not set in relation to the main truck, root or stem, but that it is related to gravity.
If you place a plant in a pot on its side the branches will start a new phase of growth that is related to gravity. The new growth shifts away from the original angle between the branch and the trunk. It reorientates to a new angle to mirror the trunk to branch angle relative to gravity.
If the trunk angle changes, the branch angle changes as well and both are related to the vertical angle of gravity.
How to Tree Trunks and Main Stems of Plants Grow Vertically
The mechanism responsible for the vertical growth of the main stem or trunk and how gravity controls it is well understood:
- Statocytes - Special gravity sensing cells detect when the plant has been shifted away from vertical (aligned with gravity).
- This triggers a boost in the amount of the growth hormone auxin on the under side of the shoot or root, that is the side towards which the shoot has tilted down. This boost in the amount of hormone causes the shoot to grow vertically once again.
- The statocytes provide a constant feed-back mechanism keeping growth vertical in relation to gravity.
What Controls the Branch Angles?
For branch growth another controlling mechanism has been discovered.
This has the effect of generating an 'anti-gravity offset'. In a sense it adjusts the level of auxin by a fixed amount to generate an offset.
This sustains more growth on one side of the branch than the other and stops the branch growing vertically.
The mechanism is similar to the way a paddle steamer is steered without a rudder. The turn is initiated by applying more power to one side than the other. If you want to straighten the direction of the paddle steamer you change the speed on both sides so they are balanced once again.
For a branch growing out from the main stem the anti-gravity offset is kept constant. The statocytes still sense gravity and make adjustments if the tree tilts over, but the offset maintains the orientation in relation to gravity.
There you have it. Plants are smart, aren’t they?