GHRE: Gravitropism

The word growth is rooted in the proto-Germanic word GHRE. It refers to early spring when the first signs of green emerge as young leaves and grasses start to sprout. The GHRE-series investigates if and how our generally positive understanding of growth was influenced by its specific seasonal plant-based origins, as well as try to understand some of the principles of plant-based growth. In this articles we zoom in on gravitropism: the ability of plants to respond to the force of gravity.

The biological phenomenon indicating the movement of a plant towards an environmental stimulus, such as gravity, is called a tropism. As plants grow, some cells move away from gravity’s pull and develop stems and branches. This proces is called negative gravitropism, as these cells seem to negate gravity. Other cells move towards gravity’s force to form roots, a proces that is called positive gravitropism. The mechanism behind this directional plant response was first described in the Cholodny-Went model. The theory predicts that growth suppressing phyto-hormones move towards gravity’s pull where they limit growth in cells, while growth stimulating factors move away from gravity where they stimulate the production of larger cells. As a result the roots bend towards the gravitational force, and towards earth. The illustration shows the accumulation of growth suppressors in the bottom part of a root. The growth of stems and branches works pretty much the other way around but is directed mostly by the detection of sunlight. And that is slightly confusing since the sun is easier for us to observe. That probably influences our understanding of growth as a positive response to sunlight. We say that plants grow towards the light, but it is equally true that they grow towards gravity’s pull. It is about the balance between a sunny UP and a gravitational DOWN.

As people and economies continue to grow bigger, they mostly negate the force of gravity while celebrating the metaphor of growing towards the light. This negative gravitropism may eventually cause our downfall much like it did for Icarus when he flew to close to the sun. Gravity is best kept as a friend.