Skototropism

All plants grow toward sunlight, except when then don’t.

In nearly all cases, plant stems, vines, and bines grow upward, reaching for the sun’s energy. Plants deprived of sunlight will often grow longer than they can support, in an effort to reach that energy source. This is called etiolation. But, sometimes, it is better for a plant to grow away from sunlight. This behavior is called skototropism.

Sun avoidance?
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How could avoiding sunlight be a good idea for a plant? Now, we are not talking about plants taking it upon themselves to avoid a sunburn. That would be something else, entirely. Skototropism is a type of negative phototropism. Phototropism refers to the way plants grow toward the sun. This is done by elongating plant cells on the side of the plant that is not receiving sunlight, hence the bend. Negative phototropism means the bend (and ensuing growth) goes in the opposite direction, away from the sun.

​Jungle behavior

Skototropism is most commonly found among tropical vining plants. One of the biggest challenges to a plant in the jungle is to get enough sunlight. They have plenty of moisture and soil nutrients. But sunlight, that’s the rub. The jungle canopy blocks most of the sunlight to the lower growing plants. Vines climb up the trees that make up the canopy, but how does a vine find a tree?

Devil’s ivy growing up tree (Nyanatusita) CC BY-SA 3.0

The dark side

Light levels are pretty dim at ground level in a thick jungle. If a vine does not find a tree to climb, it will die. If it heads toward visible light (an opening in the canopy), it will never find a tree to climb. Instead, these vines must grow toward the darkest place they can find (the base of a large tree) in order to find something big enough, strong enough, and tall enough, to provide support. What’s really interesting, is that larger trees attract more vines, while smaller trees attract less vines. There are even mathematical formulas that describe skototropism among certain jungle seedlings!

Once a climbing plant has found a support structure, the skototropism behavior is turned off and upward growth (phototropism) begins in earnest.

[Some scientists believe that roots grow down because of skototropism, while others believe it is something called gravitropism. You decide.]