A New Category

One of my readers recently had a good idea for a new category on the blog. His idea was to explore the fascinating world of Tropisms. What are they? Tropisms are the ability to respond, usually by growing toward or away from a particular stimulus. The Latin root “tropism” means “turning.” This refers to the plant turning towards, known as positive, or turning away, known as negative. Plants and fungi have many tropisms and are fascinating.

One of the most common tropisms in plants is positive phototropism, the ability of plants to grow towards light. Everyone knows about this phenomenon but we seldom think about how and why they do it. Usually plants respond to a stimulus because it will increase their chances for reproduction. At the simplest level, a plant grows toward the light so it will get more sunshine and grow bigger and stronger and bear a more prolific cone or fruit or berry with lots more chances to make little baby plants. But how is it done, exactly?

Well, we know some ways it is done and other ways we can only guess. The first and most obvious lesson in tropisms is the classic experiment where a seed is germinated on its side and we can observe the radical (shoot) grow up and the root, grow down. This experiment is done in the dark. This is geotropism, also known as gravitropism. The words mean earth-turning and gravity-turning, respectively. It is usually called geotropism in most classes. The shoot responds by growing in the negative direction from the earth and the root grows toward the earth. The shoot is negatively geotropic and the root is positively geotropic.
This is very handy for a seed covered with soil and starts the little plant on its way to having the roots down and the shoots up.

How does the plant detect the gravity? No one knows. We can guess that the plant hormone auxin is somehow associated with starch molecules that settle on the downward side of the plant. Strictly speaking the starch molecule theory has been disproven but some mechanism tells the auxin hormone to go to the bottom side of the plant. We do know that auxins have opposite effects on the plant parts. The shoots respond by growing away from the auxin heading the new shoot upward away from gravity.

The auxin hormone, on the other hand, makes roots grow towards itself. Down the root tip goes, down into the soil where it belongs, if you are a root. This is the way new plants get headed in the correct directions. This experiment is a classic botany lesson first described by Charles Darwin.

There are many other tropisms in plants and fungi involving different stimuli. I will try to do a occassional post on some of the more interesting ones.

Thank you to my reader who has started an interesting area of blogging.