「植物看得見你」公開課筆記／2.2 Darwin's Experiment

One of the first people to study how plants respond to light and how they bend, was Charles Darwin. While you're all familiar with Darwin's work on evolution, what you're probably less familiar with is that the last 20-30 years of his life, most of Darwin's research had to do with plant biology. One of his most important works was published in 1880, together with his son, Francis Darwin. It's called The Power of Movement in Plants.

He was studying many types of plants, and a certain type of grass called canary grass. Darwin wrote about that he put the canary grass in a room with only a very dim candle on one side of the room, it was so dim that he couldn't even see the clock on the wall, yet the grass bent towards the candle. Darwin did this experiment on tens of types of plants, and he came to the following conclusion, and this is in his own words:

Heliotropism, that's what we call the bending towards the light, prevails so extensively among higher plants, that there are extremely few of which some part, either the stem, flower, peduncle, petiole, or leaf, does not bend towards a lateral light.

Almost every type of plant is sensitive to light, and will bend towards the light.

Darwin was not only an observer of nature, he was also an excellent experimentalist. And Darwin asked the following question. Where is the eye of the plant? Which part of the plant is sensing the light in order to allow it to bend towards it?

Darwin's hypothesis was that the part of the plant that senses the light, what we would call the eye, at the tip of the plant. So what experiments could he do in order to test his hypothesis? So the first thing he did, was he chopped off the tip. The result is that the plant lost the ability to bend towards the light.

We needed a few more experimental controls. So his first control that he did, was he now covered the top of the plant with a hat, that was impermeable to light. And in this case, the plant also didn't bend.

But it could also be that this cap was so heavy that it inhibits the plant from responding. So he did another control. He covered the tip of the plant with a glass cap, which allowed light signals to go through. Under this conditions, the plant does bend.

In a very simple experiment with no huge technology that was published in 1880, Darwin successfully showed that it's the tip of the plant that senses the light signal. And somehow or another, it transfers this light signal, down to the bottom part of the plant. And that's where the bending occurs.

Here's an experiment that was carried out by one of Darwin's colleagues, Jules von Sachs in Germany in the late 19th century. He asked the question, can plants differentiate between different colors? And what he showed, is that the same seedlings that Darwin studied, will bend to blue light, but not to red light.