Darwin : Phototropism

Darwin was fascinated by the effects of light on plant growth, as was his son Francis. In his final book, The Power of Movement in Plants, Darwin wrote: ‘There are extremely few [plants], of which some part … does not bend towards lateral light.’ Or in less verbose modern English: almost all plants bend towards the light.

In a very simple experiment, published in 1880, Darwin and his son showed that this bending was due to some inherent sensitivity to move towards the light.

For their experiment, Darwin and his son, Francis, grew a pot of canary grass (Phalaris canariensis) in a totally dark room for several days. Then they lit a very small gas lamp twelve feet (3.5 metres) from the pot and kept it so dim that they ‘could not see the seedlings themselves, nor see a pencil line on paper.’ But after only three hours, the plants had obviously curved towards the dim light. The curving always occurred at the same part of the young plant, an inch or so below the tip.

This led them to question which part of the plant saw the light. The Darwins carried out what has become a classic experiment in botany.  They hypothesized that the ‘eyes’ of the plant were found at the seedling tip and not at the part of the seedling that bends.

They checked phototropism in five different seedlings, illustrated by the following diagram:

a. The first seedling was untreated and shows that the conditions of the experiment are conducive to phototropism.

b. The second had its tip pruned off.

c. The third had its tip covered with a lightproof cap.

d. The fourth had its tip covered with a clear glass cap.

e. The fifth had its middle section covered by a lightproof tube.

They carried out the experiment on these seedlings in the same conditions as their initial experiment, and of course the untreated seedling bent towards the light.  Similarly, the seedling with the lightproof tube around its middle bent towards the light. If they removed the tip of a seedling, however, or covered it with a lightproof cap, it went blind and couldn’t bend towards the light. Then they witnessed the behaviour of the plant in scenario four (d): this seedling continued to bend towards the light even though it had a cap on its tip.  The difference here was that the cap was clear. The Darwins realized that the glass still allowed the light to shine onto the tip of the plant.

In this experiment, the Darwins proved that phototropism is the result of light hitting the tip of a plant’s shoot, which sees the light and transfers this information to the plant’s midsection to tell it to bend in that direction.

Extracted from What a Plant Knows – A Field Guide To The Senses Of Your Garden And Beyond, 2012, Daniel Chamovitz