Light as the Stimulus

Plants respond to light as a stimulus.



A13.2 A Trunk showing Phototropism
Have you ever noticed a plant that is bent? This often occurs because the plant was at an odd angle at one point but continued to grow toward the light. Look at image A13.2. The trunk of the tree is bent at the base because as the tree emerged from the cliff face, it started to grow toward the sunlight. The plant did not want to grow straight out from the cliff, as only the part of the tree facing upward would get sun. By bending and growing toward the sun, all the leaves get equal access to the sun.

This reaction to the sun is called phototropism. “Photo” means light and “tropism” means the direction of growth in response to a stimulus. Together, “phototropism” means the response of the plant’s growth to the sun. Positive phototropism means the plant is growing toward the light. The plants stem, trunk, and leaves show positive phototropism. Negative phototropism means the plant is growing away from the light. The plant’s roots show negative phototropism. Phototropism ensures the parts of the plant that perform photosynthesis (the leaves) are closest to the sun.

Some key scientists were involved in discovering the mechanism for phototropism. All the scientists used the same plant at the same growing stage to carry out their experiments. This ensured their results were due to the reactions of the plant to the sun, not due to the type of plant used.

A13.3 Roots growing away from the sun

A13.4 Darwin and Darwin’s experiment
Charles Darwin and his son Francis were the first to investigate phototropism. They wanted to know which part of the plant detected light and which part reacted to the light. They took some oat grass seedlings and modified the tip of the seedlings. They found that if the tips of the seedlings were covered, the plant did not respond to the sunlight; but if the tips were not covered, the plant showed phototropism. They also found they could cover the stem of the seedling and the seedling would still grow toward the light. This showed the Darwins that while the tip of the seedling was detecting the light, the growth was happening in the stem of the plant. They concluded the cells in the tip must be communicating with the cells in the stem.

Peter Boysen-Jensen was determined to discover how the cells in the tip of the seedling were communicating with the cells in the stem. To do this, he cut the tips off two seedlings. With the first seedling, he glued the tip back on using gelatin, and with the second seedling, he placed a piece of mineral mica between the tip and the rest of the seedling. He found the seedling with the gelatin still grew toward the light, while the seedling with the mineral mica did not. Based on this observation, he suggested it must be a chemical produced in the tip that caused the stem to grow toward the light. This chemical could diffuse through the gelatin to the stem but could not diffuse through the mineral mica.
A13.5 Boysen-Jensen’s Experiment

A13.6 How auxin works
F. W. Went wanted to figure out what the chemical substance was that caused the stem to grow toward the light. His work allowed him to isolate the substance so future scientists could continue to test it. It was later determined this chemical substance is a hormone, or a chemical messenger called auxin. Cells with a high concentration of auxin grew longer, causing the stem to bend. The cells farthest from the sun were the ones containing the highest concentration of auxin, and so they were the cells that grew the longest. This causes the stem to bend toward the light. Note: The way your textbook has worded this explanation makes it sounds like the plant elongates the cells where there is a low concentration of auxin. This is not the case, the cells elongate where there is a HIGH concentration of auxin.

  Try This


Houseplants and Phototropism

If you have a houseplant in your home, try moving it to a window with a strong beam of sunlight. Rotate it every week and note how the plant grows. You should observe that after a length of time, the plant can be seen to bend toward the light. Plants that grow quicker will show this bend faster, while slower growing plants will take longer. Cacti tend to grow slowly, so it will take a couple of weeks to see a difference, while plants like a shamrock or a seedling will bend toward the light within a couple of hours.

©tuchodi via flickr
A13.7 Shamrock showing phototropism
 

  Read This

Please read pages 323, 326 and 327 in your Science 10 textbook. Make sure you take notes on your readings to study from later. You should focus on what phototropism is and how it works. Remember, if you have any questions or you do not understand something, ask your teacher!

  Practice Questions

Complete the following practice questions to check your understanding of the concept you just learned. Make sure you write complete answers to the practice questions in your notes. After you have checked your answers, make corrections to your responses (where necessary) to study from.

  1. What is the difference between positive and negative phototropism? What parts of the plant exhibit each of these?
Positive phototropism is when the plant grows toward the sunlight. The leaves and stem of a plant perform positive phototropism. Negative phototropism is when the plant grows away from the sunlight. The roots of a plant perform negative phototropism.
  1. Complete the following chart outlining the three scientists that contributed to discovering how phototropism works.

Scientist(s) Experimental Question Experiment Performed Results

Scientist(s) Experimental Question
Experiment Performed
Results
Charles and Francis Darwin What part of the plant detects and reacts to light? They modified the tips and the stems of seedlings by covering them or cutting them off. They found the that tip detects the light and the stem bends to grow toward it.
Peter Boysen-Jensen What is the signal that causes the plant to react to the light? He cut off the tips of seedlings and glued them back on using gelatin or mica. He found the tips with gelatin still showed phototropism while the tips with mica did not.
F. W. Went What is the specific substance that causes the plant to react to light? He isolated the exact substance that causes plants to perform phototropism. He found this substance causes cells to elongate and higher concentrations of it are found on the shaded side of the plant.