Biology
Growth and tropisms
Why plants stretch toward the light and send their roots downward, even though they cannot move around.
A plant cannot run away and cannot look for a better spot. Even so, it reacts to its surroundings. It senses where the light comes from and where down is, and it aims its growth exactly at that. Such directed growth movements are called tropisms.
Growth toward the light
Put a plant by the window and after a few days it leans toward the light. This is called phototropism. The shoot grows faster on the side turned away from the light than on the bright side. Because one side gets longer, the shoot bends toward the light.
Growth with gravity
also sets a direction. The root always grows downward and the shoot always grows upward. This holds even if you place a seed the wrong way round into the soil. This response to gravity is called gravitropism.
Where the plant grows
A plant does not get longer everywhere, only in certain places. These growth zones sit at the tips of the shoot and the root and just behind them. There the cells divide and then stretch lengthwise. That is why a new leaf always appears at the top, and why a nail hammered into a tree trunk stays at the same height for decades.
A messenger substance steers the bending
To make the right side grow faster, the plant spreads a messenger substance. It is called auxin, it gathers on the shaded side, and it prompts the cells there to stretch. This turns the reaction to light into real growth, entirely without nerves or muscles.
Exercises
0 of 6 solvedTime to try it yourself. You can't break anything, every attempt counts.
What is the growth of a plant toward the light called?
In which direction does the root grow because of gravity?
A shoot grows 2 centimeters a day. How many centimeters does it grow in 5 days?
Match each technical term to the right description.
In one day a shoot grows by 8 millimeters on the shaded side and by 5 millimeters on the bright side. How many millimeters difference is that?
When growing toward the light, the shoot gets longer on the … and thereby bends toward the light.