Above Image: History of experiments leading to the discovery of the hormone that changes the direction of a plant's growth. Source: Let's Talk Science
One of the major differences between plants and animals is that plants cannot move from place to place to get away from an unfavourable environment or situation. But this does not mean that plants do not move at all. A tropism is plant movement in response to an external stimulus in the environment. If a plant moves towards a stimulus, then it exhibits positive tropism; if a plant moves away from a stimulus, then it exhibits negative tropism.
How Do Plants React to Their Environment?
Plants can respond to gravity, water, touch, and light in their environment.
- Plant parts can grow with or against gravity. This type of tropism is called gravitropism. Roots of a plant grow downward and exhibit positive gravitropism. Stems, on the other hand, exhibit negative gravitropism since they grow upwards and against the force of gravity (see Figure 1). If you place a plant on its side or even upside down, the stem will curve its way up and the roots will curve their way down. During germination, this helps ensure that the seedling's stem will eventually find its way up out of the ground and grow towards the Sun while the roots will keep growing downward into the soil. To learn more about germination, see Seeds and Germination.
- Hydrotropism is plant growth response towards or away from water. This usually happens in the roots, where the plant typically absorbs water. If a plant senses that there is water close by, the roots will grow in that direction.
- A plant’s response to light is known as phototropism. A great experiment to do at home is to give any house plant a single source of light. Soon you will begin to see the plant bending towards the light source (see Figure 2), growing in that direction to get to more light for photosynthesis.
- Thigmotropism is plant growth response to touch. An example of this tropism is the curling of a vine tendril around objects that it touches. This helps the plant securely position itself and keep growing, as these types of plants do not usually have a strong stem to keep themselves upright. Mimosa pudica or “sensitive” plant responds to touch by completely folding its leaf when touched. Scientists think that this response is to help protect the plant from insects. The Venus fly trap is another plant that responds to the external touch stimulus. When the hairs on the inside of the leaf are triggered, usually when an insect has landed, the leaf closes shut. Why? Insects provide nutrients to the plant. Over time, the insect is dissolved and the nutrients will be absorbed by the plant.
Charles Darwin and the History Behind Phototropism Discovery
What does Charles Darwin, the father of the Theory of Evolution, have to do with tropisms? In 1880, Charles Darwin and his son Francis Darwin discovered that the presence of light is detected by the tip of a plant shoot. They studied grass seedlings and found that covering the tip with an opaque cover prevented the bending of the plant toward the source of light (see Figure 3). They also determined that the bending of the stem towards the light or the response to the stimulus occurs lower, below the tip.
In 1913, Peter Boysen-Jensen (Danish plant physiologist) found that the phototropic signal from the tip of the plant is mobile. His experiments involved removing the tip of the stem and putting a gelatin block in between the tip and the rest of the plant (see Figure 3). Since this block allows chemicals to pass through, the plant produced a phototropic response towards light. When he separated the tip and placed a piece of mica or a piece of rock in place of the gelatin block, nothing happened. The plant did not curve toward the light, as it normally would, because the chemicals that are moving in the stem cannot pass through the rock.
In the late 1920s, Nicolai Cholodny and Francis W. Went independently proposed a mechanism by which plants bends towards a light source; later named the Cholodny-Went theory. They found that a plant hormone called auxin helps control the direction of a plant’s growth in response to environmental stimuli. Auxin is made in the meristematic regions of the plant including the very tips of stems and roots. They showed that there was a buildup of auxin on the shaded side of the plant. Due to this buildup, the cells on the darker side elongate faster than the cells on the side towards the light source. This results in the stem bending towards light.
Tropic Responses in Space
Gravity has been known to be one of the major forces directing the growth of plants, including light and water. If gravity plays such a major role in tropic responses, what happens to plants in space where there is microgravity? Scientists discovered that the presence of gravity may not be as important as previously thought. They found that even when there is limited gravity; roots still grow downward and exhibit positive gravitropism. Other factors are also important to directing root growth. Fundamentally, roots grow away from where the seed is planted and away from light in search of water and nutrients. The discovery that plants in space can show similar growth to plants growing on Earth is very exciting for future long duration space flights and the establishment of mission bases on planets such as Mars.
A plant hormone responsible for directing plant growth and causing the elongation of cells in stems.
(1809-1882) An English naturalist who proposed the Theory of Evolution.
A theory that proposes that the accumulation of auxin on the shaded part of the plant is responsible for an elongation response to light as an external stimulus.
The movement of a plant in response to gravity.
The movement of a plant in response to water.
Meristematic cells or tissues in plants are those that have not differentiated. These types of cells produce various plant organs and are found where growth occurs within a plant.
This is when things appear to be weightless or float in space and indicates that g-forces are not zero, just very small.
The movement of a plant away from an external stimulus.
The process used by plants to change light energy into biochemical energy (sugar). Light energy is used to change carbon dioxide and water chemically into oxygen and sugar.
The movement of a plant in response to light.
The movement of a plant towards an external stimulus.
The movement of a plant in response to touch.
The movement of a plant in a certain direction in response to an external stimulus.
- Controlling plant growth (Retrieved August 9, 2016). This article describes various types of tropisms.
- Darwin, Design, and Phototropism (Retrieved August 9, 2016). This article from Evolution News discusses the discovery of phototropism and Darwin’s involvement.
- Geotropism (Retrieved August 10, 2016).
This article describes gravitropism. Other links on this page link to descriptions of other tropisms.
- How Venus Flytraps Work (Retrieved August 9, 2016). This article from How Stuff Works describes the mechanism by which Venus flytrap captures prey.
- Plant growth strategies are remodeled by spaceflight (Retrieved August 9, 2016). This article from BMC Plant Biology discusses growth patterns in plants that are grown in space.
- Plants Grow Fine Without Gravity (Retrieved August 10, 2016). This article from National Geographic News discusses results of plant growth experiments performed in space.
- Plant tropisms: The ins and outs of auxin (Retrieved August 10, 2016). This article from Current Biology discusses the role of auxin to plant growth responses.
- Tropisms (Retrieved August 9, 2016). This article from Biology Pages discusses tropisms and the experiments that led to their discoveries.
- Understanding phototropism: from Darwin to today (Retrieved August 15, 2016). This article from the Journal of Experimental Botany discusses the history of phototropism research.
- What happens to plant growth when you remove gravity? (Retrieved August 10, 2016).
This article form ScienceDaily discusses results of plant growth experiments performed in space.
- Why Do Touch-Me-Not (Mimosa Pudica) Leaves Close When Touched? (Retrieved August 10, 2016). This article from Science ABC describes the reason behind the plant’s response to thigmotropism.
- How do plants find ‘up’? (Retrieved August 9, 2016). This activity from Fizzics Education explains an investigation to study gravity in plants.
- How Does Gravity Affect Root Growth? (Retrieved August 15, 2016). This website from BioEd Online allows students to experimentally test effects of gravity on root growth.
- The Mysterious Venus Flytrap (Retrieved August 9, 2016). This article from Botanical Society of America describes how Venus flytrap works and how to grow it.
- ISS Update: Plants in Space (Retrieved August 15, 2016). This video from YouTube discusses the impact of gravity in space.
- Plants in Space (Retrieved August 10, 2016). This booklet from NASA discusses classroom activities that investigate growth responses of plants.
- The Power of Movement in Plants (Retrieved August 9, 2016). This website from Darwin Online showcases Darwin’s works on phototropism.