C3.1.17 Observations of tropic responses in seedlings (HL) Notes

Tropic Responses in Seedlings Allow Plants to Grow Toward Light and Gravity

1. Unlike animals, plants lack a nervous system to control their growth and responses to the environment.
2. However, they have coordination mechanisms that regulate their development, allowing them to sense and respond to external stimuli such as light and gravity.
3. These responses are known as tropic responses and are controlled by plant hormones, primarily auxins.

Example

• Imagine planting a seed in a pot and placing it on a windowsill. After a few days, you notice the shoot bending toward the light.
• This is phototropism, a type of tropic response where plants grow in response to environmental stimuli.

Key Tropic Responses in Seedlings

1. Tropism refers to the directional growth response of a plant to an environmental stimulus.
2. Several types of tropic responses can be observed in seedlings:

1. Phototropism

1. This type of tropism is growth in response to light.
2. Seedlings often grow toward the light, a process driven by auxins that accumulate on the side of the plant furthest from the light source, causing the cells there to elongate more than those on the light-facing side.

2. Gravitropism (Geotropism)

1. This type of tropism is growth in response to gravity.
2. Roots typically exhibit positive gravitropism (growing downward), while shoots show negative gravitropism (growing upward).

3. Thigmotropism

1. This type of tropism is growth in response to touch.
2. This response is often observed in climbing plants or vines, which wrap around supports as they grow.

Types of Observations: Qualitative vs. Quantitative

Understanding the distinction between qualitative and quantitative observations is crucial in scientific investigations.

Qualitative observations

1. Qualitative observations refer to descriptive data that involve the senses (e.g., observing the direction of growth or the pattern of bending in response to light).
2. Example of Qualitative Data:
   1. The seedling bends toward the light.
   2. The roots grow downward.

Quantitative observations

1. Quantitative observations involve measurable data, such as measuring the angle of curvature of seedlings or recording the rate of growth over time.
2. Example of Quantitative Data:
   1. The seedling bent at a 30° angle towards the light after 24 hours.
   2. The roots grew 4 cm downward in the first 3 days.

Distinguishing Between Qualitative and Quantitative Observations

1. Qualitative Observations
   1. Descriptive and non-numerical.
   2. Focus on characteristics, such as color, shape, or direction of growth.
   3. Useful for capturing overall patterns or behaviors.
2. Quantitative Observations
   1. Numerical and measurable.
   2. Provide specific data, such as angles, lengths, or growth rates.
   3. Allow for precise comparisons and statistical analysis.

Strategies for Reliable Measurements

1. Standardize Procedures: Follow a consistent method for all measurements.
2. Increase Sample Size: Observe multiple seedlings to account for natural variation.
3. Document Uncertainties: Record the margin of error for each measurement (e.g., ±$1^\circ$).

Factors That Affect Precision and Accuracy of Measurements

When conducting experiments to observe tropic responses, it’s important to consider the factors that influence the precision and accuracy of your measurements:

1. Light Intensity: The intensity and angle of light can affect phototropism, so it’s important to keep these consistent in the experiment.
2. Gravity Orientation: For gravitropism experiments, ensure that the seedlings are placed in a uniform orientation so that gravity’s effect is consistent.
3. Measurement Consistency: Ensure that all measurements (e.g., angle of curvature) are taken in the same way each time to avoid variability in data.
4. Environmental Conditions: Factors such as temperature, humidity, and soil type can also impact the tropic response, so these should be controlled as much as possible.

Strategies to Increase Measurement Precision, Accuracy, and Reliability

1. Use consistent measurement tools: Always use the same type of protractor or measuring device to record angles of curvature for consistency.
2. Control environmental variables: Ensure that factors such as light intensity, temperature, and humidity are kept constant throughout the experiment to avoid influencing the tropic response.
3. Replicate the experiment: Perform multiple trials and use multiple seedlings to gather reliable data. This helps to account for any variability in the response.
4. Ensure clear observation conditions: When measuring the angle of curvature, ensure that the seedlings are viewed in a straight line to minimize parallax errors in your measurements.