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Measuring Plant Growth

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Plant Biology Project Ideas

To capture enough data on the overall health of your plants, we recommend that you record at least one final weight measure, one measure of root health, and all of the observation measurements that pertain to the type of plant you are using.

Weighing Plants: Fresh vs. Dry Weight (1)

Measuring Fresh Weight: While you can technically measure the fresh weight of plants without harming them, the simple act of removing a plant from its growing "medium" can cause trauma and affect the ongoing growth rate and thus your experiment. Measuring the fresh weight of plants is tricky and should probably be saved as a final measure of growth at the end of the experiment. Here is the process for measuring fresh weight:
1. Remove plants from soil and wash off any loose soil.
2. Blot plants gently with soft paper towel to remove any free surface moisture.
3. Weigh immediately (plants have a high composition of water, so waiting to weigh them may lead to some drying and therefore produce inaccurate data).
Measuring dry weight: Since plants have a high composition of water and the level of water in a plant will depend on the amount of water in it's environment (which is very difficult to control), using dry weight as a measure of plant growth tends to be more reliable. You can only capture this data once as a final measure at the conclusion of your experiment.
1. Remove the plants from the soil and wash off any loose soil.
2. Blot the plants removing any free surface moisture.
3. Dry the plants in an oven set to low heat (100 degrees) overnight.
4. Let the plants cool in a dry environment (a Ziploc bag will keep moisture out) - in a humid environment the plant tissue will take up water. Once the plants have cooled weigh them on a scale.
5. Plants contain mostly water, so make sure you have a scale that goes down to milligrams since a dry plant will not weight very much.

Root Mass

Root mass is recommended as a final measurement as the plant must be removed from it's growing medium in order to capture accurate data. There are quite a few different methods for measuring root mass depending on the type and structure of the roots

Grid intersect technique:

Remove the plant from the soil.
If you are working with thin or light roots, you may want to die the roots using an acidic stain.
Lay the roots on a grid pattern and count the number of times the roots intersect the grid.

Trace the roots on paper, measure each of the tracings, and calculate root length from the tracings.
Count the number of roots.
Measure the diameter of the root. This is especially useful for root vegetables such as beets, carrots, potatoes, etc. that have a large root.

Root Shoot Ratio (2)

Roots allow a plant to absorb water and nutrients from the surrounding soil, and a healthy root system is key to a healthy plant. The root:shoot ratio is one measure to help you assess the overall health of your plants. Your control group of plants will provide you with a "normal" root:shoot ratio for each of your plant types, any changes from this normal level (either up or down) would be an indication of a change in the overall health of your plant. It is important to combine the data from the root:shoot ratio with data from observations to get an accurate understanding of what is happening with your plants. For example, an increase in root:shoot ratio could be an indication of a healthier plant, provided the increase came from greater root size and NOT from a decrease in shoot weight. To measure the root:shoot ratio:

Remove the plants from soil and wash off any loose soil.
Blot the plants removing any free surface moisture.
Dry the plants in an oven set to low heat (100 degrees) overnight.
Let the plants cool in a dry environment (a Ziploc bag will keep moisture out) - in a humid environment the tissue will take up water. Once the plants have cooled weigh them on a scale.
Separate the root from the top (cut at soil line).
Separately weigh and record the root and top for each plant. (Dry weight for roots/dry weight for top of plant = root/shoot ratio)
The root/shoot ratio can be calculated for each treatment.
Plants contain mostly water, so make sure you have a scale that goes down to milligrams since a dry plant will not weight very much.

Observation (3)

There are many different features of a plant that can be measured through observation to determine the extent of plant growth/health. The following table describes some of the measures that you can make and also recommends how frequently you should make these observations during the course of your experiment.

	Measurement	Procedure	Frequency of Measurement
When starting with seeds	First Cotyledon	Record the number of days from planting to the emergence of the first cotyledon ("seed leave(s)" that are the first to emerge from the ground).	Once
	Percentage of seeds that germinate	Calculate the percentage of seeds that germinated under each of the variables in your experiment.	Once
When starting with young plants	Plant height	Measure the height of the main plant from the border of the container to the top of the main plant stem. Note: you do not want to measure from the top of the soil, as the soil may condense with watering over time.	Every 2-3 days
	Number of leaves (indicates a plant's physiological age)	Counting Leaves: (4) Count and record the number of leaves on each plant. Count every visible leaf on the plant, including the tips of new leaves just beginning to emerge. You may want to place the plant over some graph paper to avoid counting errors.	Every 2-3 days
	Surface area of leaves	Method 1 (5): Trace the leaves on graph paper and count the squares covered to give you an estimate of the surface area for each leaf. Repeat this for each leaf on a plant and for each plant in your experiment. Method 2 (6): Trace out each leaf on paper. Make sure to use the same type of paper every time AND make sure that the paper is not wet. Cut out the leaf tracings and weigh them. Weigh the cutouts and divide the total weight by the number of leaves to give you the average leaf area for each plant. Repeat this for each of the plants in your experiment. Method 3 (7): Digital image analysis: Using a digital camera capture an image of a plant. Using special software, analyze the surface area of the leaves.	Every 2-3 days
	Plant color	Record any observations on changes or differences in plant color.	Every 2-3 days
When you are using flowering plants these two measurements serve as an additional indication of plant health	1st Flowering	Record the number of days since initial planting to the first flower.	Once
	Number of Flowers	Record the number of flowers on each of the plants. Buds should be included in your flower count.	Every 2-3 days

Endnotes

(1) "Measuring Duckweed Growth." The Charms of Duckweed. http://www.mobot.org/jwcross/duckweed/duckweed-measuring-growth.htm, accessed February 7, 2005.

(2) Mulanax, Matt. Science Buddies Advisor, email correspondence February 4, 2005.

(3) "Botany: Setting Up for RCB Lab." Rhodes College. http://www.rhodes.edu/biology/stinemetz/FPlants.htm, accessed February 4, 2005.

(4) "Measuring Duckweed Growth." The Charms of Duckweed. http://www.mobot.org/jwcross/duckweed/duckweed-measuring-growth.htm, accessed February 7, 2005.

(5) "Measuring Duckweed Growth." The Charms of Duckweed. http://www.mobot.org/jwcross/duckweed/duckweed-measuring-growth.htm, accessed February 7, 2005.

(6) "A Simple & Nondestructive Technique for Measuring Plant Growth & Development." The American Biology Teacher. http://www.nabt.org/sub/htdi/s2a, accessed February 1, 2005.

(7) "Measuring Duckweed Growth." The Charms of Duckweed. http://www.mobot.org/jwcross/duckweed/duckweed-measuring-growth.htm, accessed February 7, 2005.