I. Observation: Plants usually grow in lighted areas.
II. Question: Can plants grow without light?
III. Hypothesis: Plants cannot grow without light for photosynthesis.
IV. Prediction: Seeds germinated inside a box (without light) will not grow well, will be smaller than plants grown in light, and may even die.
V. Experiment: You planted seeds of the 'Little Marvel' cultivar of pea (Pisum sativum) some time ago. One pot of plants was left in the greenhouse and a second pot was placed in a dark box for growth. These have been watered as needed over the subsequent weeks and have grown under the lighted and dark conditions. You will now carefully dump out the soil, separate the plants, and measure their growth.
A. Height of shoots. Make sure that each pot has the same number of seedlings (5). Cut at soil level and completely discard any extras from a pot. You must do this or all your efforts go down the drain!!! Measure the height of the five shoots remaining from each pot. For this, measure each shoot and then calculate the mean.
| Dark: | | | | | | Mean: | cm |
| Light: | | | | | | Mean: | cm |
B. Size of leaf. Your instructor will tell you which leaf to measure: Leaf # __________.
Measure the length of this leaf on each plant and calculate an average leaf length for each treatment. During your observations of the leaves, note the color of the leaves. CAUTION: The leaves are compound and may end in a tendril!
| Dark: | | | | | | Mean: | cm | Leaf Color: | |
| Light: | | | | | | Mean: | cm | Leaf Color: | |
C. Number of leaves. Your instructor will demonstrate the various types of leaves on the pea plant. Count the number of leaves on each plant from base to tip and calculate an average number of leaves for each treatment.
| Dark: | | | | | | Mean: | |
| Light: | | | | | | Mean: | |
D. Fresh weight of shoot.
1. Using a razor blade, cut the shoots from each pot just above the cotyledons. Take care not to damage the cotyledons. IMMEDIATELY fold the shoots gently and place them squarely on the balance pan.E. Dry weight of shoot.2. Weigh each collection of shoots as accurately as possible.
Fresh Weight of shoots: Light: _____________ g Dark: _____________ g
3. Clean and dry the balance pan for the next use.
4. Place the shoots on a labeled sheet of smooth paper in the plant press to dry, or on a labeled sheet of aluminum foil to be baked dry.
1. Remove the shoots from the plant press and reweigh each collection of shoots as accurately as possible. Be sure the balance is zeroed when the pan is clean, dry, and empty.F. Fresh weight of cotyledon. Repeat the steps in part D using the fresh cotyledons from the plants in each pot. Be sure to remove all soil, the seed coat, and other attachments.Weight of Dried Shoots: Light: _____________ g Dark: _____________ g
2. Discard the dried shoots into the compost bin and clean the balance for the next use.
Fresh Weight of cotyledons: Light: _____________ g Dark: _____________ g
G. Dry weight of cotyledon. Repeat the steps in part E using the fresh cotyledons weighed in part F. If the balance reads 0.0 g, remember the cotyledons do weigh something, they are just below the accuracy of the balance (0.1 g). So instead of recording 0.0 g, record 0.1 g.
Weight of Dried Cotyledons: Light: _____________ g Dark: _____________ g
VI. Analysis:
A. On average, under which treatment do shoots have... (Neither means <5% difference!)
| more height? | Light Dark Neither | longer leaves? | Light Dark Neither |
| more internodes? | Light Dark Neither | greener leaves? | Light Dark Neither |
| longer internodes? | Light Dark Neither | more fresh weight? | Light Dark Neither |
| more leaves? | Light Dark Neither | more dry weight? | Light Dark Neither |
B. On average, under which treatment do cotyledons have... (Neither means <5% difference!)
| more shriveled appearance? | Light Dark Neither |
| more fresh weight? | Light Dark Neither |
| more remaining dry weight? | Light Dark Neither |
C. Did the plants growing in light produce any solid matter by photosynthesis? To crudely assess this question, calculate the total plant dry weights, and calculate the ratio of dry weight in light to the dry weight in the dark.
Combined dry weight of shoots and cotyledons (E+G): | Light ______ g | Dark ______ g |
| Ratio of dry weight of plants in light to plants in dark | Light -------- Dark: | |
| (YES! You must divide it out and round to two decimal places.) | ||
| Calculate the % Increase (+) or Decrease (-) in weight | (Ratio - 1) * 100 ): | % |
D. Did the shoots grow by merely taking up water? To analyze for this question, you must calculate the percent water in each shoot. This is done by comparing difference between the fresh weight (part D) and dry weight (part E) for the shoots, and compare the difference to the total weight (fresh weight).
| Water found in shoots (D-E): | Light ______ g | Dark ______ g |
| Percent water in shoots (= 100 x (D-E)/D): | Light ______% | Dark ______% |
| Under which treatment do the plants have more water content? | Light Dark Neither | |
VII. Decision:
| The hypothesis, "Plants cannot grow without light for photosynthesis" is: | rejected not rejected |
Go back to the Course Schedule.