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score10097.895.693.391.188.986.784.482.280.077.875.673.3.      

Light and Growth Name________________________

Observation: Plants usually grow in lighted areas.

Question: Can plants grow without light?

Hypothesis: Plants cannot grow without light for photosynthesis.

Prediction: Seeds germinated inside a box (without light) will not grow well, will be smaller than plants grown in light, and may even die.

Experiment: You planted seeds of the dwarf '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 observe the plants and measure their growth.

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 to the nearest whole centimeter and then calculate the mean.

Mean (cm)
Dark:                 .  
Light:                 .  

Size of leaf. Your instructor will tell you which leaf to measure: Leaf # ____. Measure the length of this leaf to the nearest cm 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!

Mean (cm)Leaf Color
Dark:                 .    
Light:                 .    

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.

Mean
Dark:                 .  
Light:                 .  

 
-   /15

Fresh weight of shoot. 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. Weigh each collection of shoots as accurately as possible.

 LightDark
Fresh Weight of shoots: A ____.________g A ____.________g

Place the shoots on a labeled sheet of aluminum foil to be baked dry.

Fresh weight of cotyledon. Locate both fresh cotyledons from each of the plants. Be sure to remove all soil, the seed coat, and other attachments. Weigh each collection of cotyledons as accurately as possible.

 LightDark
Fresh Weight of cotyledons: B ____.________g B ____.________g

Place the cotyledons on a labeled sheet of aluminum foil to be baked dry.

Dry weight of shoot. Remove the shoots from the foil and reweigh each collection of shoots as accurately as possible. Be sure the balance is zeroed when the pan is clean, dry, and empty.

 LightDark
Dry Weight of shoots: C ____.________g C ____.________g

Dry weight of cotyledon. Remove the cotyledons from the foil and reweigh each collection of cotyledons as accurately as possible. Be sure the balance is zeroed when the pan is clean, dry, and empty.

 LightDark
Dry Weight of cotyledons: D ____.________g D ____.________g

Discard the dried shoots, cotyledons and foil into the trash. Clean the balance and put it away.

Analysis:

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  

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  

 
-   /19

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.

 LightDark
Combined dry weight of shoots and cotyledons (C+D): ____.________g ____.________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): ________%

Did photosynthesis result in at least a 5% increase in dry matter?   yes       no  

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 (A) and dry weight (C) for the shoots, and compare the difference to the total weight (fresh weight).

 LightDark
Water found in shoots(A-C): ____.________g ____.________g
Percent water in shoots(= 100 x (A-C)/A): ________% ________%
Under which treatment do the plants have at least 5% more
water content than do those of the other treatment?
  Light     Dark     Neither  

Since we pooled our shoots in the weighing operation, we cannot carry out any kind of statistical test to determine whether the differences you have calculated are statistically significant or not. There would be no degree of freedom to carry out the test. However, your instructor may collect the Percent Water in Shoots information from each team in the class and subject the several calculated results to a t-test to help determine whether the differences observed in the class as a whole are statistically significant.

Decision: The hypothesis, "Plants cannot grow without light for photosynthesis"

is:   rejected       not rejected  


 
-   /11

 

This page © Ross E. Koning 1994.

 

 

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