6. Place box 2 in the light for 20 minutes on the fifth day only. 



7. Place box 3 under the light for 2 hours each day, and box 4 for a period of at least 8 hours per day. 



8. Remove all five boxes from the dark chambers on the ninth or tenth day from planting. Measure and 

 record the length of each internode and the length of the leaves. 



9. Calculate the average length of the internodes and the average length of the leaves for each treatment. 

 10. Slice or mince the leaves and place in a known volume of ethyl alcohol. Use the same volume of alcohol 



for each treatment irrespective of the size of the leaves. A better method is to use 10 milliliters of alcohol 

 for each gram of leaves. The relative amounts of chlorophyll can be estimated by assigning a numerical 

 value to each sample based on the visual greeness of the extract, or by measuring the optical density 

 of each sample. 



OBSERVATIONS: 



The plants grown in complete darkness (box 5) should have long hypocotyls, short first internodes, small 

 leaves, and no chlorophyll. Boxes 1, 2, 3, and 4 should contain plants that have shorter hypocotyls, longer 

 first internodes, and perhaps more internodes than the plants of box 5. They should also have much larger 

 leaves. Plants of box 1 should contain no chlorophyll, and those of box 2 none or very little. Plants in boxes 

 3 and 4, however, should contain a greater amount of chlorophyll, with those plants of box 4 having more 

 than those of box 3. 



SUPPLEMENTARY READING: 



Downs, R. J. Photoreversibility of leaf and hypocotyl elongation of dark-grown red kidney bean seedlings. 



Plant Physiol. 30: 468-473. 1955. 

 Textbooks of plant physiology. 



DEMONSTRATION B-3: Why plants bend toward light (phototropism). 



MATERIALS: 



1. A chamber or box that can be made completely dark. (See Demonstration B-2, steps 1, 2, and 3 of 

 Materials, for details.) 

 PROCEDURE: 



1. Plant the bean seeds; then water. No nutrient solution is required even when the seeds are planted in 

 sand, Vermiculite, or Perlite. 



2. The best temperature is 80° to 85° F. Lower temperatures will suffice, but the rate of germination and 

 growth will be slower. 



3. After the beans are planted, place one box in the dark chamber and one in the light, where the plants 

 should receive 8 hours of light per day. 



4. When the dark-grown beans are about 6 days old, open the door of the dark chamber so that the plants 

 receive some light. Better results are obtained by placing a desk lamp 3 to 4 feet from the open door 

 of the chamber. 



5. When the plants in the light have expanded their first pair of leaves, place them in the dark chamber. 

 Again, open the door of the dark chamber and place a desk lamp 3 to 4 feet from the door. 



OBSERVATIONS: 



After a few hours the leaf blades will have twisted around until they are perpendicular to the light. 

 SUPPLEMENTARY READING: 



Textbooks of plant physiology. 



DEMONSTRATION B-4: Effect of red and far-red light on elongation of stems 



of light-grown plants. 



MATERIALS: 



1. Bean plants (preferably Pinto bean). 



2. A light-equipped chamber large enough to hold at least three pots. Illuminate the chamber with at 

 least two 40-watt fluorescent lamps. 



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