XV. THE INFLUENCE OF EXTERNAL CONDITIONS ON THE 



PLANT 



A. The Influence of Gravity on the Direction of Growth: 

 Experiment 40. To find how gravity affects the direc- 

 tion of growth of roots and shoots. 



1 . Choose two or three young seedlings of the pumpkin 

 or lupine, with radicles about 10 mm. long. 



2. Pin the seedlings horizontally on a cork and place 

 in a moist chamber in the dark (why in the dark?) 

 until the next period. A Petri dish will furnish a 

 simple moist chamber. 



3. Make a drawing of the seedlings in the horizontal 

 position. 



4. At the next laboratory period observe the position 

 of both root and shoot. Draw. 



5. Do the results give any evidence that the root grew 

 downward and was not pulled down by gravity? 

 Explain. 



B. Influence of Light on the Rate of Growth of Stems: 



1 . Compare the lengths of the stems of seedlings of the 

 same age that have grown, one in the dark, the other 

 in the light. State the exact length of each of the 

 stems in centimeters. 



2. What do you infer is the effect of light on the rate 

 of growth of stems of the plants observed? 



3. Do you think this is true of all plants? (This point 

 should be discussed with the instructor, in the light 

 of more recent investigations on the subject. See 

 especially, MacDougal, "The E/ecis of Light and 



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