and Laboratory Methods. 2085 



iodine solution. The presence of starch in the illuminated leaves, and its 

 absence in the shaded ones, shows the necessity of light for starch formation. 



3. Local Effect of Light on Carbon Assimilation. Partially cover leaves of 

 Tropaelum,' tobacco {Nicotiana tobacuni), or sunflower {HeliantJms), with thin 

 disks of cork. These disks are to be placed opposite one another on either side 

 of the leaf, and are to be secured with weak wire clips. Place the leaves so that 

 they will receive perpendicular rays of light for much of the day. The experi- 

 ment will continue twenty-four to thirty-six hours, and be ended between four 

 and six in the afternoon. The final treatment will be the same as in the 

 preceding experiment. 



4. Relation of Variegated Leaves to the Assimilation of Carbon. In the late 

 afternoon of a bright day, gather some variegated leaves which have received 

 strong light for most of the day. Make a sketch of the color pattern of each 

 leaf, then kill and apply the iodine test as in the preceding work. Note that the 

 starch pattern coincides with the chlorophyll-bearing surface of the leaf. 



5. Translocation of Starch. Late in the afternoon of a bright day, gather 

 some leaves from a Tropaelum, or other suitable plant that has stood in strong 

 sunlight. At the same time cover the plant so that no light can reach it until 

 other leaves are plucked early the next morning. The leaves that are gathered 

 in the evening are to be killed immediately by immersion in boiling water. They 

 may then be put into alcohol for preservation. The leaves taken in the morning 

 are to be killed immediately, and both sets tested for the presence of starch. 

 The results are to be explained by the fact that the starch is transformed into 

 sugars by the action of enzymes and translocated to other parts of the plant 

 during the night. 



6. Formation of Starch in the Absence of Light. Place some plants of Elodea 

 canadensis in tap-water and allow to stand in the dark until the iodine test shows 

 that the starch has all been consumed. Then select two healthy stems, deter- 

 mine their length, and count the leaves on each ; place one stem in a large flask 

 of a 2 per cent, solution of cane sugar, the other in a similar flask of clean tap- 

 water. The preparations should be watched to prevent the growth of fungi, 

 and observations made after three or four days for growth and the presence of 

 starch. In the absence of light the plant is able to manufacture starch from any 

 available carbohydrates. 



7. Effect of Light-waves of Different Lengths on Carbon Assimilation. Fill a 

 double-walled bell-jar with a saturated solution of potassium dichromate, fill a 

 similar bell-jar with a weak solution of ammoniacal copper sulphate ; under each 

 bell-jar place a small plant of Tropaelum, and allow them to stand in strongly 

 diffused light. At the end of forty-eight to seventy-two hours detach some 

 leaves from each plant and determine the relative amount of starch present by 

 the iodine test. It will be seen that the long waves of the red end of the 

 spectrum are more efficient than the short ones of the blue. 



University of Michigan. HOWARD S. Reed. 



