CHAPTER VIII. 



STARCH AND SUGAR CONCLUDED. ANALYSIS OF 

 PLANT SUBSTANCE. 



1 . Translocation of Starch. 



152. Translocation of starch. It has been found that leaves of many 

 plants grown in the sunlight contain starch when examined after being in 

 the sunlight for several hours. But when the plants are left in the dark for 

 a day or two the leaves contain no starch, or a much smaller amount. This 

 suggests that starch after it has been formed may be transferred from the 

 leaves, or from those areas of the leaves where it has been formed. 



To test this let us perform an experiment which is often made. We 

 may take a plant such as a 

 garden tropaeolum or a clover 

 plant, or other land plant in 

 which it is easy to test for the 

 presence of starch. Pin a 

 piece of circular cork, which 

 is smaller than the area of 

 the leaf, on either side of the 

 leaf, as in fig. 72, but allow 

 free circulation of air between 

 the cork and the under side of 

 the leaf. Place the plant 

 where it will be in the sunlight. 

 On the afternoon of the fol- 

 lowing day, if the sun has been shining, test the entire leaf for starch. The 

 part covered by the cork will not give the reaction for starch, as shown by 

 the absence of the bluish color, while the other parts of the leaf will show it. 

 The starch which was in that part of the leaf the day before was dissolved 

 and removed during the night, and then during the following day, the 

 parts being covered from the light, no starch was formed in them. 



73 



Fig. 72- 



Leaf of tropaeolum 

 with portion covered 

 with corks to pre- 

 vent the formation 

 of starch. (After 

 Detmer.) 



Fig. 73- 



Leaf of tropaeolum treated 

 with iodine after removal of 

 cork, to show that starch is 

 removed from the leaf dur- 

 ing the night. 



