112 THE FIRST BOOK OF FARMING 



ten cents worth of tincture of iodine from a drug 

 store diluted to about the color of weak tea will 

 be sufficient for these leaf experiments.) 



Experiment. If a potted plant was used for the 

 last experiment, set it away in a dark closet after 

 taking the leaves for the experiment. A day or two 

 after, take leaves from it before removing it from 

 the closet. Boil these leaves and treat them with 

 alcohol as in the previous experiment. Then wash 

 them and test them with iodine as before. No starch 

 will be found in the leaves (Fig. 62). The starch 

 that was in them when placed in the closet has dis- 

 appeared. Now paste some thick paper labels on 

 some of the leaves of a plant exposed to the sun- 

 light. After a few hours remove the leaves that 

 have the labels on them, boil, treat with alcohol and 

 test with the iodine. In this case starch will be 

 found in all parts of the leaf except the part over 

 which the label was pasted (Fig. 63). If the sun- 

 light is intense and the label thin, some starch will 

 appear under it. 



According to these last experiments, leaves con- 

 tain starch at certain times, and this starch seems 

 to appear when the leaf is in the sunlight and to 

 disappear when the light is cut off. The fact is 

 that the leaves manufacture starch for the plant 

 and sunlight is necessary for this work. The starch 

 is then changed to sugar which is carried by the 

 sap to other parts of the plant where it is again 

 changed to starch to be built into the plant struc- 

 ture or stored for future use. 



