20 



A TEXTBOOK OF BOTANY 



[Ch. Ill, 3 



in their sap. Under action of sunlight this sugar increases 

 in quantity, but in darkness it lessens, because removed 

 through the veins to the stem. Furthermore, in most leaves, 

 when this sugar increases beyond a certain percentage the 

 surplus becomes automatically transformed into starch, which 

 returns again to grape sugar as the percentage thereof once 

 more falls. Now it happens that starch (unlike sugar) is 

 readily recognizable by a striking and easily applied test, 

 viz., addition of iodine in solution, which turns starch dark 

 blue ; and thus we are provided with a convenient means of 



proving the increase 

 of sugar, as manifest 

 in its transformation 

 to starch, under action 

 of light. The experi- 

 ment is well-nigh 

 classic, and every 

 student should see it. 

 One has only to keep 

 a thin-leaved potted 

 plant for a day or 

 two in the dark (to 

 cause the disappear- 

 ance of its starch) : 

 cover part of a leaf, 

 in a way not to pre- 

 vent its ordinary func- 

 tions, with some kind of contrasting light-and-dark screen, 

 such, for example, as shown in our picture (Fig. 3) : expose the 

 plant to strong, but not intense, light for two or three hours : 

 place the leaf in warm alcohol until the chlorophyll is re- 

 moved : and cover the blanched leaf with a solution of iodine. 

 Then a striking result appears, for the parts left in light 

 by the screen all turn dark blue, and the parts which were 

 shaded remain white, or at most a little browned by the 

 iodine (Fig. 4). Thus it is clear that the starch, and there- 



Fig. 3. — A light screen for experiment; 

 in starch formation by leaves ; X §• 



The star is cut from tinfoil attached to 

 glass, and the box excludes light but admits air. 



