FUNCTIONS 



233 



lost; if it were, the resulting substance would not be 

 starch. 



Iodine is a substance which turns starch blue. It is 

 common to use a solution of iodine in order to demonstrate 

 the presence of starch in leaves which have been actively 

 at work in sunlight. First, the leaves are soaked in alco- 

 hol which dissolves out the chlorophyll. Then they are 

 dipped in iodine. The action of 

 iodine turns such leaves dark, 

 proving the presence of starch, 

 while upon similar leaves which 

 have been shaded for some time 

 iodine produces no such effect. 

 (See Figure 82.) The application 

 of iodine to a leaf which has been 



. Fig. 82. — A geranium leaf tested 



partly shaded and partly illumi- with iodine for starch. The 

 nated shows that starch is present t^^^e"^ portion had been ex- 



, , 1 • 1 . I , 11 . . posed to light, while the lighter 



in the hghted part and absent in portion had been screened from 



the darkened part, but evidently light. Both parts of the leaf, 



.r* ' i. J J. however, were exposed to air. 



this experiment does not prove 



that this result is due to differences in light alone unless 

 care has been taken that the shaded part receives carbon 

 dioxide as well as the lighted part. 



A good definition for photosynthesis is the following : 

 "It is the manufacture of carbohydrates by chloroplasts 

 in the presence of light, water and carbon dioxide being 

 used, and oxygen being given off as a waste product." 

 (J. M. Coulter, Textbook of Botany.) 



E. Summary as to Gaseous Exchanges. — The following 

 sums up what we have been discussing : — 



" The chief gas movements in plants are associated with 



