FUNCTIONS OF LEAVES 139 



the green layer of the bark, one of the best evidences of 

 the intimate relation of chlorophyll to starch-making is 

 derived from the fact that plants which contain no chloro- 

 phyll cannot make starch from water and carbon dioxide. 

 Parasites, like the dodder, which are nearly destitute of 

 green coloring matter, cannot do this ; neither can sapro- 

 phytes or plants which live on decaying or fermenting 

 organic matter, animal or vegetable. Most saprophytes, 

 like the molds, toadstools, and yeast, are flow.erless plants 

 of low organization, but there are a few, such as the curi- 

 ous pine-sap (frontispiece) which flourishes on rotten wood 

 or among decaying leaves, that bear flowers and seeds. 



165. Detection of Starch in Leaves. — Starch may be 

 found in abundance by microscopical examination of the 

 green parts of growing leaves, or its presence may be 

 shown by testing the whole leaf with iodine solution. 



EXPERIMENT XVIII 



Occurrence of Starch in Nasturtium Leaves. — Toward the close of 

 a very sunny day collect some bean leaves or leaves of nasturtium 

 {Troipmolum). Boil these in water for a few minutes to kill the 

 protoplasmic contents of the ceUs and to soften and swell the starch 

 grains. 



Soak the leaves, after boiling, in strong alcohol for a day or two 

 to dissolve out the chlorophyll, which would otherwise make it diffi- 

 cult to see the blue color of the starch test, if any were obtained. 

 Rinse out the alcohol with plenty of water and then place the leaves 

 for ten or fifteen minutes in a solution of iodine, rinse off with water, 

 and note what portions of the leaf, if any, show the presence of starch. 



If convenient try the test with a leaf treated as shown in Fig. 96. 



What might this prove about importance of sunlight ? 



If starch disappeared from between the corks, where did it go ? 



Review Sects. 17-21, 95-97. Read Sect. 166. 



