FUNCTIONS OF LEAVES 189 
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 flowerless 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 
(Tropeolum). Boil these in water for a few minutes to kill the 
protoplasmic contents of the cells 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. 
