46 A TEXTBOOK OF GENERAL BOTANY 
other was more or less shaded. The leaves of many plants, if 
grown in different habitats, show much greater differences than 
those represented in this illustration. | : 
Demonstration. Starch is formed in leaves very soon after pho- 
tosynthesis begins. This is of great advantage in demonstrating 

Fig. 35. Cross section of a shade leaf (left) and of a sun leaf (right) of a 
rain tree (Hnterolobium saman). (x 485) 
the general facts of photosynthesis, as the test for starch is 
very simple. It consists in treating the material to be tested. 
with a solution of iodine, which gives starch a blue color. In 
working with leaves the chlorophyll is first extracted, after 
which the leaves are placed in the iodine solution. 
The necessity of light for photosynthesis can be shown by 
keeping a leaf or a part of a leaf in the dark (being careful 
