256 LEAVES 
Starch occurs in the form of starch grains, which are light in 
color and have a characteristic shape and structure as shown in 
Figure 234. When starch grains are treated with iodine, they 
turn dark blue, and this color test can be applied directly to the 
leaf to indicate the amount of starch present and, therefore, the 
rate of photosynthesis. In applying the test, the leaf is first 
treated with hot alcohol to remove the chlorophyll. The leaf, 
Fic. 234.— Starch grains from a Potato tuber. A, simple grain; B, 
half-compound grain; C and D compound grains. c, hilum. Enlarged 
540 times. After Strasburger. 
now almost white, is immersed in the iodine solution which turns 
it blue, if starch is present, with the depth of blue roughly indicat- 
ing the amount of starch present. If no starch is present, then 
the leaf takes only the brownish color of the iodine solution. 
This test is of considerable service in experiments on photosyn- 
thesis as its application in Figure 235 shows. 
Proteins are made in leaves, but in what part of the leaf 
they are made is not known. The main evidence that they 
are formed in leaves is that large quantities of them are being 
continuously carried away through the veins to the stem. That 
light is essential in the formation of proteins is doubtful, for 
there is considerable evidence that the energy employed in their 
synthesis comes from chemical action and not directly from sun- 
light. Although proteins are of many kinds, all are formed by 
