124 BOTANY 
leaves of the nasturtium. The iodine test will show 
that starch is present only in parts that were green. 
By sketching the leaf before and after the experiment, 
it will be seen that the areas without chlorophyll. 
exactly coincide with the areas without starch. Chloro- 
phyll is essential to starch formation. This is indirectly 
illustrated in another way. Where a plant is grown 
in the dark, as for instance where a patch of grass is: 
eovered with a sack, the leaves lose all their chloro- 
phyll, which is replaced by a yellow substance called 
etiolin. The plant is said to be etiolated. Since, owing’ 
to the absence of light, starch formation is impossible, 
the plant does not waste energy in forming’ 
ehlorophyll. 
The carbon dioxide that supplies the carbon comes: 
from the air. This may be proved by keeping a plant 
in an atmosphere devoid of that gas. The most 
satisfactory way of doing this is to obtain a suitable 
plant growing in a pot and place it on a stand under 
a bell jar standing in a dish containing a little lime 
water. By means of an aspirator the air is drawn 
into the jar through a solution of caustic soda, this 
substance having the power to absorb carbon dioxide: 
(Fig. 85). At the beginning of the experiment the 
aspirator should be filled and emptied several times, 
so that the air originally in the jar may be drawn 
out and replaced by that which has passed through 
the caustic soda. The lime water in the dish absorbs 
any carbon dioxide given off by the respiration of 
the plant. During the day, the tap of the aspirator 
is allowed to run slowly and the vessel refilled as 
required. By testing the leaves ag in the previous’ 
experiments, it will be found that no starch has been 
formed. The plant, then, obtains the carbon for starch 
formation from the carbon dioxide of the air. 
Oxygen is given off in photosynthesis. This may 
be shown by growing some water plant submerged 
