PLANT PHYSIOLOGY 185 
SYNTHESIS. Only the green parts of plants can assimi- 
late, and they can only do it under certain conditions. 
They must have light, either daylight, or bright electric 
light, and they must have a certain temperature. As- 
similation is not only important for the plants them- 
selves, being their chief method of obtaining carbon, 
but it is also immensely important to us, as by its means 
the air is kept pure and fit to breathe. All animals 
breathe out carbon di-oxide, and if it were not for plants 
taking it in there would soon be an excess of this gas in 
the air. 
The first visible product of assimilation in plants 
is starch, hence if we can show that starch has been 
formed we shall know that assimilation has taken place. 
103. Experiment 11.—To see if green leaves contain 
starch. 
Apparatus.—Iodine solution, alcohol, beakers, leaves. 
Method.—Boil some leaves to kill them, and then 
soak them in alcohol or methylated spirits till all the 
green colour has come out This may take some time, 
but nasturtium leaves are fairly quick. Then place the 
whitened leaves in iodine. 
Result,—They turn dark purple or black. 
Deduction—They must contain starch, since this 
substance always turns purple with iodine. 
104. Experiment 12.—To show that assimilation does 
not take place in the dark. 
Apparatus.—A plant in a pot, alcohol, iodine, dishes. 
Method (i-).—Pick a leaf from a potted plant, and test 
for starch as above, then place the plant in a dark cup- 
board for forty-eight hours, pick another leaf and test 
