170 PRACTICAL COURSE IN BOTANY 
formed in the tube of the funnel? Move back into the sunlight and 
leave for a few hours; what happens when you thrust a glowing splinter 
into the tube? 
Experiment 69. Is ANY FOOD PRODUCT FOUND IN LEAVES ? — Crush 
a few leaves of bean, sunflower, or tropxolum, and soak in alcohol until all 
the chlorophyll is dissolved out. Rinse them in water, and soak the 
leaves thus treated in a weak solution of iodine for a few minutes, then 
wash them and hold them up to the light. If 
there are any blue spots on the leaves, what are 
you to conclude? If a test for sugar is to be 
made, use sap pressed from fresh leaves; for 
oils and fats, leaves should be dried without 
being placed in alcohol. 
ExPERIMENT 70. HAS THE PRESENCE OR 
ABSENCE OF LIGHT ANYTHING TO DO WITH THE 
OCCURRENCE OF STARCH IN LEAVES ? — Exclude 
the light from parts of healthy leaves on a grow- 
ing plant of tropxolum, bean, etc., by placing 
patches of black cloth or paper over them. 
Fic. 227.—Leaf arranged Leave in a bright window, or preferably out of 
with a piece of tin foil to ex- doors, for several hours, and then test for starch 
clude light from a portion of gs in the last experiment; do you find any in 
the surface. 
the shaded spots? 
ExpeRiMENT 71. Is THE PRESENCE OF AIR NECESSARY FOR THE 
PRODUCTION OF STARCH ?— Cover the blades and the petioles of several 
leaves with vaseline or other oily substance so as to exclude the air, and 
after a day or two test as before. 
185. Influence of plants on the atmosphere. — These 
experiments show that leaves cannot do their work without 
light and air. The particular element of the atmosphere 
used by them in the process of food making is carbon dioxide. 
Their action in absorbing this gas and giving off oxygen 
tends to counterbalance the opposite action of respiration, 
decomposition, and combustion of all kinds, by which the 
proportion of it in the atmosphere tends to be constantly 
increased. In this way they help to regulate the quantity 
of it present and have a beneficial effect in ridding the air of 
one source of impurity. 
