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THE LEAF 
the most important work of the leaf, as well as the vital 
process on which all other vital processes, both in plants and 
animals, depend. 
A second function of the leaf is to perform respiration. 
Respiration in plants is exactly the same as respiration in 
animals. That is, every living cell uses oxygen, combining 
it with some of its protoplasm, releasing energy for the work 
of the plant and forming carbon dioxide and other wastes in 
the process. Respiration is most easily studied in plants 
like toadstools which cannot perform photosynthesis, and in 
sprouting seedlings which have not developed chlorophyll. 
HOME WORK ON PHOTOSYNTHESIS 
Make a list of the plants that are used for food. Look up in an 
encyclopedia the amounts of each produced in a given year. Make 
a list of the industries which are dependent on agriculture. Obtain 
information concerning the number of men employed in each. 
What uses are made of starch and sugar aside from food? 
What are the sources of the protein used for food? Which of these 
depend directly upon photosynthesis? which indirectly upon it? 
LABORATORY STUDY 
To show that light is necessary for photosynthesis, fasten thin 
discs of cork to the upper and under sides of a leaf with clips, completely 
shutting off the light. Stand the plant in the bright light for half a 
day, then remove the corks from the leaf and the leaf from the plant. 
Heat enough 60 per cent alcohol to cover the leaf in a shallow glass 
dish. This may be done by setting the dish into hot water. Keep 
hot for half an hour, or till the chlorophyll is removed from the leaf. 
Turn off the alcohol and put drops of weak iodine on the leaf. Note 
that the circle covered by the cork discs show little starch or none, as 
indicated by the faint blue color or by lack of color. 
Make a similar test by comparing a leaf from a plant that has been 
in the dark twelve hours with one that has stood in the bright light 
for the same time, using alcohol to remove the chlorophyll and iodine 
for a test for the presence of starch, as before. 
To test the nature of the gas given off by growing algae collect a 
quantity of algae and place it in a deep glass jar in the sunlight. When 
