form oxygen they have to get energy to make it possible. Where did the
energy come from?
A Dutch scientist, Jan Ingenhousz (1730-1799), found the answer. He
studied the way in which plants form oxygen. In 1779, he noticed that it
happened only in the presence of light.
In an experiment with the aquatic
plant, Hydrilla, he observed that in bright sunlight, small bubbles formed
around the green parts while in the dark they did not form. He also found
that the gas present in the bubbles was oxygen.
It was further confirmed when Engelman in the early 20th century
ingeniously detected the point of maximum rate of photosynthesis. He
used a strand of algae and exposed it to different colours of light
(the
colours that we see in a rainbow) Then he used oxygen sensitive bacteria
and found that they crowd around areas illuminated with red and blue rays
of light.
This led to more studies on effect of light on photosynthesis, the
role of different coloured compounds called pigments in plants and the
utilization of light energy.
Make
two identical sets.
Place some water plant like Elodea or Hydrilla in
a short stemmed funnel and keep it in a beaker.
Then pour the water in the beaker.
Invert a test-tube full of water over the stem of the
funnel. Ensure that the level of water in the beaker
is above the level of stem of the inverted funnel.
Place one apparatus in the sun and the other in the
dark for at least 2-3 hours.
You would see that in place
of water there is air that fills in the set up kept in sun.
It
is actually a gas that will be collected in the test-tube.
Observe the other set up kept in dark.
Is there any
difference in the amount of gas collected?
Test the gas in the test-tube by inserting a glowing match stick or
incense stick which would burst into flames.
This shows the presence of
oxygen.
What precautions do you need while removing test tube from the beaker.
Discuss with your teacher.