When the
varnish is dry, puncture it with a needle, and immerse the stem in
the water in the test tube, keeping the varnished larger end
uppermost. If the submerged plant be now exposed to the strong rays
of the sun, bubbles of oxygen gas will begin to pass off at a rapid
and even rate, but not too fast to be easily counted. If the simple
apparatus has begun to give off a regular succession of small
bubbles, the following experiments can be at once conducted:
(1) Substitute for the fresh water some which has been boiled a few
minutes before, and then allowed to completely cool: by the boiling,
all the carbonic acid has been expelled. If the plant is immersed in
this water and exposed to the sun's rays, no bubbles will be evolved;
there is no carbonic acid within reach of the plant for the
assimilative process. But,
(2) If breath from the lungs be passed by means of a slender glass
tube through the water, a part of the carbonic acid exhaled from the
lungs will be dissolved in it, and with this supply of the gas the
plant begins the work of assimilation immediately.
(3) If the light be shut off, the evolution of bubbles will presently
cease, being resumed soon after light again has access to the plant.
(5) Place round the base of the test tube a few fragments of ice, in
order to appreciably lower the temperature of the water.
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