34 NEW ZEALAND NATURE-STUDY BOOK 
The last experiment merely proves that air is able to enter 
the plant through its leaves. 
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We have yet to show that this 
Fig. 29.—The inside of a leaf. Below the upper skin a, are cells of 
living green stuff 6. Beneath these is the spongy part c, where the 
surplus water evaporates. In the lower skin d are many breathing 
pores. These are shown in Fig. 30. 
air contains carbon-dioxide. 
This can be done as follows :— 
Obtain a beaker of lime-water and breathe into it through a 
piece of fine glass tubing. The water quickly becomes milky 
in appearance owing to the presence of carbonate of lime. 
The air expired from the lungs contains carbon-dioxide and 
the union of this with the lime held in solution forms the 
insoluble salt carbonate of lime. 
This shews the presence 
of carbon-dioxide in air expired by breathing animals and 
Fig. 30.—The lower skin of a leaf 
showing three breathing-pores. 
as the number of such 
animals is so enormous, we 
may reasonably infer that 
the atmosphere contains 
an appreciable amount of 
the gas. Further proof 
will be obtained by ex- 
posing to the air some 
lime-water in a _ saucer. 
At the end of a few hours 
