To do this we must set up an apparatus which will allow only air freed from carbonic acid gas to surround the plant. Such an apparatus is shown in the figure 12. The plant is grown in the closed bell jar D, which stands over the dish C filled with lime-water, which prevents carbonic acid gas entering through the cracks between the foot of the jar and the table. All the air which enters the jar D must come first through jar A, which is filled with a solution of caustic potash that has the power of absorbing the carbonic acid gas, and then through jar B with lime-water. You can draw plenty of air through jar D for the use of the plant by sucking at the indiarubber tube G, which must be carefully shut with a clamp when you stop the current. The bell-jar D will now be filled with air which is quite free from carbonic acid gas, and the small quantity which is breathed out by the plant itself will be absorbed by the lime-water in dish C. Place the whole in a light or sunny position, and change the air every day or two in the way you filled it, that is by drawing at G so that the fresh air comes in through A and B, and is free from carbonic acid gas.
Fig. 12. Apparatus used to keep a plant without any carbonic acid gas. A, jar of caustic potash, B, jar of lime water, which absorb the carbonic acid gas, through which all the air entering jar D must pass; C, basin of lime water to absorb any of the gas given out by the plant growing in D; G, indiarubber tube which can be closed or attached to a siphon to draw air through D.
If you keep the plant growing under these conditions for some time you will find, in comparison with another quite similar plant growing in the open near it, that its growth is very slow. The leaves it forms are smaller, and finally its growth almost ceases. Further, if you test the leaves of the plant growing out in the air for starch (see pp. [24] and [25]), you will find that they contain plenty, but that the leaves on the plant in the bell-jar are empty of starch. Now all healthily growing green leaves contain starch, so that this is a good proof that something is seriously wrong with the plant, which has been deprived of the supply of carbon in the air. This shows us that plants use the carbonic acid gas in the air for their growth.
Carbonic acid gas is composed of a union of carbon and oxygen gas. If, then, the carbon is used by the plant, what happens to the oxygen?
Fig. 13. Jar of Elodea in water, giving off bubbles of oxygen gas in the sunlight.
You must have noticed bubbles rising from the “pond scum” and water-plants when they are in the sunlight, the little bubbles sometimes coming up in a quick, regular succession from the leaves and stems. Let us collect this gas and test it to find out what it is. This is more easily done if the plants are living in glass jars, where you can see them and get at them readily. A very good plant to use is the common Canadian water-plant (Elodea), which you can buy in aquarium shops if you cannot get it from the ponds for yourself. Place a handful of this plant in a tall, glass jar filled with fresh water, and cover it with a glass funnel, so as to collect the bubbles as they rise. See that the funnel is well under the water and support over it a test tube full of water, as in fig. 13. Place the jar in as bright sunlight as possible, when you should see the bubbles beginning to come off quite quickly. As the bubbles rise in the tube A, the water is forced out till the whole vessel is filled with gas. Then place your thumb over the mouth of the tube of gas, and remove it quickly from the water. Test it by plunging into it a splinter of wood which has been burning, but just blown out, so that it is still glowing. If you plunge it quickly enough into the tube, it should catch fire and burn brilliantly. Now this is the test for oxygen gas, so that we have proved that the tube was full of oxygen. This oxygen is the part of the carbonic acid gas which is given off by the plant as it uses the carbon and frees the oxygen it does not need.