Carbon dioxide gas -- CO2 -- can be used for enriching the growing conditions for plants. Increasing the amount of CO2 in the growing environment produces larger plants. Plants can utilize about four times the amount of CO2 found in the atmosphere. You can make your own CO2 generator with yeast and a sugar solution. The yeast feed on the sugar and produce CO2 as a product of respiration. One bottle of yeast doesn't produce much more CO2 than an animal does by exhaling, so if you want to fill your greenhouse, you'll need many such generators.
Drill a hole with a slightly smaller diameter than a length of aquarium tubing into the bottle cap of a 2-liter bottle. If you use 1/4-inch tubing, drill the hole with a 3/16-inch drill bit.
Feed enough tubing through the cap so to extend two inches into the bottle when the cap is on. Secure the tubing with a small amount of silicon sealant -- allow the sealant to dry overnight.
Fill the bottle halfway with water; it should be lukewarm to avoid killing the yeast. Add two cups of sugar.
Cover the bottle opening with an extra cap and shake the bottle to dissolve the sugar. Remove the cap and fill the bottle with more lukewarm water. The level should come to an inch below the tubing when you place the cap with the tubing on the bottle.
Add one tablespoon baker's yeast and a pinch of baking soda to the bottle. Put on the cap and screw it on lightly. It shouldn't be airtight, because excess gases in the bottle may need to escape.
Place the other end of the tubing near some plants in the greenhouse. It may take up to three days, but the yeast should start producing CO2 to feed the plants..