Carbon dioxide (CO2) is the most important element your plants need. Plants absorb CO2 from the air viasmall pores in their leaves called stomata and use it as a building block to grow and bloom. CO2 is naturallypresent in the air at around 360-395 PPM. If you don’t keep CO2 levels topped up (either by ventilatingwith fresh air or supplementing with a CO2 cylinder or burner) your plants will run out of food. If thishappens you can expect stretched, leggy growth, smaller fruits and airy, wispy flowers.