[kabar]

In the dark, however, they take in oxygen and give off CO2.

So at night plants are CO2 generators? Does the amount of CO2 produced equal the amount consumed during the day? Would that make them neutral in terms of their impact on the atmosphere?

I have never heard that one before nor has my daughter who was a biology major. Are you sure?

[maryz]

I'm sure that's the way I learned it in 10th grade. (Also in 7th grade, I think.) Of course, when I was in 10th grade, no one worried about global warming or carbon credits!

I'm having trouble finding an appropriate link -- either they're oversimple or overtechnical (a lot of things that come up in my search are quizzes!). Here's a bit from an indoor gardening site: gardenweb.com

Plants have two distinct phases in their metabolism, one in bright light where they are making sugars from sunlight and carbon dioxide, and one in the dark where they are burning those sugars in a very similar way to animal metabolisms.

There's a much more technical one here, comparing the processes of photosynthesis (gives off oxygen) and respiration (gives off CO2) in plants. PHYSIOLOGY: PLANT GROWTH AND DEVELOPMENT