Re: how do plants exchange gases?

    Plants do all of their gas exchange passively, through diffusion.  
Plants have tiny pores called stomata (sing. stoma or stomate) that allow 
carbon dioxide and oxygen exchange while preventing liquid water from 
flooding the air spaces inside of the leaf.  Plants are able to open and 
close the stomata depending on their environmental conditions.  When they 
need CO2 for photosynthesis, the guard cells around each stoma swell with 
water and pull back, opening the passageway.  Because the concentration 
of CO2 is higher outside of the leaf than inside, it diffuses into the leaf 
along a concentration gradient. When the stomata are open, however, the 
plant loses a lot of moisture - because there is a higher concentration of 
water vapor inside of the leaf than outside, water also diffuses outward 
along a concentration gradient. In order to conserve water, the guard cells 
are able to close around the stoma, preventing water loss when the need for 
CO2 is low or when it is overridden by water stress.
    For more detailed information on stomata and gas exchange in plants, 
check out Introduction to Plant Physiology by William G. Hopkins (John 
Wiley & Sons, 1995) or any other introductory plant physiology text.  For 
some good images of stomata on the web, check out: 

http://www.microscopy-uk.org.uk/schools/images/stomata.html
 
http://www.science.widener.edu/~grant/esa/exp2/stomata.jpg

http://www.wisc.edu/botit/img/bot/130/Leaf/Zea_leaf_cross_section/Stoma.jpg