"Fieldnotes," December 22nd & 23rd, 2013: "Evergreen Needles," by Peter Lesica / Annie Garde.
http://www.montananaturalist.org/
"When you turn a pine or fir needle over and look at the underside you can often see two white lines. These are hundreds of little mouth-like openings all lined up. Stomates open to take in the carbon dioxide that all plants need to make food, and they close up when it's dry to keep water loss as low as possible...These characteristics help conifers deal with the desiccating cold common during winter in Western North America."