Ecological notes: Those two white lines are actually a bunch of little white dots, which are actually the stomata (the mouths, or sites of gas exchange, in leaves).
Because conifers weather out the winter with their needles still on the branches, they have to compete with harsh winter winds for water. They effectively have a water input of zero. With the ground frozen, the tree can't replace any of the water it might lose to transpiration (basically evaporation of water out of any openings in a leaf). Transpiration increases with wind that would carry water vapor away (much like how you can tell wind direction by licking your finger and holding it up; it feels cool on the windy side because the water is evaporating even though it's well below the boiling temperature where you'd expect it to go from a liquid to a gass).
Hemlocks would risk desiccation (severe drying) in the leaves unless it could somehow protect itself from the ways it loses water. It does this in a couple of ways. First, it has a super waxy cuticle. The waxy cuticle prevents transpiration from occurring on the top side of the leaf. Second, it protects the stomata (those openings) from being exposed to the wind by putting them on the underside of the leaf and having them in grooves.