It also help the tree survive the cold, dry air of winter. In the warm seasons, leaves use sun light, water, and air to make the tree's food. In that process, the tree losses a lot of water through tiny holes in the leaves. In winter, the tree does not get enough water to replace what it would lose through the leaves. If the tree did not seal the spots where the leaves grow, it would die. When springs brings warm air and fresh water, tree will sprout new leaves and start to growing again.