As discussed in previous articles, a tree that sheds all of its leaves each season is known as deciduous. On the other hand, a tree that is not leafless at any time is known as an evergreen tree. These new leaves will be retained for several years. Evergreen leaves are particularly well designed to withstand the rigors of cold weather and lack of water that result from frozen earth during harsh winters. One may not think of a pine needle as being a leaf, but it's photosynthetic factory function just as well as those of the oak, apricot, cottonwood, etc.
Every leaf has to allow a flow of gases in and out of the plant, as well as the exit of water vapor.
This flow proceeds through openings in the leaves called stomata. Stomata usually open during the day and close at night. Cells known as guard cells encircle them. Guard cells contain chloroplasts and so are able to conduct photosynthesis. This little factory produces glucose.