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. Photosynthesis occurs during sunlit hours and a plant exposed to sunlight must be able to open the stomata to transpire water vapor and maintain the optimum temperature for the plant's survival. This arrangement helps to regulate the loss of water vapor from the leaf, as well as the exchange of gases. An evergreen will conduct photosynthesis on a sunny day, even in the coldest days of winter. This peculiarity increases the total area of chloroplasts.