Leaves are the principle sites of photosynthesis in plants. Sunlight is utilized by the chloroplasts within the mesophyll cells within the leaves, and carbon dioxide becomes incorporated into complex carbohydrate molecules. Leaves are also the principal site of water loss, through a process called transpiration during which water vapour is lost to the atmosphere through stomata. The evapotranspirational water loss can help lower the leaf temperature several degrees below that of the ambient air temperature.  Nature's 'air-conditioning systems' serve an important function not only in temperature regulation, but in ensuring a constant 'pull' of water from the soil through the roots, to the leaves, thus ensuring a continuous supply of nutrient during this process. The Ligustrum leaf shown at left, is a typical mesomorphic dicotyledonous leaf.