Green leaves capture the light energy available to them by means of the most important process for life on earth, at least life as we know it today. This process is photosynthesis. It involves the trapping of energy in sugar molecules that are constructed from ordinary water and from carbon dioxide present in the atmosphere. All the energy needs of living organisms ultimately depend on photosynthesis from the first day of their existence to the last. The lower surfaces of leaves as well, sometimes the upper one, are dotted with tiny pores called the stomata, which not only permit air circulation but are also involved in cooling the interior of the leaf, as water evaporates from the moist cell surfaces and passes out of them into the air in vapour form.
Leaves also perform other functions such as respiration, the movement of water throughout the plant.
Most of the water reaching the leaves evaporates into the atmosphere by a process known as transpiration. Many of the approximately 275,000 different kinds of plants that produce leaves can be distinguished from one another by their leaves alone. In addition many plants such as lilies, some aquatic plants, ferns and pine trees produce on the same plant different forms of leaves , including papery scales. The variety of shapes, sizes and textures of the leaves seems to be infinite. In fact leaves may assume virtually any form.