The first and ultimate goal of every plant is photosythesis. If a plant is multi-cellular, it usually develops relatively large, flat structures which goal is to catch sun rays. Terrestrial plants are no exception; most probably, they started to build their body with organs similar to present day leaves.
A leaf is lateral photosynthetic organ of shoot with restricted growth. Its functions are photosynthesis, respiration, transpiration, and synthesis of secondary chem-icals. Features of a leaf (i.e., characters help to distinguish it) include having a bud in the axil, not growing by apex, not producing new leaves or shoots, and having hierarchal morphology (see below).