Over the geological eons, as structural changes in stems and roots made larger and taller plants possible, the branching systems that served as the first leaves—the sites of photosynthesis—changed, too. Effective mechanical tissues, especially those that function with dead cells, make poor tissues in which to conduct the bio‐chemical process of photosynthesis; hence, the success of parenchyma‐filled leaves.
Leaves arose independently in the lineages of flowering plants and other major plant groups and, in most instances, became the specially modified sites for photosynthesis. Modern leaves come in all sizes and shapes, and their diversity provides an oft‐used means to identify kinds of plants. While genetics determines the way leaves are arranged on stems (their phyllotaxy) and their basic form, physical factors of the environment such as light and water are responsible for many of the variations in their appearance.