Trees commonly are divided into two groups on the basis of their reproductive structures. Gymnosperms, the more primitive of the two groups, bear seeds on modified leaf structures called scales, which may be aggregated into cones. These are often referred to as conifers. Angiosperms, the more highly evolved, have flowers and bear their seeds enclosed in a fruit, which is the ripened ovary of a flower. These are referred to as deciduous trees. Trees have four major structural components: stem or trunk, branches, and twigs; leaves; roots; and reproductive structures.