Gymnosperms are seed-bearing, vascular plants. The term gymnosperm comes from the Greek word gumnospermos, meaning literally "naked seed". This term is applied because the seeds of these plants are not formed in an enclosed ovulary (that is, a pistil with one or more carpels, developing into a fruit as in the angiosperms), but naked on the scales of a cone or cone-like structure. The production of seeds distinguishes the gymnosperms (along with the angiosperms) from other members of the vascular plants. Gymnosperms are heterosporous, producing microspores that develop into pollen grains and megaspores that are retained in an ovule. After fertilization (joining of the micro- and megaspore), the resulting embryo, along with other cells comprising the ovule, develops into a seed. The seed is a sporophyte resting stage.
At one time, the gymnosperms were considered to be a class (Class Gymnospermae), first within the seed plants (Division Spermatophyta; 1883~1950) and later within the vascular plants (Division Tracheophyta; 1950~1981). The class essentially encompassed the conifers and their allies (by which term is meant "related species of plants"), including several groups of extinct plants known only from fossils. In these earlier classification schemes, the "naked seed" plants were clearly set off from the other classes of higher plants (that is, the ferns and flowering plants), essentially as they are today. However, fossil evidence suggests that the angiosperms evolved from a gymnosperm ancestor, which would make the gymnosperm taxon paraphyletic. Modern cladistics attempts to define taxa that are monophyletic, traceable to a common ancestor and inclusive therefore of all descendants of that common ancestor. So, while the term gymnosperm is still widely used to distinguish the four taxa of non-flowering, seed-bearing plants from the angiosperms, plant species once treated as gymnosperms are distributed among four groups given equal rank as divisions within the Kingdom Plantae. These groups are: