The Gymnosperms comprise of that group of plants in which a naked, (i.e., exposed and not encased in a closed ovary) seed with a single integument is present. A seed is a mature integument megasporangium. The megasporangium is now called the ovule and is usually associated with a sporophyll or carpel.
The ovule permanently retains within it the megaspore which develops the female gametophyte (embryosac) within a tissue (nucellus) formed inside the megasporangium. The female gametophyte is a complete parasite within the sporophyte and has no independent existence — thus completely reversing the condition in the Bryophytes.
The integument develops from the base of the megasporangium. The archegonium structure is present but it disintegrates in the most advanced Gymnosperms, the Gnetales.
The embryo develops within the seed. The endosperm is still a gametophytic tissue as in the Pteridophytes. The seed passes through a short or long period of dormancy before germination. Fertilisation is siphonogamic through pollen tubes, by motile sperms in the primitive Gymnosperms and by non-motile male gametes in the more evolved ones.
The stigma and style passage is not yet evolved in the Gymnosperms.
The Gymnosperms arose in the Palaeozoic, dominated the World during the Mesozoic— the age of the dinosaurs, and the earlier members of the group have become extinct today.
Although the Angiosperms form the dominant vegetation today the Gymnosperms are not less important as they form a wide belt of forests in the North Temperate to Subarctic regions which is of the greatest economic importance as a source of timber, lumber and wood pulp. They are mostly arboreal, sometimes shrubby in habit.
The Gymnosperms had been considered as a group co-ordinate with the Angiosperms within the Phanerogams or Spermaphytes. There is now, however, a tendency to divide the Gymnosperms into a number of divisions as in the case of Pteridophytes.
J. D. Hooker placed the Gymnospermae in between the Dicotyledons and the Monocotyledons. Eichler (1883) proposed Gymnospermae and Angictpermac as the two divisions of Phanerogamae. Bessey (1911) divides the Spermatophytevinto three phyla (divisions) parallel to his three divisions of Pteridophyta.