The gymnosperms are plants in which the ovules are not enclosed by any ovary wall and remain exposed.
The giant redwood tree Sequoiais one of the tallest tree species belongs to gymnosperms.
The roots are generally tap roots, in some genera have fungal association in the form of mycorrhiza (Eg.,Pinus), while in some others (Eg.Cycas) small specialized roots called coralloid roots are associated with N2- fixing cyanobacteria.
The stems are unbranched (Eg.,Cycas) or branched (Eg.,Pinus).
The leaves may be simple or compound, such as pinnate leaves in Cycas and needle-like leaves in conifers.
The gymnosperms are heterosporous, haploid microspores and megaspores are produced within sporangia on sporophylls which are arranged spirally along an axis to form compact strobili or cones.
The strobili bearing microsporophylls and microsporangia are called microsporangiate or male strobili.
The microspores develop into a male gametophyte which is highly reduced called as a pollen grain.
The cones bearing megasporophylls with ovules or megasporangia are called macrosporangiate or female strobili.
The megaspore mother cell is differentiated from one of the cells of the nucellus, protected by envelopes and the composite structure is called an ovule.
The megaspore mother cell divides meiotically to form four megaspores, one of the megaspores enclosed within the megasporangium (nucellus) develops into a multicellular female gametophyte that bears two or more archegonia or female sex organs.
The male and the female gametophytes and the pollen grain are released, carried in air currents and come in contact with the opening of the ovules borne on megasporophylls
The pollen tube carrying the male gametes grows towards archegonia in the ovules and discharge their contents near the mouth of the archegonia for fertilization which leads to formation zygote and finally uncovered seeds.