General characteristic features

Most gymnosperms are evergreen, woody trees or shrubs (a few, like Gnetum, are lianas), with a sporophyte differentiated into root, stem and leaves. A well-developed tap root system is usual, with special adaptations in places — the coralloid roots of Cycas host a symbiotic blue-green alga, while Pinus roots associate with fungal mycorrhizae. The stem is aerial, erect, and either branched or unbranched (as in Cycas), typically marked with old leaf scars; conifers additionally show two branch types, dwarf shoots (limited growth) and long shoots (unlimited growth). Leaves are dimorphic — green, photosynthetic foliage leaves (often xerophytic) and protective scale leaves.

The xylem is built mainly of tracheids, though Gnetum and Ephedra also have true vessels. Secondary growth is present, and the resulting wood is either manoxylic (soft, porous, with plenty of parenchyma and wide medullary rays, e.g. Cycas) or pycnoxylic (compact, with narrow medullary rays, e.g. Pinus). All gymnosperms are heterosporous, and a species may be monoecious (Pinus) or dioecious (Cycas); microsporangia and megasporangia develop on microsporophylls and megasporophylls respectively, and both sexes are usually organised into cones. Pollination is anemophilous (wind-driven), and fertilization is siphonogamous — a pollen tube physically delivers the male nuclei to the egg. Polyembryony (more than one embryo forming per seed) is common …