Seed Plants – Gymnosperms

The non-flowering seed plants (commonly called gymnosperms) include four clades that have similar life cycles. Microsporangia (and ultimately pollen) is formed on the surface of modified leaves called microsporophylls.  The microsporophylls are grouped into microsporangiate cones.  Microsporangiate cones are often small and ephemeral.  Megasporangia (in ovules) are formed on the surface of modified leaves called megasporophylls or (in Ginkgo) on sporangiophores.  Megasporophylls are grouped into megasporangiate cones (also called ovulate cones).  Seeds are therefore formed on the surface of cone scales or on sporangiophores.  Megasporangiate cones are often larger and woody.

All gymnosperms are woody plants, either shrubs or trees.  All have roots, stems, and leaves.  Leaves are of the megaphyll type.  Most gymnosperms have substantial fossil histories.

Pinophyta (Conifers)

Conifer sporophytes are typically large, long-lived and form copious amounts of wood (the stems and roots of older sporophytes are mostly wood.  Anatomists describe them as pycnoxylic.   Conifer leaves are typically reduced and needle-like or scale-like and often evergreen.  Sporophytes are heterosporous.  Microsporangia are formed in microsporangiate cones where microspores develop into pollen grains (male gametophytes/microgametophytes).  Megasporangia are formed in megasporangiate cones.  Megagametophytes develop within ovules.  Following pollination and fertilization, ovules develop into seeds on the upper surface of cone scales.  Most conifers are monoecious (both pollen and ovules are formed on each individual).  This familiar group includes pines, spruces, firs, redwoods, junipers, and hemlocks (as well as others).  About 625 species.

Cycadophyta (Cycads)

Cycad sporophytes produce only small amounts of wood (they are manoxylic).  Stems are unbranched or sparingly branched and armored with persistent leaf bases.  Leaves are large, (usually) pinnately compound, and borne on the crown of the plant.  Plants often form coralloid roots at the soil surface (roots which contain an endosymbiotic nitrogen-fixing cyanobacterium).  Pollen is formed in microsporangiate cones, seeds are formed in megasporangiate cones.  Plants are dioecious (individuals form microsporangiate or megasporangiate cones, not both).  About 185 species.

Ginkgophyta (Ginkgoes)

Ginkgo sporophytes are deciduous woody (pycnoxylic) trees which bear broadly triangular leaves which have open dichotomous venation.  Pollen is formed in microsporangiate catkin-like structures.  Seeds are formed at the tips of a forked structure (sporangiophore).  Plants are dioecious.  This division is monotypic, with a single extant species (Ginkgo biloba).  Ginkgo is widely planted, but may be extinct in the wild.

Gnetophyta (Gnetophytes)

Gnetophyte sporophytes are pycnoxylic shrubs or vines whose wood contains vessels.  Other sporophyte features are highly variable.  Pollen is formed in microsporangiate cones and seeds are formed in megasporangiate cones.  Plants are dioecious.  Pollination results in a double fertilization (but endosperm is not formed).  This division contains three very dissimilar genera (Gnetum, Ephedra, and Welwitschia) and about 80 species.