The gymnosperms are a group of seed-producing plants that includes conifers, cycads, Ginkgo,
and Gnetales.
The term "gymnosperm" comes from the Greek composite word γυμνόσπερμος, meaning
"naked seeds", after the unenclosed condition of their seeds.
Their naked condition stands in contrast to the seeds and ovules of flowering plants,
which are enclosed within an ovary.
Gymnosperm seeds develop either on the surface of scales or leaves, often modified to form
cones, or at the end of short stalks as in Ginkgo.
The gymnosperms and angiosperms together compose the spermatophytes or seed plants.
By far the largest group of living gymnosperms are the conifers, followed by cycads, Gnetophytes,
and Ginkgo.
Classification
In early classification schemes, the gymnosperms were regarded as a "natural" group.
There is conflicting evidence on the question of whether the living gymnosperms form a clade.
The fossil record of gymnosperms includes many distinctive taxa that do not belong to
the four modern groups, including seed-bearing trees that have a somewhat fern-like vegetative
morphology When fossil gymnosperms such as Bennettitales, Caytonia and the glossopterids
are considered, it is clear that angiosperms are nested within a larger gymnosperm clade,
although which group of gymnosperms is their closest relative remains unclear.
For the most recent classification on extant gymnosperms see Christenhusz et al..
Subclass Cycadidae Order Cycadales
Family Cycadaceae: Cycas Family Zamiaceae: Dioon, Bowenia, Macrozamia,
Lepidozamia, Encephalartos, Stangeria, Ceratozamia, Microcycas, Zamia.
Subclass Ginkgoidae Order Ginkgoales
Family Ginkgoaceae: Ginkgo
Subclass Gnetidae Order Welwitschiales
Family Welwitschiaceae: Welwitschia
Order Gnetales Family Gnetaceae: Gnetum
Order Ephedrales Family Ephedraceae: Ephedra
Subclass Pinidae Order Pinales
Family Cupressaceae: Cunninghamia, Taiwania, Athrotaxis, Metasequoia, Sequoia, Sequoiadendron,
Cryptomeria, Glyptostrobus, Taxodium, Papuacedrus, Austrocedrus, Libocedrus, Pilgerodendron,
Widdringtonia, Diselma, Fitzroya, Callitris, Neocallitropsis, Thujopsis, Thuja, Fokienia,
Chamaecyparis, Cupressus, Juniperus, Calocedrus, Tetraclinis, Platycladus, Microbiota
Family Pinaceae: Cedrus, Pinus, Cathaya, Picea, Pseudotsuga, Larix, Pseudolarix, Tsuga, Nothotsuga,
Keteleeria, Abies Family Podocarpaceae: Phyllocladus, Lepidothamnus,
Prumnopitys, Sundacarpus, Halocarpus, Parasitaxus, Lagarostrobos, Manoao, Saxegothaea, Microcachrys,
Pherosphaera, Acmopyle, Dacrycarpus, Dacrydium, Falcatifolium, Retrophyllum, Nageia, Afrocarpus,
Podocarpus Family Sciadopityaceae: Sciadopitys
Family Taxaceae: Austrotaxus, Pseudotaxus, Taxus, Cephalotaxus, Amentotaxus, Torreya
Order Araucariales Family Araucariaceae: Araucaria, Wollemia,
Agathis
Diversity and origin There are more than 1000 extant or currently
living species of gymnosperms in 88 plant genera belonging to 14 plant families.
It is widely accepted that the gymnosperms originated in the late Carboniferous period.
This appears to have been the result of a whole genome duplication event around 319
million years ago.
Early characteristics of seed plants were evident in fossil progymnosperms of the late
Devonian period around 380 million years ago.
It has been suggested that during the mid-Mesozoic era, pollination of some extinct groups of
gymnosperms were by extinct species of scorpionflies that had specialized proboscis for feeding
on pollination drops.
The scorpionflies likely engaged in pollination mutualisms with gymnosperms, long before the
similar and independent coevolution of nectar-feeding insects on angiosperms.
Conifers are by far the most abundant extant group of gymnosperms with six to eight families,
with a total of 65-70 genera and 600-630 species.
Conifers are woody plants and most are evergreens.
The leaves of many conifers are long, thin and needle-like, others species, including
most Cupressaceae and some Podocarpaceae, have flat, triangular scale-like leaves.
Agathis in Araucariaceae and Nageia in Podocarpaceae have broad, flat strap-shaped leaves.
Cycads are the next most abundant group of gymnosperms, with two or three families, 11
genera, and approximately 300 species.
The other extant groups are the 75-80 species of Gnetales and one species of Ginkgo.
Uses Gymnosperms have major economic uses.
Pine, fir, spruce, and cedar are all examples of conifers that are used for lumber.
Some other common uses for gymnosperms are soap, varnish, nail polish, food, gum, and
perfumes.
Life cycle Gymnosperms, like all vascular plants, have
a sporophyte-dominant life cycle.
The gametophyte is relatively short-lived.
Two spore types, microspores and megaspores, are typically produced in pollen cones or
ovulate cones, respectively.
Gametophytes, as with all heterosporous plants, develop within the spore wall.
Pollen grains mature from microspores, and ultimately produce sperm cells.
Megagametophytes develop from megaspores and are retained within the ovule.
They typically produce multiple archegonia.
During pollination, pollen grains are physically transferred between plants, from pollen cone
to the ovule, being transferred by wind or insects.
Whole grains enter each ovule through a microscopic gap in the ovule coat called the micropyle.
The pollen grains mature further inside the ovule and produce sperm cells.
Two main modes of fertilization are found in gymnosperms.
Cycads and Ginkgo have motile sperm that swim directly to the egg inside the ovule, whereas
conifers and gnetophytes have sperm with no flagella that are conveyed to the egg along
a pollen tube.
After syngamy, the zygote develops into an embryo.
More than one embryo is usually initiated in each gymnosperm seed.
The mature seed comprises the embryo and the remains of the female gametophyte, which serves
as a food supply, and the seed coat.
Genetics The first published sequenced genome for any
gymnosperm was the genome of Picea abies in 2013.
References
External links Gymnosperm Database
Gymnosperms on the Tree of Life