Gymnosperms (from the Greek, γυμνόσπερμος, meaning "naked seed" because the seeds do not develop within an ovary).
The following table describes the relative hierarchy of gymnosperm taxonomy and provides links to descriptions of each order and family.
This database provides information on taxa belonging to the gymnosperms. The gymnosperms have long been recognized as a distinctive group of plants, but there has also been a long (and ongoing) debate about whether the principal groups within the gymnosperms share a common ancestor, and if so, whether that common ancestor is or is not shared with the flowering plants, also called angiosperms. If they do share a common ancestor, distinct from the flowering plants, then the gymnosperms are a unique lineage of plants. Otherwise, the term "gymnosperm" has no taxonomic significance. In any event, the term "gymnosperm" has practical significance because it refers to a relatively small and highly distinctive assemblage of plants that are for the most part very ancient in lineage and reduced to a largely relictual distribution, but which nonetheless have great cultural and ecological importance at a global scale—a significance far greater than their species diversity alone would imply.
The principal point of contention in the taxonomic debate is whether the gnetophytes Ephedra, Gnetum and Welwitschia share a common ancestor with the angiosperms or the conifers (which are usually, but not always, assumed to share a common ancestor with the cycads and Ginkgo). The former opinion is called the "anthophyte" hypothesis, where anthophytes are defined to be a clade including both angiosperms and gnetophytes. The latter opinion is the gnetifer (or the variant 'gnepine,' gnetophytes plus Pinaceae) hypothesis, where gnetifers are a clade including both gnetophytes and the conifers. A succinct online review of the subject is provided by Castillo (2001), and cladograms clarifying the anthophyte and gnetifer hypotheses are provided by Burleigh and Mathews (2004). Every year or so a new paper is published supporting one hypothesis or another, and the accumulation of molecular data seems to be swinging in favor of the gnetifer or gnepine hypothesis, but there are still recent morphology studies supporting the anthophyte hypothesis (e.g., Friis et al. 2007). Christenhusz et al. (2011), reviewing studies available to date, simply assign each of the four gymnosperm groups to a subclass under the seed plants, Equisetopsida. They also note that under the gnepine hypothesis it would be appropriate to put the gnetophytes and Pinaceae in one subclass, and all other confers in another subclass. However, few taxonomists would be so rash as to assert that an end is in sight. It appears likely that Darwin's "abominable mystery", the origin of the angiosperms, will remain a mystery for a few more years. Until it is resolved, we won't quite know what to make of the gnetophytes.
I largely adopt, for taxa at the rank of family and above, the classification proposed by Christenhusz et al. (2011). It is still a conservative view, for instance in not assuming validity of the gnepine hypothesis, but is well supported by both molecular and morphological evidence. I should note that much different classification schemes exist. For instance, Reveal (1998) subdivided the gymnosperms into 4 divisions, 6 classes, and 12 orders. I suppose he has reconsidered, since he is a co-author on the Christenhusz paper.
Although molecular studies have done much to improve our understanding of evolutionary relationships between extant gymnosperms, changing relationships at evolutionary timescales are best elucidated through the fossil record. The following points summarize the chronology of gymnosperm evolution:
Although morphologically, ecologically and taxonomically diverse, and individually very numerous, the gymnosperms are not represented by a great many species. Here is a table summarizing diversity at the family, genus and species level. In this treatment there are 947 species (as of 2002.07.07), or about as many as may be found in the largest genera of flowering plants (such as Acacia). Of this total, one species represents the Ginkgos, 68 represent the Gnetophytes, 289 are Cycads, and the remaining 589 species are Conifers. At this time, those taxon figures are debatable within ±5-10%. There are not likely to be many more species discovered. There have lately been notable discoveries (Cupressus vietnamensis, discovered in 2001 in Vietnam), and the affinity of certain cycads, some podocarps and many gnetophytes for tropical rainforest environments means that new discoveries may be made in those taxa as well.
As a practical matter, this database generally treats species as valid if there is decent ground for debate as to whether a species is distinct, or should be treated as a subspecies or variety. Such debates are common because, for many taxa, up-to-date taxonomic treatments are simply not available. Where I can find such a treatment, then I tend to give it considerable credence; but there are some taxa that have not received monographic treatment for many decades.
I am also sometimes questioned about the distinction between subspecies and varieties. Generally I prefer to discriminate subspecies if they represent spatially and ecologically disjunct groups that show minimal evidence of recent introgression or other genetic exchange. I prefer to use the term variety to represent morphologically distinctgroups that may be local in extent, share a common range or ecological setting, or display evidence of active gene flow between the taxa. Of course there are many intermediate conditions and there is still an element of subjectivity. Nonetheless I feel it is better to have scientific criteria in designation of varieties and subspecies; in the past, the two seem to have been discriminated mainly on the basis of personal preferences.
Plants are classified into two large groups, vascular plants and nonvascular plants. Nonvascular plants lack tissues that have been specialized for the transport of water and nutrients between distant parts of the plant. They include all algae, mosses, and their allies. Vascular plants have such transport systems. They include the vast majority of familiar terrestrial plants and can be divided into three major groups according to major features of their reproductive systems. The simplest and oldest of these is the Pterophytina or ferns and fern allies. The next group is the Pinophyta or Gymnospermae, which are the focus of this presentation. The last and by far the largest group (about 250,000 described species), is the Angiospermae.
Gymnosperms are woody plants, either shrubs, trees, or, rarely, vines (some gnetophytes). They differ from flowering plants in that the seeds are not enclosed in an ovary but are exposed within any of a variety of structures, the most familiar being cones. The gymnosperms display four architectural models of tree growth, which determine the basic form and appearance of the plant; see Tomlinson (1983) for more detail.
A more anatomically detailed discussion of the comparative morphology of gymnosperms and related vascular plants can be found in the On-Line Biology Book or by clicking on Google Search (Gymnosperm Life Cycle). It seems odd, but apart from that naked seed, the gymnosperms really do not share many attributes. Wood and leaf, habit and habitat, physiology and anatomy - all are quite variable between the four major groups. We might have chosen to put "the gymnosperms" together in one group because they are all relicts, survivors of an ancient flora. They all seem strange to children of the Cenozoic savannas.
Living gymnosperms are distributed worldwide (excepting Antarctica), with a majority, particularly the conifers, in temperate and subarctic regions.
Gymnosperms include many of the largest trees on earth. Ginkgo can grow very large. Among the conifers, Sequoiadendron giganteum is the largest of all trees, but many other giants exist as well; see the sidebar for A Tale of Big Tree Hunting In California.
The age of a gymnosperm is usually difficult to determine. Precise numbers are only available if someone recorded the date of planting. Many trees have (approximately) annual rings, permitting approximate determination of the age of an individual, but in harsh environments many years may pass before the first ring is made. For example, Ken Lertzman (pers. comm. 1990) found mountain hemlock seedlings up to 18 years old (based on bud scar counts) that had yet to form their first ring. The greatest age known from ring counts is about 5000 years, for Pinus longaeva. Several other species, such as Thuja plicata and Taxus baccata, might achieve comparable ages but we cannot tell because they live in moist climates where the tree's woody heart rots away while the rest of the tree is in the prime of life, so that no record of the rings is left behind. For further detail on the oldest known gymnosperms, see the article How Old Is That Tree?.
Many other trees, including most tropical species, and all non-tree gymnosperms (Cycadales and Gnetales), do not have regular annual rings. Age estimates for such plants are usually based on extrapolation of observed growth rates. Such extrapolation may lead to estimation errors of several hundred percent (usually overestimation, because trees grow more slowly as they age).
Many species of plants are clonal, so that numerous different individuals are genetically identical. Among the oldest, a clone age of 43,600 years has been estimated for Lomatia tasmanica, a Tasmanian tree (DEWHA 2010). A clone age estimate of 10,500 years has been proposed for Huon pine, Lagarostrobos franklinii, and a comparable age may be reached in the redwood, Sequoia sempervirens, which is only one of many gymnosperms that can sprout from fallen stems or cut stumps. Some cycads produce clones by producing buds at the base, which may become separated from the main plant and then root. The oldest gymnosperm clones are likely to be found among species prone to vegetative reproduction in an area of extraordinarily stable climate, such as cycads of the genus Encephalartos growing in South Africa.
Dendrochronology, the study of tree rings, is necessarily restricted to trees that form annual rings. This includes hundreds of species. Nearly all gymnosperms living in temperate or boreal climates have been investigated for use in dendrochronology, and most have proven suitable, with the exceptions found mainly in areas where extreme stress from cold, drought or other factors frequently prevents the formation of an annual ring. Tropical gymnosperms have been less widely studied, but generally, those growing in climates with pronounced wet and dry seasons form annual rings and those with year-round rainfall are not usable.
There are few gymnosperms that were not used by aboriginal peoples; the main exceptions seem to have been in areas where there were few or no people, or in tropical areas where angiosperm diversity far exceeded gymnosperm diversity. Two outstanding examples include:
Today, of course, gymnosperms provide the world with softwood lumber and with most of the wood pulp supply. Most of this timber comes from a handful of species, notably Pinus radiata, the most widely planted tree in the world, although it has one of the smallest natural distributions.
Gymnosperm diversity is best observed at botanical gardens and arboreta. Conifers, at least, are also very well represented in ornamental collections throughout the temperate zones of the world.
All trees, indeed all vascular plants, possess a fundamental architecture prescribed by their genetic code. This architecture remains constant throughout the plant's life, providing a uniform functional structure even as the plant experiences immense changes in size, from seed to tree, and even in spite of calamities that may befall the plant during its life. Every tree can be defined by one of 23 different architectural models. This is a complex topic, not yet fully explored, and I have not yet found a source that classifies the gymnosperms according to their various architectural models; for further information see this excellent blog and the additional sources cited therein.
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Chaw, S.M., C.L. Parkinson, Y. Cheng, T.M. Vincent, and J.D. Palmer. 2000. Seed plant phylogeny inferred frrom all three plant genomes: monophyly of extant gymnosperms and origin of Gnetales from conifers. Proc Natl Acad Sci USA 97:4086-4091. Available online HERE (2007.11.26).
The University of Sydney School of Biological Sciences provides a good overview of the gymnosperm life cycle (2006.03.20).
Wang, Xiao-Quan, and Jin-Hua Ran. 2014. Evolution and biogeography of gymnosperms. Molecular Phylogenetics and Evolution 75:24-40.
White 1994 gives a beautifully illustrated account of the fossil origins of the major gymnosperm and angiosperm groups.
Last Modified 2018-01-13