Group of cone-bearing seed plants
Conifers are a group of cone-bearing seed plants, a subset of gymnosperms. Scientifically, they make up the division Pinophyta (), also known as Coniferophyta () or Coniferae. The division contains a single extant class, Pinopsida. All extant conifers are perennial woody plants with secondary growth. The great majority are trees, though a few are shrubs. Examples include cedars, Douglas-firs, cypresses, firs, junipers, kauri, larches, pines, hemlocks, redwoods, spruces, and yews. As of 2002, Pinophyta contained seven families, 60 to 65 genera, and more than 600 living species.
Although the total number of species is relatively small, conifers are ecologically important. They are the dominant plants over large areas of land, most notably the taiga of the Northern Hemisphere, but also in similar cool climates in mountains further south. Boreal conifers have many wintertime adaptations. The narrow conical shape of northern conifers, and their downward-drooping limbs, help them shed snow. Many of them seasonally alter their biochemistry to make them more resistant to freezing. While tropical rainforests have more biodiversity and turnover, the immense conifer forests of the world represent the largest terrestrial carbon sink. Conifers are of great economic value for softwood lumber and paper production.
Names and taxonomy.
"Conifer" is a Latin word, a compound of "conus" (cone) and "ferre" (to bear), meaning "the one that bears (a) cone(s)".
The division name Pinophyta conforms to the rules of the "International Code of Nomenclature for algae, fungi, and plants" (ICN), which state (Article 16.1) that the names of higher taxa in plants (above the rank of family) are either formed from the name of an included family (usually the most common and/or representative), in this case Pinaceae (the pine family), or are descriptive. A descriptive name in widespread use for the conifers (at whatever rank is chosen) is Coniferae (Art 16 Ex 2).
According to the ICN, it is possible to use a name formed by replacing the termination "-aceae" in the name of an included family, in this case preferably Pinaceae, by the appropriate termination, in the case of this division "-ophyta". Alternatively, "descriptive botanical names" may also be used at any rank above family. Both are allowed.
This means that if conifers are considered a division, they may be called Pinophyta or Coniferae. As a class, they may be called Pinopsida or Coniferae. As an order they may be called Pinales or Coniferae or Coniferales.
Conifers are the largest and economically most important component group of gymnosperms, but nevertheless they comprise only one of the four groups. The division Pinophyta consists of just one class, Pinopsida, which includes both living and fossil taxa. Subdivision of the living conifers into two or more orders has been proposed from time to time. The most commonly seen in the past was a split into two orders, Taxales (Taxaceae only) and Pinales (the rest), but recent research into DNA sequences suggests that this interpretation leaves the Pinales without Taxales as paraphyletic, and the latter order is no longer considered distinct. A more accurate subdivision would be to split the class into three orders, Pinales containing only Pinaceae, Araucariales containing Araucariaceae and Podocarpaceae, and Cupressales containing the remaining families (including Taxaceae), but there has not been any significant support for such a split, with the majority of opinion preferring retention of all the families within a single order Pinales, despite their antiquity and diverse morphology.
There were seven families of conifers c. 2011, with 65–70 genera and over 600 living species (c. 2002). The seven most distinct families are linked in the box above right and phylogenetic diagram left. In other interpretations, the Cephalotaxaceae may be better included within the Taxaceae, and some authors additionally recognize Phyllocladaceae as distinct from Podocarpaceae (in which it is included here). The family Taxodiaceae is here included in the family Cupressaceae, but was widely recognized in the past and can still be found in many field guides. A new classification and linear sequence based on molecular data can be found in an article by Christenhusz et al.
The conifers are an ancient group, with a fossil record extending back about 300 million years to the Paleozoic in the late Carboniferous period; even many of the modern genera are recognizable from fossils 60–120 million years old. Other classes and orders, now long extinct, also occur as fossils, particularly from the late Paleozoic and Mesozoic eras. Fossil conifers included many diverse forms, the most dramatically distinct from modern conifers being some herbaceous conifers with no woody stems. Major fossil orders of conifers or conifer-like plants include the Cordaitales, Vojnovskyales, Voltziales and perhaps also the Czekanowskiales (possibly more closely related to the Ginkgophyta). 
Multiple studies also indicate that the Gnetophyta belong within the conifers despite their distinct appearances, either placing them as a sister group to Pinales (the 'gnepine' hypothesis) or as being more derived than Pinales but sister to the rest of the group. Most recent studies favor the 'gnepine' hypothesis.
Phylogeny.
The earliest conifers appear in the fossil record during the Late Carboniferous (Pennsylvanian), over 300 million years ago. Conifers are thought to be most closely related to the Cordaitales"," a group of extinct Carboniferous-Permian trees and clambering plants whose reproductive structures had some similarities to those of conifers. The most primitive conifers belong to the paraphyletic assemblage of "walchian conifers", which were small trees, and probably originated in dry upland habitats. The range of conifers expanded during the Early Permian (Cisuralian) to lowlands due to increasing aridity. Walchian conifers were gradually replaced by more advanced voltzialean or "transition" conifers. Conifers were largely unaffected by the Permian–Triassic extinction event, and were dominant land plants of the Mesozoic era. Modern groups of conifers emerged from the Voltziales during the Late Permian through Jurassic. Conifers underwent a major decline in the Late Cretaceous corresponding to the explosive adaptive radiation of flowering plants.
Description.
All living conifers are woody plants, and most are trees, the majority having a monopodial growth form (a single, straight trunk with side branches) with strong apical dominance. Many conifers have distinctly scented resin, secreted to protect the tree against insect infestation and fungal infection of wounds. Fossilized resin hardens into amber, which has been commercially exploited historically (for example, in New Zealand's 19th-century kauri gum industry).
The size of mature conifers varies from less than one metre to over 100 metres in height. The world's tallest, thickest, largest, and oldest living trees are all conifers. The tallest is a coast redwood ("Sequoia sempervirens"), with a height of 115.55 metres (although one mountain ash, "Eucalyptus regnans", allegedly grew to a height of 140 metres, the tallest living angiosperms are significantly smaller at around 100 metres.) The thickest (that is, the tree with the greatest trunk diameter) is a Montezuma cypress ("Taxodium mucronatum"), 11.42 metres in diameter. The largest tree by three-dimensional volume is a giant sequoia ("Sequoiadendron giganteum"), with a volume 1486.9 cubic metres. The smallest is the pygmy pine ("Lepidothamnus laxifolius") of New Zealand, which is seldom taller than 30 cm when mature. The oldest non-clonal living tree is a Great Basin bristlecone pine ("Pinus longaeva"), 4,700 years old.
Foliage.
Since most conifers are evergreens, the leaves of many conifers are long, thin and have a needle-like appearance, but others, including most of the Cupressaceae and some of the Podocarpaceae, have flat, triangular scale-like leaves. Some, notably "Agathis" in Araucariaceae and "Nageia" in Podocarpaceae, have broad, flat strap-shaped leaves. Others such as "Araucaria columnaris" have leaves that are awl-shaped. In the majority of conifers, the leaves are arranged spirally, the exceptions being most of Cupressaceae and one genus in Podocarpaceae, where they are arranged in decussate opposite pairs or whorls of 3 (−4).
In many species with spirally arranged leaves, such as "Abies grandis" (pictured), the leaf bases are twisted to present the leaves in a very flat plane for maximum light capture. Leaf size varies from 2 mm in many scale-leaved species, up to 400 mm long in the needles of some pines (e.g. Apache pine, "Pinus engelmannii"). The stomata are in lines or patches on the leaves and can be closed when it is very dry or cold. The leaves are often dark green in colour, which may help absorb a maximum of energy from weak sunshine at high latitudes or under forest canopy shade.
Conifers from hotter areas with high sunlight levels (e.g. Turkish pine "Pinus brutia") often have yellower-green leaves, while others (e.g. blue spruce, "Picea pungens") may develop blue or silvery leaves to reflect ultraviolet light. In the great majority of genera the leaves are evergreen, usually remaining on the plant for several (2–40) years before falling, but five genera ("Larix", "Pseudolarix", "Glyptostrobus", "Metasequoia" and "Taxodium") are deciduous, shedding their leaves in autumn. The seedlings of many conifers, including most of the Cupressaceae, and "Pinus" in Pinaceae, have a distinct juvenile foliage period where the leaves are different, often markedly so, from the typical adult leaves.
Tree ring structure.
Tree rings are records of the influence of environmental conditions, their anatomical characteristics record growth rate changes produced by these changing conditions. The microscopic structure of conifer wood consists of two types of cells: parenchyma, which have an oval or polyhedral shape with approximately identical dimensions in three directions, and strongly elongated tracheids. Tracheids make up more than 90% of timber volume. The tracheids of earlywood formed at the beginning of a growing season have large radial sizes and smaller, thinner cell walls. Then, the first tracheids of the transition zone are formed, where the radial size of cells and the thickness of their cell walls changes considerably. Finally, latewood tracheids are formed, with small radial sizes and greater cell wall thickness. This is the basic pattern of the internal cell structure of conifer tree rings.
Reproduction.
Most conifers are monoecious, but some are subdioecious or dioecious; all are wind-pollinated. Conifer seeds develop inside a protective cone called a strobilus. The cones take from four months to three years to reach maturity, and vary in size from long.
In Pinaceae, Araucariaceae, Sciadopityaceae and most Cupressaceae, the cones are woody, and when mature the scales usually spread open allowing the seeds to fall out and be dispersed by the wind. In some (e.g. firs and cedars), the cones disintegrate to release the seeds, and in others (e.g. the pines that produce pine nuts) the nut-like seeds are dispersed by birds (mainly nutcrackers, and jays), which break up the specially adapted softer cones. Ripe cones may remain on the plant for a varied amount of time before falling to the ground; in some fire-adapted pines, the seeds may be stored in closed cones for up to 60–80 years, being released only when a fire kills the parent tree.
In the families Podocarpaceae, Cephalotaxaceae, Taxaceae, and one Cupressaceae genus ("Juniperus"), the scales are soft, fleshy, sweet, and brightly colored, and are eaten by fruit-eating birds, which then pass the seeds in their droppings. These fleshy scales are (except in "Juniperus") known as arils. In some of these conifers (e.g. most Podocarpaceae), the cone consists of several fused scales, while in others (e.g. Taxaceae), the cone is reduced to just one seed scale or (e.g. Cephalotaxaceae) the several scales of a cone develop into individual arils, giving the appearance of a cluster of berries.
The male cones have structures called microsporangia that produce yellowish pollen through meiosis. Pollen is released and carried by the wind to female cones. Pollen grains from living pinophyte species produce pollen tubes, much like those of angiosperms. The gymnosperm male gametophytes (pollen grains) are carried by wind to a female cone and are drawn into a tiny opening on the ovule called the micropyle. It is within the ovule that pollen-germination occurs. From here, a pollen tube seeks out the female gametophyte, which contains archegonia each with an egg, and if successful, fertilization occurs. The resulting zygote develops into an embryo, which along with the female gametophyte (nutritional material for the growing embryo) and its surrounding integument, becomes a seed. Eventually, the seed may fall to the ground and, if conditions permit, grow into a new plant.
In forestry, the terminology of flowering plants has commonly though inaccurately been applied to cone-bearing trees as well. The male cone and unfertilized female cone are called "male flower" and "female flower", respectively. After fertilization, the female cone is termed "fruit", which undergoes "ripening" (maturation).
It was found recently that the pollen of conifers transfers the mitochondrial organelles to the embryo, a sort