Pinophyta

 

(pī'nŏf'ətə) , division of the plant kingdom consisting of those organisms commonly called gymnosperms. The gymnosperms, a group that includes the pine, have stems, roots and leaves, and vascular, or conducting, tissue (xylem and phloem). In these plants the ovules, or young seeds, are exposed to the air at the time of pollination, hence the term gymnosperm, meaning naked seed. Pollination is always by wind. Because the seed-bearing structures of many gymnosperms are organized into a cone, or strobilus, these plants have been called conifers; because the leaves of many species are perennial, they have also been called evergreens.

Class Cycadopsida

The class Cycadopsida, or cycads, are only the small evolutionary vestige of a large and varied group of plants that flourished in late Paleozoic and Mesozoic time. The only order of living cycads, the Cycadales, is dioecious, i.e., male and female cones are borne on separate plants. Although cycads resemble the palms in form and usually have erect stems that reach 50 ft (15 m) in height, they have very little wood; rather, they are supported largely by a hard outer layer of the stem. They have large, fernlike leaves and produce seeds in terminal cones. In their reproduction, pollen grains, or microspores, are transported by wind to the female spore case, or megasporangium. Within the microspore wall, motile flagellated sperms are produced, unlike the nonmotile sperms of the higher gymnosperms.

Class Pinopsida

The class Pinopsida is characterized by generally small, always simple leaves and by the active secondary growth of stem and root. Many members of this group flourished from Lower Carboniferous times to the Permian age. Plants of the order Pinales (conifers) occur in the Northern Hemisphere; a few species occur within the tropics at sea level. Conifers are the most numerous of living gymnosperms and form large and relatively pure forests. Common examples of conifers are the pines, firs, spruces, redwoods, cedars, junipers, hemlocks, and larches. The wood of conifers is used extensively for construction of all kinds. It has no vessels and thus differs from the wood of angiosperm trees. Although conifers are called softwoods and angiosperm trees hardwoods, the wood of some pines is much harder than that of some angiosperms. Most conifers are monoecious, i.e., the male and female cones occur on the same tree. The microspores, or pollen grains, are produced in such vast abundance that clouds of pollen, carried on the wind, have settled on ships far at sea. In plants of the order Taxales (yews) the seeds, produced individually on short shoots, are surrounded by a conspicuous, fleshy covering.

Other Classes

The class Ginkgoopsida contains the contains the ginkgo, Ginkgo biloba, the last surviving species of a once large and flourishing group of gymnosperms. The class Gnetopsida contains three genera in separate orders, all of great botanical and evolutionary interest. Gnetum is a tropical tree or shrub with broad leaves much like those of an angiosperm. Ephedra is a low shrub with scalelike leaves that grows in arid regions of western North America and in China; from it is produced the traditional Chinese herbal medicine ma huang and the drug ephedrine. Welwitschia, a desert plant of SW Africa, typically has only two large, leathery leaves that persist for the life of the plant, which can be as long as 1,500 years.

Pinophyta Fossil range: Late Carboniferous - Recent
Trees of a Pinophyta species: Araucaria heterophylla (Araucariaceae)
Scientific classification
Kingdom: Plantae Division: Pinophyta Class: Pinopsida
Orders & Families
Cordaitales † Pinales   Pinaceae - Pine family   Araucariaceae - Araucaria family   Podocarpaceae - Yellow-wood family   Sciadopityaceae - Umbrella-pine family   Cupressaceae - Cypress family   Cephalotaxaceae - Plum-yew family   Taxaceae - Yew family Vojnovskyales † Voltziales †

The conifers, division Pinophyta, also known as division Coniferae, are one of 13 or 14 division level taxa within the Kingdom Plantae. They are cone-bearing seed plants with vascular tissue; all extant conifers are woody plants, the great majority being trees with just a few being shrubs. Typical examples of conifers include cedars, cypresses, douglas-firs, firs, junipers, kauris, larches, pines, redwoods, spruces, and yews. Species of conifers can be found growing naturally in almost all parts of the world, and are frequently dominant plants in their habitats, as in the taiga, for example. Conifers are of immense economic value, primarily for timber and paper production; the wood of conifers is known as softwood. The division contains approximately 700 living species.

Evolution

The earliest Conifers date back to the late Carboniferous (Pennsylvanian) period.

Taxonomy and naming

The division name Pinophyta conforms with the rules of the ICBN, which state (Art 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, in this case Pinaceae (the pine family), or are descriptive. In the latter case the name for the conifers (at whatever rank is chosen) is Coniferae (Art 16 Ex 2), which is also in widespread use. Older scientific names (no longer allowed) are Coniferophyta and Coniferales.

According to Article 16 of the ICBN such "descriptive botanical names" may be used at any rank above family. Alternatively, it is also possible to use a name formed by replacing the termination -aceae in the name of an included family, preferably Pinaceae, by the appropriate termination. Both are allowed.

This means that if the conifers are regarded to be an order they may be called Coniferae or Pinales (but see also Coniferales); if regarded as a class they may be called Coniferae or Pinopsida; if regarded as a division they may be called Coniferae or Pinophyta.

Commonly the conifers are considered equivalent to the Gymnosperms, particularly in areas with a temperate climate where they may be the only commonly occurring gymnosperms. However, these are two different groupings; conifers are the largest and economically most important component group of the 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 regarded as 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.

The conifers are now accepted as comprising six to eight families, with a total of 65-70 genera and 600-650 species. 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 recognise Phyllocladaceae as distinct from Podocarpaceae (in which it is included here). The family Taxodiaceae is here included in family Cupressaceae, but was widely recognised in the past and can still be found in many field guides.

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 recognisable 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).

Morphology

All living conifers are woody plants, and most are trees, the majority having monopodial growth form (a single, straight trunk with side branches) with strong apical dominance. The size of mature conifers varies from less than one meter, to over 100 metres. The world's tallest, largest, thickest and oldest living things are all conifers. The tallest is a Coast Redwood (Sequoia sempervirens), with a height of 115.2 metres. The largest is a Giant Sequoia (Sequoiadendron giganteum), with a volume 1486.9 cubic metres. The thickest, or tree with the greatest trunk diameter, is a Montezuma Cypress (Taxodium mucronatum), 11.42 metres in diameter. The oldest is a Great Basin Bristlecone Pine (Pinus longaeva), 4,700 years old.

Foliage

The leaves of many conifers are long, thin and needle-like, 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. In the majority of conifers, the leaves are arranged spirally, 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, the leaf bases are twisted to present the leaves in a flat plane for maximum light capture (see e.g. photo of Grand Fir Abies grandis). 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) have a very strong glaucous wax bloom 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 the leaves in autumn and leafless through the winter. 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.

Reproduction

See conifer cones for a more detailed discussion.

Most conifers are monoecious, but some are subdioecious or dioecious; all are wind-pollinated. Conifer seeds develop inside a protective cone called a strobilus (or, very loosely, "pine cones", which technically occur only on pines, not other conifers!). The cones take from four months to three years to reach maturity, and vary in size from 2 mm to 600 mm 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 coloured, 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 which produce yellowish pollen. 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. When a pollen grain lands near a female gametophyte, it undergoes meiosis and fertilizes the female gametophyte. The resulting zygote develops into an embryo, which along with its surrounding integument, becomes a seed. Eventually the seed may fall to the ground and, if conditions permit, grows 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).

Life cycle

1. To fertilize the ovum, the male cone releases pollen that is carried on the wind to the female cone.
2. A fertilized female gamete (called a zygote) develops into an embryo.
3. Along with integument cells surrounding the embryo, a seed develops containing the embryo.
4. Mature seed drops out of cone onto the ground.
5. Seed germinates and seedling grows into a mature plant.
6. When mature, the adult plant produces cones.

Other facts

Although the total number of species is relatively small, conifers are of immense ecological importance. They are the dominant plants over huge areas of land, most notably the boreal forests of the northern hemisphere, but also in similar cool climates in mountains further south.

Many conifers have distinctly scented resin, secreted to protect the tree against insect infestation and fungal infection of wounds. Fossilised resin hardens into amber.

External links

• ToLweb: Conifers
• 300 million-year-old conifer in Illinois - 4/2007

Pinophyta families
Pinaceae · Araucariaceae · Podocarpaceae · Sciadopityaceae · Cupressaceae · Cephalotaxaceae · Taxaceae

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