Ecology and human uses
The conifers comprise about 560 species of woody plants, mainly occurring in temperate and boreal regions. They form extensive forests and include the tallest, oldest and most massive of trees. They are important as sources of timber and wood pulp. Leaves decay slowly and accumulate on the ground forming an acid litter; periodic fires may be characteristic of conifer forests. The fossil record extends to the Carboniferous period.
Stems and roots
The stems resemble those of angiosperms except that the xylem has only tracheids and the phloem only one specialized type of cell, the sieve element. The tracheids have bordered pits in lines. The roots of most have ectomycorrhiza associated with them.
The leaves are mostly needle-like or scale-like with a single central vein resembling microphylls but have a vascular trace in the stem. A few are broader and fossil leaves are forked. Resin canals are often present.
Male reproductive structures
Anthers are borne in pairs on the underside of scales forming male cones. The pollen grains usually have air bladders mainly involved with orientation at the micropyle for fertilization. All are wind-pollinated and pollen is produced in enormous quantities. In most conifers, the male gametophyte consists of four cells contained within the pollen grain.
Female reproductive structures
Ovules (megasporangia) are borne on ovule scales in female cones, except for yews that bear seeds individually. The female gametophyte initially has a free nuclear stage before forming cells and archegonia. A drop of fluid is secreted by the micropyle to trap pollen when it is mature.
Fertilization and the seed
Pollen tubes penetrate the archegonium and the largest gamete fertilizes the egg. Several embryos may be formed initially but one occupies the mature seed. It has several cotyledons. Seeds are released by the cone scales separating, and some are an important food source for animals.
Conifers, the Pinopsida, are the largest and most important seed plant class except for the flowering plants. Most conifers are trees, a few are shrubs; all are woody. They are found throughout the world but achieve their greatest abundance as the dominant growth form of the boreal forests of the northern hemisphere with high diversity mainly in western North Americaand east Asia. They also cover extensive areas in temperate parts of the southern hemisphere. They are much less common in the tropics with only a few tropical genera, mainly in the mountains. In total there are seven living families and about 560 species. They are the tallest and most massive of trees, at least three species reaching to well over 100 m, all taller than any angiosperm. They can live to a great age and are among the oldest known organisms, with the bristlecone pines of California over 4000 years old and the huon pine of Tasmania possibly reaching ≥10 000 years of age. Most have specialist reproductive structures, the familiar cones that give the class its name. A few, such as the yew, bear seeds individually. They are an ancient group with fossil conifers first appearing in Carboniferous rocks .
Conifers are enormously important economically since their wood is widely used for furniture and other constructions and they are one of the main sources of pulp for paper. They have been planted extensively throughout the temperate world, particularly on poor soils, and huge part-managed coniferous forests occur in Eurasia and north America. They are extensively used for ornaments and many are resinous, with the resins widely used in gums and varnishes. Turpentine is derived from pine resin.
The decay of many conifer leaves is slower than their production by the tree, leading to an accumulation of fallen leaves in a conifer forest that can acidify the ground and provide a fire hazard. Periodic fires are characteristic of many conifer woodlands and one of the main ways in which the accumulation of fallen leaves can be removed.
The stems of conifers are all woody and have xylem made of tracheids with no vessels (Topics C1 and C4). There is extensive secondary thickening and the secondary tracheids form regular ranks. In temperate species, tracheids that are formed in the spring are wider than later ones resulting in clear annual rings. The tracheids have lines of large circular pits with conspicuous borders in their cell walls. The xylem is interlaced with rays containing living parenchyma cells and sometimes resin cells . There are differences in xylem structure between different conifers that are well preserved in fossils, e.g. wood similar to that of the living Araucaria (monkey-puzzle and its relatives) first appeared in the Carboniferous; pine-like wood did not appear until the Cretaceous period.
The phloem has a simpler structure than that of flowering plants with only one specialized type of cell, an elongated sieve element , with parenchyma cells rich in starch and mitochondria next to them. A pith is present
in the young stages of stem growth.
Many conifers have a single straight stem with much smaller side branches, giving rise to the characteristic narrow pyramidal shape. The straightness of the stem has greatly helped foresters use the timber. Others, such as most pines, have a much more rounded crown with more even branching, and a few, such as the junipers, are shrubs with gnarled stems or several stems arising from the base.
The roots of conifers resemble those of angiosperms (except that the xylem has only tracheids). Ectomycorrhizae are associated with all except the southern hemisphere families Podocarpaceae and Araucariaceae, and the fungi aid in the decay of the leaf litter. Araucariaceae have endomycorrhizae with a similar function and Podocarpaceae have nodules in their roots like those of legumes .
The leaves of living conifers are all simple and most are shaped like needles, sometimes long, or scales, semicircular or flattened in cross-section. A few southern hemisphere conifers in the Araucariaceae and Podocarpaceae families have larger, broader leaves, up to 5 cm wide; a few fossils show forked leaves.
The great majority of conifers retain their leaves throughout the year, each leaf living for 2 years or more, and up to 15 years in Araucaria. A few have deciduous leaves, such as the larches (Larix) and swamp cypress (Taxodium).
Leaves are borne either directly on branches or on small pegs or scales and sometimes in bundles or whorls. Some species have two different forms of leaf growing together, needle-like and scale-like (e.g. pines), and some cypresses have needle-like leaves in juvenile stages, with the scale leaves appearing later. In the pines, the coast redwood (Sequoia) and some others, the small leaf-bearing lateral branches are shed with the leaves.
The leaf structure resembles that of flowering plants with a thick cuticle, palisade and spongy mesophyll, but all conifers have simple leaf venation, either with one central vein or a few parallel veins. In this they resemble microphylls (Topics Q1 and R1), although the traces in the stem vascular tissue are not similar to those of clubmosses and other microphyll-bearing plants, and their origin remains anomalous. Most conifers have resin canals throughout the mesophyll, this resin consisting of acidic phenols, terpenes and other complex molecules. It can be present in large quantities and may protect the leaves from insect attack and make the leaves resistant to decay. A few conifers have other aromatic oils in their leaves, e.g. Thuja, giving characteristic scents, often species-specific, when the leaves are crushed.
The microsporangia, or anthers, are borne on the underside of specialized fertile leaves in short strobili (cones) (Fig. 1). These leaves often have expanded tips to which the anthers are attached. There are two anthers per fertile leaf in many conifers but in some there are more. The cones themselves are produced in the axils of scale-leaves in pines, or at the tips of lateral shoots in other families.
The anthers have a thin wall, of one or a few cell layers. The outer cells have uneven thickening, in rings or a reticulate pattern, which are involved in dehiscence. They take about a year to develop to maturity and to release the pollen. The pollen grains resemble those of flowering plants except that pines and some other conifers have characteristic air bladders formed from an extension of the outer pollen wall giving a most distinctive appearance (Fig. 2). These bladders may aid with wind dispersal but are mainly involved with the orientation of the pollen as it fertilizes the ovules. All conifers are wind-pollinated and pollen can be produced in enormous quantities, frequently coming off in visible clouds when the cones are mature. By lakes they can form yellowish lines in the water or ‘tide-lines’ as the water recedes.
The male gametophyte is much reduced and formed within the outer wall of the pollen grain. The largest male gametophytes are found in Agathis, the southern hemisphere white pines of the Araucariaceae family that have a male prothallus with up to 40 cells. In pines there are four cells by the time the pollen is shed, two vegetative prothallus cells, the pollen tube nucleus and the generative cell. The generative cell gives rise to a sterile cell and two unequal sized sperm cells (after another division). In the cypresses there are no vegetative cells at all, only the generative cell and tube nucleus. In all conifers the sperms have no flagellum and are not motile.
The female reproductive branch is the familiar pine or fir cone (Fig. 3). It has two ovules attached to each fertile scale leaf, but the cone differs markedly from the male in that each fertile scale has a bract underneath it, in some partially fused with the ovule scale. This, and evidence from fossil conifers, suggests that the cone is a compound structure with each fertile scale derived from a whole shoot. The cone can take 2 years to mature. A few conifers, notably the yew and its relatives, Taxaceae, do not have cones and the ovule is solitary, borne at the tip of a minute shoot in the leaf axils.
The megasporangium has a single integument, and four megaspores are produced, though only one is functional. The megaspore starts to divide to produce the megagametophyte at an early stage, and in northern species there is usually a dormant period in the first winter. Eventually many free nuclei are produced and cell walls form once there are around 2000 nuclei. Between one and six archegonia are produced next to the micropyle and, in these, the egg is surrounded by neck cells and a canal cell as in other vascular plants and bryophytes .
When the ovule is receptive, the cone scales open apart slightly and a drop of sticky liquid is exuded from the micropyle. Pollen grains are caught in this drop which is then reabsorbed, and after pollination the cone scales may close up again. The air bladders on the pollen grains serve to orient the pollen grain as it approaches the micropyle.
Once a pollen grain has reached the micropyle, the two male gametes are formed and the pollen tube grows actively through the nucellus to reach the archegonia. The male gametes are discharged into the egg along with the tube nucleus and the sterile nucleus. Fertilization is achieved by the fusion of the larger of the male gametes with the egg nucleus, the other three male nuclei degenerating.
If more than one archegonium has been fertilized several embryos may be formed, and each fertilized egg may divide to form several embryos, so competition can occur. Eventually, one embryo outcompetes the others and absorbs the nutrients from the female gametophyte, filling the seed when it is mature.The embryo has several cotyledons (the food stores and first leaves;). In most conifers the seeds are released when the cone scales separate at maturity. The seeds may be nutritious and can be a major food for some birds, such as crossbills, and small mammals. Some species have a resistant integument that allows the seed to lie dormant, and a few only germinate after a fire has passed over them. The yew has a fleshy outgrowth from the integument which is attractive to birds.
In comparison with most flowering plants the stages of reproduction are slow, taking about a year from female cones being receptive to the mature seed, but in pines fertilization does not occur until a year after the female cones become receptive and the cycle takes 2 years.
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