- Related Topics:
- Podocarpaceae
- Cupressaceae
- Pinaceae
- Cordaitales
- Cordaitopsida
The sporangia of vascular plants are technically asexual, but in the seed plants, because the gametophytes are wholly dependent upon the sporophyte and the female gametophyte even remains within the megasporangium, sexual terminology continues to be erroneously extended to the sporophyte and sporangium-bearing organs. In all conifers the organs containing microsporangia (“male”) are separate from those bearing megasporangia (“female”), and in Cephalotaxus, some junipers (Juniperus), and the family Taxaceae they are found on different individuals.
The microsporangia of all conifers are attached to the scales of a simple pollen cone, or microstrobilus. The pollen cones usually consist of thin, parchmentlike scales (microsporophylls), each carrying two or more microsporangia on the lower surface. The number of scales and their size is quite variable, so that the overall length of the microstrobilus ranges from about 2 mm (0.08 inch) in some cypress (Cupressus) species to more than 20 cm (8 inches) in some Araucaria species.
Wide variations in the female (megasporangiate) reproductive structures among the conifers are the main basis for their classification. Most living conifers have a seed cone that is interpreted as a compound strobilus; each cone scale, inserted in the axil of a bract, is equivalent to an entire simple pollen cone. Fossil evidence shows how each ovule-bearing dwarf shoot of ancestral conifers was reduced and fused to form a single cone scale. Like the leaves, the bracts and scales are spirally arranged or occur in pairs or trios on the axis, and modern conifers have at least some fusion between each bract and its scale. The bracts and scales, or combined scales, vary in texture from woody to leathery, or even fleshy in bird-dispersed junipers (Juniperus) and the family Podocarpaceae. The number and size of cone scales varies widely among conifers, leading to seed cones that range from 3 mm (0.1 inch) long and less than 1 gram (0.04 ounce) in Microbiota of the Amur region of Russia to more than 40 cm (16 inches) long in the sugar pine (Pinus lambertiana) of California and more than 2.2 kg (4.9 pounds) in some Araucariaceae and the coulter pine (Pinus coulteri) of California.
The megasporangiate strobili of Cephalotaxus and most Podocarpaceae have the same basic structure as other conifer cones, but are so reduced and dominated by their much larger seeds that they do not look like cones. Even greater modification in the family Taxaceae has completely eliminated any trace of strobilar organization, and the solitary seeds sit at the tip of a short branch in a fleshly aril, a cup-shaped outgrowth of the seed stalk.
Classification
Distinguishing taxonomic features
Extant conifers differ from other gymnosperms in combining simple pollen cones with compound seed cones (or solitary terminal seeds in family Taxaceae). Although not possessed by all species, only conifers have needle leaves (of a variety of shapes) and pollen with bladders. Some other features, although not exclusive to conifers, are also more common in them than in other gymnosperms. Those include flattened, winged seeds (also in Welwitschia), scalelike foliage leaves (also in Ephedra), and the growth habit of a normal tree or shrub (also in Ginkgo).
Annotated classification
With 7 extant families, 68 genera, and 545 species, classification of the extant conifers remains controversial. Disagreements exist throughout the classification, so that the numbers of orders, families, genera, and species are all disputed. The classification outlined here reflects current opinion for living conifers but simplifies extinct groups because the number of families to be recognized among the fossils is so uncertain. Extinct groups are indicated by a dagger (†).
- †Class Cordaitopsida
- Paleozoic; strap-shaped leaves, up to 1 metre (3 feet) long, much larger than those of true conifers; both pollen and seed cones were compound and open, each bract with an axillary branch bearing numerous scale leaves surrounding pollen sacs or ovules; generally considered the ancestors of the Coniferales.
- Class Pinopsida
- Order Pinales
- Contains the extant coniferophytes and a number of fossil families; ovules attached to the scales of a condensed compound seed cone; families defined by seed-cone structure.
- †Families Walchiaceae and Voltziaceae
- Paleozoic and Mesozoic; show many stages in the transformation of the seed-bearing dwarf shoots of cordaiteans into the unified, flattened seed scales of modern conifers; foliage resembled that of araucarians; include Walchia, Voltzia, and Voltziopsis.
- Family Pinaceae
- Largest and most widespread and abundant modern conifer family in the Northern Hemisphere; woody, usually thin, cone scales carry 2 winged seeds and are fused to the bracts only at their bases; bracts usually hidden by the scales in mature seed cones but may be prominently exserted in some species; leaves are most often needlelike and spirally arranged, either singly or in clusters; pine (Pinus), spruces (Picea), firs (Abies), and larches (Larix) are all found across the Northern Hemisphere, while Douglas firs (Pseudotsuga) and hemlocks (Tsuga) are restricted to North America and Asia, Cathaya, Keteleeria, Nothotsuga, and Pseudolarix are restricted to China, and the true cedars (Cedrus) occur from Morocco to the Himalayas; 11 extant genera; about 200 species.
- Family Araucariaceae
- From Triassic; massive seed cones with a single large seed on each cone scale; highly reduced scales completely fused to the much larger bracts; species of Araucaria have branches densely clothed with prickly, spirally arranged, clawlike-to-wedge-shaped leaves; dammars (Agathis) have well-separated, oppositely arranged oval or oblong leaves; found in South America, Southeast Asia, and Australasia; 3 extant genera and 33 species.
- Family Sciadopityaceae
- Umbrella pine (Sciadopitys verticillata) usually included in the Cupressaceae, but recent work confirms its isolation from that family; seed cones superficially resemble those of the giant sequoia (Sequoiadendron giganteum), but the equal-sized scales and bracts fused for only about two thirds of their length, each having 5 to 9 seeds; foliage consists of whorls of about 15 to 20 double needles separated by stem segments with spirally arranged nonphotosynthetic scale leaves; endemic to Japan.
- Family Cupressaceae
- Although species of this family are traditionally divided between 2 families, Cupressaceae for the cypresses (Cupressus) and similar genera and Taxodiaceae for the much more-varied genera allied to the bald cypress (Taxodium) and redwood (Sequoia), present evidence shows that all belong to a single family containing 30 genera and 133 species; scales of seed cone intimately fused to the bracts; scale complexes vary in texture, shape, and arrangement on the cone; most have 3 to 5 seeds per scale, but the number ranges from 1 to about 20; leaves vary in shape from scales to clawlike or needlelike and are spirally arranged or in opposite pairs or whorls of 3; several genera, usually referred to as cedars (such as Calocedrus, Chamaecyparis, Libocedrus, and Thuja), have flattened sprays of frondlike branches closely covered with scale leaves; considerable diversity in both the Northern (18 genera) and Southern (11 genera) hemispheres; 50 or more species of junipers (Juniperus) are widespread, exceeding even the pines in their coverage of the Northern Hemisphere; other genera include Athrotaxis, Callitris, Cryptomeria, Cunninghamia, Diselma, Fitzroya, Metasequoia, Microbiota, Neocallitropsis, Sequoiadendron, and Widdringtonia.
- Family Podocarpaceae
- Seed cone is reduced, with 1 to few highly modified, brightly coloured, fleshy scales, each called an epimatium and surrounding a single seed; most with spirally arranged, yewlike needles, but scale leaves and opposite, broad, oblong blades (up to 30 cm [12 inches] long and 5 cm [2 inches] wide) are also found; nearly 200 species in about 18 genera, including Decussocarpus, Lepidothamnus, Microcachrys, Parasitaxus, Phyllocladus, Podocarpus, and Saxegothaea. Phyllocladus was for a time thought to comprise a new family, Phyllocladaceae, but genetic evidence demonstrates the genus’s affinity with Podocarpaceae.
- Family Cephalotaxaceae
- Seed cones highly modified with a few opposite pairs of small bracts, each with a greatly reduced scale remnant strongly dominated by a pair of ovules; only 1 ovule develops into a large seed with a fleshy seed coat; leaves are large yewlike needles carried in opposite pairs; found in East Asia, the plum-yews (Amentotaxus, Cephalotaxus) are the second smallest and most-local extant conifer family; 2 genera and about 11 species. Some botanists include this family in Taxaceae.
- Family Taxaceae
- Solitary ovules borne at the end of a dwarf shoot bearing densely spiraled scale leaves; mature seeds surrounded by a fleshy aril and may also have fleshy seed coats; Taxus seeds are highly toxic; pollen cones and flattened, needlelike leaves are more like those of other conifers than are their seeds; widely distributed in the Northern Hemisphere, but Torreya is found in restricted areas of both North America and East Asia, Pseudotaxus is localized in China, and Austrotaxus is endemic to New Caledonia; 6 genera and about 30 species often segregated into an order separate from Pinales because of absence of seed cone.
