Known as conifers, members of the division Pinophyta are among the most diverse of the gymnosperms, with some 630 living species across six families. Some of the oldest living things on Earth are conifers, including several bristlecone pines (Pinus longaeva) found in the White Mountains of California that approach 5,000 years in age. Additionally, conifers are also the tallest and most-massive living organisms: coast redwoods (Sequoia sempervirens) can exceed 100 metres (328 feet) in height, and the giant sequoia (Sequoiadendron giganteum) can have a girth of 31 metres (101.5 feet) in circumference. Conifers are often featured in formal gardens, and most of the commercial lumber in the Northern Hemisphere is derived from the trunks of conifers such as pine, Douglas fir, spruce, fir, and hemlock.
In most conifers the pollen-bearing and ovule-bearing components (the microsporangia and megasporangia, respectively) are borne on the same plant. A pollen-bearing cone, the microstrobilus, consists of a central axis on which are borne, in a close helical arrangement, reduced fertile leaves (the microsporophylls). On the lower surfaces of the microsporophylls are borne elongated microsporangia; two microsporangia per microsporophyll is common, but some genera have more. The ovulate cone, the megastrobilus, is more complex than the microstrobilus. The megastrobilus bears seeds on flattened dwarf branches, all parts of which are fused (ovuliferous scales). Subtending the ovuliferous scale on the cone axis is a reduced scale leaf, or bract. In some conifers the bract is unrecognizable because it has been fused to the ovuliferous scale.
Conifer leaves are always simple and often small and scalelike (e.g., Thuja) or needlelike (Abies, Picea, Pinus), though some have a broad blade (Araucaria, Agathis). Bald cypress (Taxodium), larch (Larix), and dawn redwood (Metasequoia) are deciduous, but most conifers are evergreen.
The stems of conifers are characteristically woody with a dense mass of secondary xylem. They are usually branched, with basal branches dropping off as the stem elongates, resulting in a main stem that is often tall and straight. The wood is simpler than that of angiosperms; it consists primarily of elongated tracheids (water- and mineral-conducting cells) in the xylem and vascular rays in the phloem that store materials and provide for lateral conduction. The growth tissue of the stem and branches, known as the vascular cambium, contributes more xylem each growing season, forming concentric growth rings in the wood. Tracheids produced by the vascular cambium early in the growing season are larger, and the walls thinner, than those formed later in the growing season. This results in the characteristic light and dark bands of wood. Some conifers have additional cell types, such as fibres and axially elongated xylem parenchyma cells that store food. The wood of gymnosperms is often called softwood to differentiate it from the hardwood angiosperms.
Leave a Reply