Pinophyta

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Pinophyta , 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.

Coniferophyta A phylum of seed-bearing plants comprising the conifers, including the pines, firs, and spruces. Conifers have an extensive fossil record going back to the late Devonian. The gametes are carried in male and female cones, fertilization usually being achieved by wind-borne pollen. The ovules and the seeds into which they develop are borne unprotected (rather than enclosed in a carpel, as are those of the Anthophyta). Internal tissue and cell structure of these species is not as advanced as in the angiosperms. Conifers are typically evergreen trees inhabiting cool temperate regions and have leaves reduced to needles or scales. The wood of conifers, which is called softwood in contrast to the hardwood of angiosperm trees, is widely used for timber and pulp. See also gymnosperm.

Coniferophyta (Pinophyta) The biggest division of gymnosperms, with a long fossil history, comprising trees and shrubs, nearly all of which are evergreen, commonly with monopodial crowns. Most are resinous. The wood lacks vessels (see TRACHEA and VESSEL ELEMENT). Conifers are extremely important for timber and paper production. The leaves are often needle- or scale-like. Fertile parts occur in unisexual cones, variously containing sterile scales. Stamens are borne on commonly peltate scales. The ovule and seed are naked and borne on a scale. They first appear as fossils in Carboniferous rocks. There are about 550 extant species.

Coniferophyta (Pinophyta) The biggest division of gymnosperms, with a long fossil history, comprising trees and shrubs; most are resinous. The leaves are often needle- or scale-like. Fertile parts occur in unisexual cones, variously containing sterile scales. The ovule and seed are naked and borne on a scale. They first appear as fossils in Carboniferous rocks.

Pinophyta See CONIFEROPHYTA.