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Gymnosperms is a vascular plant that reproduced by means of an exposed seeds or ovule. The gymnosperms are the most ancient seed plants that originated during the late Paleozoic era. Out of 1000 living species of gymnosperms, about 20 species of gymnosperms have been known in Nepal.
All gymnosperms are found in four major divisions of plants. The divisions are Ginkgophyta, Cycadophyta, Gnetophyta, and Coniferophyta. The Coniferales form the most conspicuous order of the living gymnosperms.
Systematic Position ( Five Kingdom Classification )
Sub - division: Spermatophyta
Occurrence: It is worldwide in distribution with nearly 20 species in tropical and subtropical regions of Asia and Australia. It is dioecious, primitive gymnosperms plant and shows xerophytic characters.
Morphology: It has a graceful palm-like evergreen shrubby habit. It lives for several hundred years with very slow growth. It has crown of leaves present at the top of the pillar-like the unbranched stem. It is sporophyte (2n ) and is differentiated into root, stem, and leaf.
Root: Taproot with lateral branches growing from a lower surface of the stem, positively geotropic and its function is fixation and absorption. Some secondary roots are also developed which is negatively geotropic and grows upward, stumpy, dichotomously branched and closed massed forming coralloid masses which are known as coralloid roots. It takes part in aeration and nitrogen fixation as it consists of symbiotic, endophytic blue-green algae like Anabene and Nostoc.
Stem: Tuberous in young but becomes columnar, unbranched, woody, stout, erect, columnar and arborescent and is covered by armor or persistent leaf bases.
Leaf: Spirally arranged leaves around the stem, dimorphic leaves i.e. foliage leaves and scale leaves foliage leaves are green, thick, leathery, and pinnately compound and consists of rhomboidal shaped leaf base, thick and wood rachis and lanceolate leaflets or pinnae. Persistent leaf bases, some leaf bases modified into spines, lateral veins are absent. Foliage leaves help in photosynthesis.
Scale leaves are small, rough, triangular in shape and brown in color, densely coated with brown colored ramenta. Lies alternate to the whorls of foliage leaves. At maturity, the male plant bears male cone and female bears female strobilus.
Systematic Position (5 Kingdom Classification)
Sub - division: Spermatophyta
Morphology: The pinus trees are 30m tall, graceful, evergreen, represent a sporophytic phase of the life cycle and plant body is differentiated into root, stem, and leaves. Stem covered with rugged scaly bark.
Roots: Primary root persists and forms elongated straight tap root having a massive root system. It does not grow very deep. It possesses protective root cap at young but disappears soon, as the root becomes fully developed they are covered with ecotropic symbiotic fungus. It helps in water absorption. Mycorrhizal roots are short, thick without root hair and root caps but are branched and covered with fungal hyphae.
Stem: Woody, erect, and covered with scaly bark that is of two types.
Long shoots: They are normal branches actively growing by an apical bud are often called branches of unlimited growth.
Dwarf shoots: Arise directly from the trunk, called branches of limited growth that are numerous and borne on the ordinary branches in the axils of scale leaves. Consists of a short axis terminating in a cluster of green needles and is covered with scale leaves or cataphylls.
Leaves: Dimorphic i.e. foliage leaves and scale leaves. Foliage leaves are green, long and narrow needle-like borne only on the dwarf shoot and take part in the photosynthesis.
The number of needles in dwarf shoots is fixed in different species of pinus.
Scale leaves are brown membranous and are protective in function, found on both types of branches and stem. They fall off as the branches mature. They are not involved in photosynthesis.
Distribution of Pinus
Some common species of Pinus are as follows: