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  The Conifers, Biology tutorial

The Conifers (Spermatophyta: the Seed-Bearing Plants)

Introduction:

Spermatophyta is a great set or phylum, the highest in vegetable kingdom, taking up seed-bearing plants. It is same as Phanerogamia of older botanists, all correctly flowering plants as well bearing seeds, and as this last quality is now regarded as more primary one, name Phanerogamia is being usually abandoned. It is also equal to Embryophyta Siphonogama of Engler. At times reduced to Spermaphyta. Form Spermophyta is no longer utilized.

Seed bearing plants are the most successful of all plants. They are very plentiful and varied. They are everywhere - in the gardens and in nature. They are very significant to man. Seed bearing plants encompass the leading sporophyte generation. Gametophyte generation is decreased. So thought of alternation of generation is not apparent in bryophytes and pteridophytes. Sporophyte makes two kinds of spores microspore (pollen grain) and megaspore (embryo sac). Embryo sac is entirely surrounded in ovule. When ovule is fertilized, it grows into the seed. They contain complicated vascular tissues in leaves, stems and roots. There are 2 phyla in this kingdom, conifer phyta and angiospermophyta.

Seeds have been known as "time capsules of life". They have skill to delay development of plant embrio until surroundings are correct for germination. This has verified to be the immense evolutionary benefit. Now 96% of the entire plants are seed plants.

Seed-bearing plants were usually divided in angiosperms, or flowering plants, and gymnosperms that include ginkgo, conifers, cycads, and gnetae. Angiosperms are now considered to have evolved from the gymnosperm ancestor that would create gymnosperms a paraphyletic group if it comprises extinct taxa. Though not the monophyletic taxonomic unit, gymnosperm is still extensively utilized to differentiate 4 living taxa of non-flowering, seed-bearing plants from angiosperms. Molecular phylogenies contain conflicted with morphologically-based facts as to whether existing gymnosperms include the monophyletic group. Few morphological data recommends that Gnetophytes are sister-group to angiosperms, but molecular phylogenies have usually illustrated the gymnosperm clade which comprises Gnetophytes as sister-group to conifers. Cantino et al. suggested name Acrogymnospermae for clade of modern gymnosperms if identified to differentiate it from traditional usage of name Gymnospermae.

Common Appearance and Habitat:

Main diagnostic characteristic of conifer is production of cones. Others are fact that seeds lie on surface of leaves and are not surrounded. The diagnostic characteristic is the trait by which one can recognize the object. As they are seed producing, they are among the most successful group of plants. They are extensively dispersed and form the third of the forest in world. Generally they are evergreen trees or shrubs. They inhabit places which are fairly high above sea level. They are discovered more in northern hemisphere. They are of commercial significance. Conifers are utilized as resins, timber, turpentine and wood pulp. They comprise larches, pines, firs, cedars and spruce. The characteristic conifer is Pinus Sylvestris (Scots Pine). Pinus Sylvestris is discovered all over central and northern Europe, Russia and North America. It is inhabitant to Scotland but has been set up to other parts. It is cultivated for its timber and as the ornament it is stately, tall (up to 36m), with the trait pink or orange-brown bark. As it produces on sandy soils on mountains, roots are shallow and spreading.

Their major branches and trunk produce yearly from apical bum. This development has been explained as unlimited. Younger branches on top are shorter whereas older ones further below on stem are longer. Branches are arranged in the whorl of spiral around stem. As older ones drop, they leave stem bare, from base upwards. Branches and trunk have spirally set scale buds that develop into extremely short branches known as dwarf shoots (with restricted growth) with 2 leaves at their tips. These drop after 2 or 3 years leaving scars.

External Morphology and Adaptations:

Once shoot has developed, scale leaves drop off to leave scars. Conifers contain needle-like leaves that decrease surface are for water loss. Leaves are covered with thick waxy cuticle. Stomata of leaves are sunken to further conserve water. Such characteristics are known as xenomorphic features. They make sure conservation of water in plant in difficulty for instance when there is snow or drought. In warm weather, tree that is sporophyte generation makes male and female cones on same tree. Male cones cultivate behind apical bud at bases of new shoots in clusters. They are curved and about 0.5cm in diameter. They develop at bases of scale leaves in place in dwarf leaves. Female cones grow at axils of scale leaves at tips of new strong shoots. They are few distance away from male cones. They differ in size by their age range from 0.5-6.0cm in tree as it takes them up to 3 years to mature. Younger ones are green while older ones are reddish brown. Both male and female cones contain spirally arranged; strongly packed sporophylls around the central axis. Sporophylls of male cone contain sporangia (pollen sacs) within which meiosis takes place to create haploid pollen grains (microspores). Haploid - explains product of nuclear division in gamete formation.

Pollen grains have male gametes. The pollen grain contains 2 large air sacs that help in wind dispersal. In May, cones turn out to be yellow due to cloud of pollen grains that they free. They drop off as then whither at end of summer. Female sporophyll is composed of the lower bract scale and larger upper ovuliferous scale. On upper surface of ovuliferous scale are 2 ovules set are formed inside the ovules. Ovules are polluted in first year but fertilized in given spring when pollen tube grows. Fertilized ovules turn out to be winged seeds that continue to mature throughout second year until they are isolated in third year. By this time cones turned out to be woody with scales bending outwards to expose seeds for dispersal by wind.

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