Gymnosperm Grade 11 Botany and It's importance (Roxburghi and Wallichiana)

GYMNOSPERMS

General Characters:

Ø They are naked seeded plants i.e. they do not have fruits and seeds are open.

Ø They are found in tropical to temperate regions.

Ø The plant body is sporophyte.

Ø The sporophyte plant body is differentiated into roots, stems, and leaves.

Ø Roots are taproots.

Ø The stem is branched and usually, two types of branches are present i.e. branch of unlimited growth called long

shoots and a branch of limited growth called the dwarf shoot.

Ø Leaves may be of one kind i.e. monomorphic or two kinds i.e. dimorphic (one is green leaf and another is

minute scale leaf).

Ø Plants have vascular tissues i.e. xylem and phloem

Ø Xylem lacks vessels and phloem lacks companion cells (except Ephedra and Gnetum).

Ø Pollination is anemophily and direct.

Ø The endosperm is haploid.

Ø Double fertilization and triple fusion are absent.

Ø Polyembryony is common.

Habit: Pinus is a xerophytic monoecious plant.

Morphology: The plant body is a sporophyte differentiated into roots, stem, and leaves.

Root: Root is a taproot. Root has a symbiotic relationship with fungi called mycorrhiza.

Stem: Stem is erect, branched, and woody. The stem bears two types of branches i.e. long shoot (which arises from the main

stem and grows indefinitely) and dwarf shoot (which arises from the long shoot and grows for a short time). Long shoot

bears only scale leaves while dwarf shoot bears scale as well as foliage leaf.

Leaves: Pinus is dimorphic i.e. possess two types of leaves: scale leaves and foliage leaves. Scale leaves are thin,

brown, flattened, and minute structures which fall off with the maturity of branches while foliage leaves are long, needle-like, and green. The dwarf shoot-bearing foliage leaves are called spurs.

Reproduction: Pinus is monoecious and bears male and female cones on different branches of the same plant. Male cone develops in

the cluster (15-140) on the base of a long shot. They arise from axils of scale leaf and develop later than a male cone. Female

cones grow very slowly thus female cones of different ages may be seen in acropetal succession in the long shoot.

Male cone:

Each male cone is small and oval-shaped. It arises in clusters from the axils of scale leaves on the dwarf shoot. The male

cone has a central axis on which 60-150 microsporophyll’s are spirally arranged around the axis. A single

microsporophyll is a membranous stalked structure with a distal expanded roughly triangular sterile part called

the apophysis. Each microsporophyll bears two sac-like microsporangia on the abaxial surface. A mature

microsporangium consists of a multilayered wall, tapetum, and microspore mother cells. Each microspore mother cell

— by meiotic division — produces four microspores or pollen grains.

The pollen grains are boat-shaped with monosulcate apertures and are bounded by two concentric wall layers: the

outer thick exine and the inner thin intine. The exine on the lateral sides of the pollen is expanded to form two wings

(sauce). Pinus is wind-pollinated (anemophilous).

Female cones

They are produced in pairs or in clusters in the axil of the scale leaves. The female cones mature very slowly.

The fully matured third-year cone is much larger (15-60 cm in length), woody, loose, and brown in color. Here

megasporophylls are separated from each other due to the elongation of the cone axis. The female cone is

composed of a central axis on which 80-90 megasporophylls, axillary to bract scale/scale leaves, are arranged spirally

The bract scale and ovuliferous scale thus form a seed-scale complex. A single megasporophyll consists of two types

of scales:

(a) a large woody ovuliferous scale or seminiferous scale bearing two ovules on the adaxial surface, and

(b) a bract scale or cone scale on the abaxial surface.

Initially, the ovuliferous scale is much smaller than that of the bract scale, but after pollination, it becomes larger than

the bract scale. The ovuliferous scale is a thick, large, woody, roughly triangular, and brownish structure. Its upper

thick exposed part is known as apophysis.

The ovules of Pinus are anatropous, unitegmic and crassinucellate. The single integument is free from the nucellus

except at the chalazal end. There is a fairly broad micropylar tube that becomes inwardly curved during prepollination

stages and becomes outwardly curved at the time of pollination fertilization

The fertilization takes place after one year of pollination. One of the male nuclei fuses with the egg cell and thus a

zygote is formed.

Pinus found in Nepal: Two species of Pinus i.e. Pinus Roxburgh and Pinus wallichiana are found in Nepal. Pinus Roxburgh is found between an altitudinal range of 1200-2100m while Pinus wallichiana is found between 1800-3300m.

P. roxburghi can be distinguished from P. wallichiana by number of needles in a spur (roxburghi-3 needles;

wallichiana-5 needles) and shape of female cone (Roxburgh-oval cone; wallichiana- cylindrical).

ECONOMIC IMPORTANCE OF GYMNOSPERMS

Gymnosperms are the small group of plants, which constitute a sub-division of Spermatophyta or

phanerogams. There are about 73 genera and 7000 species in subdivision Gymnospermae.

1. As food

· Seeds of some species are edible: Cycas, Ginko, Pinus, Gnetum

· The stem of Cycas revoluta is a good source of Sago starch

· Zamia is a rich source of starch.

· Seeds and stem of Cycas revoluta used for making wine.

2. As medicine

Leaves of Cycas circinalis, Taxus are used as medicines.

· Pollen grains of some Cycas have a narcotic effect

· Oil of Juniperus is important.

· Ephedrine is derived from Ephedra used in the treatment of cold, cough.

· An anti-cancerous drug called taxol is obtained from the bark of Taxus

3. As ornaments

· Species of Cycas are used for decoration purposes

· Ginkgo Biloba, possess beautiful ornamental leaves

· Thuja, Pinus, Taxus, etc are grown in parks.

4. In an industry

· Spruce or Picea is an important source of pulpwood.

· The wood of Juniperus is used in making pencils, scales, and holders

· The bark of Larix yields a tannin

· Turpentine is obtained from Abies balsamic.

· The wood of red spruce is especially important for the music industry.