Gymnosperm Botany Grade 11 ( Pinus )

 GYMNOSPERMS

General Characters:

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

 

Ø They are found from tropical to temperate region.

 

Ø Plant body is sporophyte.

 

Ø The sporophyte plant body is differentiated into root, stem and leaves.

 

Ø Roots are tap roots.

 

Ø Stem is branched and usually two types of branches are present i.e. branch of unlimited growth called long shoots and branch of limited growth called 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 has 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 Endosperm is haploid.

 

Ø Double fertilization and triple fusion is absent.

 

Ø Polyembryony is common.

 

PINUS

   

 

Habit: Pinus is xerophytic monoecious plant.

 

Morphology: Plant body is sporophyte differentiated into root, stem and leaves.

 

Root: Root is tap root. Root has symbiotic relationship with fungi called mycorrhiza.

 

Stem: Stem is erect, branched and woody. Stem bears two types of branches i.e. long shoot (which arise from main stem and grows indefinitely) and dwarf shoot (which arise from long shoot and grows for a short time). Long shoot bears only scale leafs 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 maturity of branches while foliage leaves are long, needle like and green. The dwarf shoot bearing foliage leaves is called spur.

 

Reproduction:

Pinus is monoecious and bears male and female cones on different branches of same plant. Male cone develops in cluster (15-140) on base of long shoot. They arise from axils of scale leaf and develops later than 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 axis of scale leaves on dwarf shoot. The male cone has a central axis on which 60-150 microsporophylls are spirally arranged around the axis. A single microsporophyll is a membranous stalked structure with a distal expanded roughly triangular sterile part called 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 (sacci). 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 colour. 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 arrange spirally.

 

The bract scale and ovuliferous scale thus form a seed-scale complex. A single megasporaphyll 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 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 which 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.