B Sc II Pinus Morphology

Pinus

General Morphology:

The Pinus tree represents the sporophytic generation. The stem displays the excurrent habitat. The main stem is branched. Branches are of two types:

(a) Long branches:

Branches with unlimited growth (grow by means of apical bud)

(b) Dwarf branches:

Branches with limited growth, arising directly from the trunk.

Leaves are also of two kinds:

(a) Foliage leaves:

These are unusual type being long, narrow, tough, green and are frequently known as Pinus-spur or Pine-needles. They are borne only on the dwarf shoots in clusters of two (P. merkusii), three (P. roxburghii) or five (P. wallichana).

Branches with foliage leaves are called spurs. Spurs could be monofoliar, bifoliar, trifoliar, pentafoliar having 1, 2, 3, 5 foliar leaves.

(b) Scale leaves:

These are brown, membranous and are protective in function. These are borne on both type of branches, but they fall off as the dwarf shoots mature. Scale leaves of dwarf shoots are called cataphylls.

Primary root persists and forms a typical elongated straight tap root. Mycorrhizal roots develop like that of cycas.

Sexual Reproduction:

Pinus is monoecious, it bears male and female reproductive cones on the same tree but on separate branches.

Male cone:

It is shortly stalked and consists of an elongated central axis, bearing a number of small spirally arranged and closely fitted scale-like microsporophylls. Numerous winged microspores are produced from microspore mother cell in the microsporangium.

Male gametophyte:

The microspore nucleus divides into a small protallus cell and a large central cell. The nucleus of large central canal cell called the antheridial cell divides into a generative cell and a tube cell.

Tube cell:

The tube cell grows out to form a delicate pollen tube which grows into the nucellar tissue upon which it now depends for its nourishment and protection. The pollen tube rests for about a year in this condition because the ovule is not yet ready for fertilization. Hence further growth of microgametophyte is arrested. It rests throughout the late summer and following winter resuming activity in the following April (second year).

The tube becomes active again and it penetrates the nucellar tissue. The generative cell divides to give rise to a barren stalk cell (sterile cell) and a fertile body cell (Spermatogenous cell). The body cell along with protoplasmic contents of the tube and stalk cell pass down the pollen tube. The body cell divides into two unequal cells, which are the male gametes. The gametes are formed only a week before the fertilization.

Female cone:

Arise singly or in a small cluster of two to four, each as a bud in the axial of the scale leaf towards the end of the new shoots of unlimited growth which do not bear the male cones. The female cones are very slow in growth. They take almost a year to be mature enough to receive the pollens. The central axis bears paired scales in a close spiral.

Bract scales or Carpellary scales (Each corresponding to a carpel or megasporophyll), lower scale, small, leathery, brownish scales.

Ovuliferous scales:

It bears two sessile ovules on its upper surface at the base. Each ovule is orthotropous, and consists of a central mass of tissue the nucellus, surrounded by single integuments made of three layers.

Pollination:

Takes place in March/April in Eastern Himalayas. The amount of pollens liberated by the pine forests at this time is prolific so that the air gets saturated with them and there is a yellow deposit of pollens on the forest floor. This phenomenon is known as 'sulphur-shower'.

Pollination drop:

As the ovule matures for the pollination, the nucellar cells dissolve just below the micropyle. The dissolved tissue becomes mucilagenous and project out through the micropyle in the form of a droplet and is called pollination drop.

Fertilization:

The pollen tube on reaching the archegonial neck (which takes place a year after pollination i.e. two years after the female cone first emerged) the pollen tube destroys the neck cell. Just before fertilization, the body cell divides into two nake male cells or gametes.

Simple polyembryony:

(A number of embroys from a number of fertilized eggs) is common is some genera of Pinaceae viz. Larix & Picea but is Pinus (also Cedrus) the proembryo tires split from one another-into 4-along the 4-cells of each tire giving rise to 4-separate embryos. This is known as cleavage polymebryony. Usually one of these embryos survive in the seed.

Source: http://www.preservearticles.com/2012032128292/short-notes-on-the-general-structure-and-reproduction-of-cycas-and-pinus.html