Book : Botany
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Posted by: CHELSEA
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Writer CHELSEA

Pinus Lifecycle

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Introduction

The Pinus tree represents the sporophytic generation. The stem displays the excurrent habitat. The main stem is branched. It is composed of small pith, a thick vascular cyclinder made up of a ring of separate collateral and open vascular bundles. Well developed resin canals are present in the stem. Primary xylem contains neither true vessels (tracheae) nor wood fibres which characteristic of angiosperms. Mesophyll cells not differentiated into palisade and spongy tissue in the leaves. Resin canals and transfusion tissue are present in the leaves. Anatomy of Root: Resembles dicots. There are 2, 3 or 4 (diarch, triarch or tetrach) exarch bundles (more or less Y shaped) with alternating xylem and phloem (radical). Pith is generally absent. In the apical root meristem there is no dermatogen.

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.


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