Adult plant of Pinus represents sporophytic phase of life cycle. Sporophytic plant body of Pinus reproduces sexually by spores and after passing from gametophytic phase again reproduces sporophytic plant. Pinus plant shows alternation of generation.
Sporpophytic phase:
The sporophyte plants of Pinus are mostly mocoecious i.e. male and female cones are found in the same plant. Plant Pinus reproduces by aexual spores which are produced in Sporangia borne on specialized structures, the sporophylls and the sporophylls together into the cones. Male and female cones are separate but produced on the same plant.
Male cone: Male cone occurs in clsters near the ends of long branches like the short branches are produced in the axils of the scale leaves on long branches. Each male cone is simple ovoid structure about one inch long with 60 to 100 spirally arranged microsporophylls or stamens. They arise from the central axis and pass out horizontally.
Each microsporophyll (stamen) has a short stalk and much longer expanded scale like part on the under surface of horizontal part are borne two microsporangia or pollen sacs. Inside the sporangia large number of microspore mother cells is present which undergo reduction division and produce haploid microspores or pollen grains.
Female cone or Ovulate strobiuls:
Female cones are developed laterally in the axxils of scale leaves. They are usually produced in clusters in place of shoots of unlimited growth and on different branches from the male cones. Female cones are much bigger, woody, dry and hard structures. Each female cone has central axis to which are attached megasporophylls.
Each meagsporphyll on its upper surface have two ovules. Each megasporophyll has an outer small bract scale and a large ovulifervous scale. Bract scale arises directly from the bract scale. The ovule arises as a group of cells which form rounded hump of tissue termed the nucellus (megasporganum). From the base of nucellus an endosing integument grows up which does not enclose nucellus completely but leaves an opening the micropyle which leads to the outer end of nucellus. Megaspore mother cell undergoes reduction division producing four megaspores arranged in a row each with haploid (x) number of chromosomes.
Gametophytic Phase:
Spores are the units as gametophytic phase of life cycle. In pinus spors are of two types microspores and megaspores which on germination give rise to male and female gametophyte.
Male gametophyte: Microspore is the unit of male gametophyte. Each microspore or pollen grain is a unicellular body covered with two layered coat. Outer layer the exine is thick and heavily cuticularized while inner layer the intine is very thin. The exine forms two balloon shaped wings on either side. Pinus is wind pollinated and the wings add to the buoyancy of their structures.
Development of male gametophyte from microspore begins while the later (mcrospore) is still within the microsporangium. Microspore divides into small Prothallial cell and much large apical cell. Apical cell further divides to form second Prothallial cell and antheridial cell which divides in a small generative cell and a large tube cell.
It is four celled stage (one generative cell) and two Prothallial cell, that the partially developed male gametophyte is shed from the microsporangium. Now these four celled male gametophytes are carried by wind and after reaching in the pollen chamber of ovule further development takes place.
The exine of four called male gametophyte now ruptures and intine grows out to form pollen tube into which tube nucleus passes. Pollen tube penetrates into the nucellar tissue and grows slowly. In second year generative cell divides into stalk cell and body cell. Now body cell migrates by tube and the nucleus of body cell dividies to form two male gametes or cell, each has its nucleus surrounded by cytoplasmic sheath. Male gametes are non flagellated i.e. non motile. Pollen tube pierces the nucellus and reaches the archegonia, which are developed on female prothallus.
Female gametophyte:
The megaspore is the first cell of female gametophyte. One megaspore from tetrad becomes functional and rest three disintegrate. Functional megaspore increases in size and its nucleus undergoes a series of free nuclear division and a vauole appears. Now wall formation begins from periphery and proceeds towards the centre. Thus a cellular female gametophyte without a central vacuole is formed and is also called endosperm.
Archegonia are formed towards micropylar side and may be 2 to 5 in Pinus. Any ell from female prothallus or endosperm tissue functions as archegonium initial cell which enlarges and divides by Periclimical wall into a primary cover cell and central cell. Primary cover cell divides into four cells and then by transverse division to form 8 celled neck, cels of which are arranged in two tiers of four cells each. The central cell enlarges in size and divides into ventral canal cell and a large egg cell. There are no neck canal cells. Thus archegonium of Pinus consists of large venter and a short neck. Around archegonium is a nutritive jacket.
Pollination:
In Pinus Pollen grains (microspores) transfer by wind in May and June. Male cones are borne in clusters and there is large number of microsporangia in each cone bearing many microspores. In day time Pollination of pine-forests are full of dusty pollen grains.
In four celled stage (two prothallial cells, one generative cell and one tube cell) the pollen grain is shed. A drop of nucilage substance secreted by upper part of nucellus at the apex of ovule and the pollen grain is caught in this pollination drop. After pollination scales of female cone bend downwards so that the cone hangs vertically. During internal various changes in the pollen grains and ovule takes place. Male cone after pollination dry up and fall.
Fertilization:
The pollen tube formed in the pollen chamber of ovule from the microspore becomes active and after piercing through the nucellus reaches the neck of archegonium. Tip of pollen tube now contains two male gametes and the tube and stalk nuclei. Pollen tube come in contact with archegonium and the tip ruptures discharging the contents into the egg. One of the amel gamete approaches the egg nucleus and unites to form oospore or zygote. Second male gamete alon with the stalk and tube nuclei disintegrate.
Fertilization takes place in June and July. More than one pollen tube may develop in association with a single ovule and more than one archegonium may be fertilized.
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