I had explained about the male cone, female cone and the structure present inside it including the pollination in Pinus in my previous note and let us begin with the reproductive structures over here.

Gametophytes

Male Gametophyte 

• Microspores or pollen grains represent the initial stage of male gametophytic generation or male prothallus.


• The mature one divides to form a three celled structure with two small prothallial cells and a large antheridial cell.


• At this three celled stage, the pollen grains are released from pollen sac.


• The further development into male gametophyte occurs in pollen chambers within the ovule.


• Each pollen grains produces a pollen tube that penetrates through the cells of nucellus towards the archegonia.


• The pollen tubes carry two unequal motile and naked male gametes or sperms along with two vegetative cells, tube cell and stalk cell.

Female Gametophyte

• Megaspore represents the initial stage of female gametophytic generation or female prothallus.


• The megaspore germinates within the nucellus in the ovule and finally forms the female gametophyte.


• The megaspore first enlarges and undergoes many free nuclear divisions followed by the formation of wall and finally forms the solid wall of tissue.


• The tissue is called female gametophyte which is also called as endosperm, sorrounded by the cells of nucellus.


• The endosperm sorrounded the embryo in the seed after fertilization.

Archegonium

• Archegonium is a multicellular female reproductive organ in Pinus that appears during its gametophytic phase.


• Usually 2-5 archegonia develop in female gametophyte towards the micropyle.


• The archegonium consists of a short neck of eight cells and a swollen venter with an archegonial chamber below the neck.


• The venter consists of a large egg cell and a venter canal cell which soon disintegrates.

Fertilization in Pinus 

After pollination, the pollen grains are deposited on the pollen chamber inside the ovule. However, fertilization takes place only after the following year as the development of archegonia is very slow. After reaching the neck of archegonium the pollen grains burst at the apex and releases two male gametes. One of them move towards the egg nucleus and soon fuses forming a zygote or oospores.The oospore is the fertilized egg that represents the first cell of saprophytic generation. The second male gamete, tube cell and the stalk cell degenerates.

Embryo 

• The oospore may give rise to four potential embryos by the phenomenon called cleveage polyembryony which is the characteristic feature of Pinus.


• Many embryos are produced. However, only one survives and goes to further development while other degenerates.


• A fully formed embryo consists of an axis with hypocotyl, a radicle and a tiny plumule with 2-15 cotyledons.

Germination

During germination the seed coat first ruptures and the radical comes out forming a tap root where as plumule comes out of a soil forming a shoot system. The young seedlings develop into a adult sporophytes having long shoot with numerous of dwarf branches.

Alternation of Generations in Pinus 

The lifecycle of Pinus gets completed in two different generations - the diploid saprophytic generation and a haploid gametophytic generation. However, the saprophytic generation is dominant.

The male cone(microsporophylls, microsporangium, microspore mother cell),  female cone (megasporophylls, megasporangium, megaspore mother cells) represent the saprophytic structures while the microspores produced in microsporangium and megaspore formed in megasporangium represent the gametophytic generation.

These spores after germination develop into gametophytes. Pollination brings pollen grains into  pollen chamber in ovule. The pollen tube brusts in the archegonial chamber releasing the male gametes. The fusion of male gametes and female gametes give arise to oospores that represents the initial phase of saprophytic generation. The oospores finally develops into embryo then to seed which germinates and gives rise to saprophytic plant.

Hence one complete lifecycle of Pinus involve alternating cycles of sporophytic and gametophytic phases and this is called alternation of generations in Pinus .