Haplo diplontic life cycle (is also referred as diplohaplontic, diplo biontic or dibiontic) shows multicellular diploid and haploid stages as dipicted in the following figure.

Life Cycle of a Pteridophyte The life cycle of a pteridophyte consists of two morphologically distinct phases

(i) The gametophytic phase

(ii) The sporophytic phase

These two phases come one after another in the life cycle of a pteridophyte. This phenomenon is called alternation of generation. The gametophyte is haploid with single set of chromosomes. It produces male sex organs antheridia and female sex organs archegonia.

(i) The antheridia may be embedded or projecting type. Each antheridium has single layered sterile jacket enclosing a mass of androcytes.

(ii) The androcytes are flask-shaped, sessile or shortly stalked and differentiated into globular venter and tubular neck.

(iii) The archegonium contains large egg, which is non-motile.

(iv) The antherozoids after liberation from antheridium, reaches up to the archegonium fuses with the egg and forms a diploid structure known as zygotes.

(v) The diploid zygote is the first cell of sporophytic generation. It is retained inside the archegonium and forms the embryo.

(vi) The embryo grows and develop to form sporophyte which is differentiated into roots, stem and leaves.

(vii) At maturity the plant bears sporangia, which encloses spore mother cells.

(viii) Each spore mother cell gives rise to four haploid spores which are usually arranged in tetrads.

(ix) The sporophytic generation ends with the production of spores.

(x) Each spore is the first cell of gametophytic generation. It germinates to produce gametophyte and completes its life cycle.

Is correct, lichen is a symbiotic association between an alga and a fungi, which live together for their mutual benefit. If both are separated from each other then they will die. The fungus holds water, provides protection and ideal housing to the alga. The alga in turn supplies carbohydrate food for the fungus. If the alga is capable of fixing nitrogen (e.g., Nostoc), it supplies fixed nitrogen to the fungus. The kind of mutual interdependence helps lichens to grow on dry, barren rocks, where the other plants fail to exist. Morever, the algae or the fungi alone cannot grow in such places. Thus, both the partners cannot survive without each other.

An angiospermic plants sexually reproduces by the formation of male and female gametes. The male gamete is a pollen which contain two male nuclei and female gamete is egg cell produced in ovule (female gametophyte).

The pollen grains germinate on the stigma of a flower and the resulting pollen tube grow through the tissues of stigma and style and reach near the egg apparatus. The two male gametes are discharged within the embryo sac. One of the male gamete fuses with the egg cell to form a diploid zygote.

This fusion is known as fertilisation or syngamy. The second male gamete fuses with the diploid secondary nucleus and forms the triploid Primary Endosperm Nucleus (PEN). This fusion is known as triple fusion.

Because of the involvement of two fusion, this event in angiosperms is termed as double fertilisation. The zygote then develops into embryo and PEN develops into endosperm which provides nourishment to the developing embryo.