Why are the pollen grains and embryo sacs of flowers sometimes considered the gametophyte generation in an alternation of generations life cycle?
1 Answer
Pollen grains and embryo sac in flowering plants are actually male and female gametohytes, respectively. It is not a question of considering sometimes as you have written.
Explanation:
Angiosperms like all other vascular plants show the phenomenon of alternation of generations.
Main plant body in all vascular plants, including Angiosperms is sporohyte (2n). The gametohytic generation is reduced.
Sporohytic generation reproduces asexually by meisospores .
All Angiosperms are heterosporous, producing 2 types of meiospores, i.e. micropsores and megaspores.
Most of the cells of sporogenous tissue inside anther lobes (microsporangia) function as microspore mother cells, each dividing by meiosis to form 4 haploid cells. Each haploid cell develops an outer thick wall exine and inner thin wall intine to become mature microspore (pollen grain). Microspore is the first step of male gametophyte.
The development of male gametophyte is endosporic and precocious, i.e. mcropspore starts developing into male gametophyte while still within the microsporangium (anther lobe). The pollen grains liberated from the anther at the time of pollination are actually microspores enclosing partially developed male gametophyte.** The remaining development of male gametophyte occurs when the pollen falls on stigma.
The megaspore mother cell inside megasporangium (ovule) divides by meiosis to form 4 haploid cells, out of which 3 degenrate and one enlarges to become megaspore. The megaspore is is the first step of female gamtophyte.
The development of female gametophyte is endosporic . The megaspore develps into female gametophyte which is permanently retained inside megasporangium (ovule). The female gametophyte is permanently enclosed and surrounded by nucellus of ovuule and popularly called embryo sac . Most common embryo sac (female gametophyte) is 7-celled, 3 cells towards micropylar end constituting egg apparatus, 3 cells towards chalazal end termed antipodals. All these cells are haploid Middle larger cell contains seconadary diploid nucleus.
