In the angiosperms or flowering plants, the pollen grains and ovules are developed in specialized structures called flowers and the seeds are enclosed by fruits.
They are divided into two classes
The dicotyledons - characterized by having two cotyledons in their seeds.
The monocotyledons- have only one cotyledon.
Fig. A dicot plant
Fig. A monocot plant
The male sex organs in a flower is the stamen, consists of a slender filament with an anther at the tip and the anthers, following meiosis, produce pollen grains.
The female sex organs in a flower is the pistil or the carpel, consists of an ovary enclosing one to many ovules.
Within ovules highly reduced female gametophytes are present, termed as embryo sacs.
The embryo-sac formation is preceded by meiosis.
Each embryo-sac has a three-celled egg apparatus – one egg cell and two synergids, three antipodal cells and two polar nuclei, the polar nuclei eventually fuse to produce a diploid secondary nucleus.
Pollen grain, after dispersal from the anthers, are carried by wind or various other agencies to the stigma of a pistil this is termed as pollination.
The pollen tubes enter the embryo-sac where two male gametes are discharged, one of the male gametes fuses with the egg cell to form a zygote (syngamy) and the other male gamete fuses with the diploid secondary nucleus to produce the triploid primary endosperm nucleus (PEN), termed as double fertilization.
The zygote develops into an embryo and the PEN develops into endosperm which provides nourishment to the developing embryo.
The synergids and antipodal cells degenerate after fertilization, during these events the ovules develop into seeds and the ovaries develop into fruit.