In flowering plants, pollination is an important mechanism that aids in transferring the pollen released from the anther to the stigma of the flower.
If the pollen is of the right type, the stigma induces it to germinate and produce a pollen tube.
The pollen tube, containing the two male gametes, then secretes hydrolytic enzymes which help the pollen tube to digest the tissues of the style as it travels down to enter the ovule through the micropyle, a minute opening in the ovule. Inside the ovule, the pollen tube encounters the embryo-sac, the female gametophyte containing three antipodals, two synergids, an egg cell and a central cell with two polar nuclei. The pollen tube now enters one of the synergids through the filiform apparatus during which it ruptures and releases the male gametes into the synergid’s cytoplasm. Of the two male gametes, one enters the egg cell and fuses with its nucleus. This fusion, known as syngamy or first fertilisation, results in the formation of a diploid zygote. The development of the embryo, also known as embryogeny, occurs only after a certain amount of the endosperm has developed; a step which assures the embryo of nutrition. Studies by botanists have revealed that during early embryogeny, the zygote divides into two cells – the basal suspensor cell towards the micropylar end and a terminal embryo cell towards the chalazal end. The structure of a mature dicotyledonous embryo is, nonetheless, different from that of a monocotyledonous embryo. A dicotyledonous embryo, for instance, consists of an embryonal axis and two cotyledons. The epicotyl, the portion of the embryonal axis above the level of the cotyledons, terminates with the plumule or stem tip. Below the level of the cotyledons is a cylindrical portion called the hypocotyl that terminates at the radical or root tip, which is covered with the root cap. On the other hand, a monocotyledonous embryo, commonly found in the grass family, has only one cotyledon called a scutellum situated towards the lateral side of the embryonal axis. The upper portion of this axis, known as epicotyl, lies above the attachment of the scutellum. The epicotyl has a shoot apex and a few leaf primordia enclosed in the coleoptile, a hollow, foliar structure. The lower end of the embryonal axis has the radical and root cap covered in an undifferentiated sheath called the coleorrhiza. Interestingly, the various parts of the embryo develop differently as the seed germinates. In this manner, the embryo as well as endosperm develops from the zygote and primary endosperm cell after fertilisation has taken place in flowering plants.