Fertilization in plants-digieduco

Definition : Fertilization or syngamy in plants (also termed fecundation) is the essentiaI biological process of sexual. reproduction, it is the union of two dissimilar sexual cells i.e. the male and female gametes resulting in the formation of a zygote.

Fertilization in plants : In angiosperms, the pollen grains after being liberated from the anther are carried to the stigma, where they germinate and produee pollen tubes which are the male gametophytes of angiosperms. The time taken by the pollen grains to germinate on the stigma varies greatly from species to species-it may be immediate or may even range from few minutes to several days. In the first step of germination, the pollen grain expands by the absorption of liquid from the moist surface of the stigma and the intine protrudes through a germ pore. The small tubular structure that arises in this way is called pollen tube. The pollen tube now continues to elongate making its way down the tissues of the stigma and style. Generally only one pollen tube comes out from each pollen grain, in some members of the family ' Malvaceae, Cucurbitaceae, Campanulaeeae etc. several ( 10-14) pollen tubes are known to come out from the same pollen grain, but only one of them finally matures. Sometimes the same pollen tube may divide into one or more branches. Only the distal part of the pollen tube bears the living cytoplasm within which the nuclei i.e. the tube nucleus and the generative nucleus or the tube nucleus and the two male nuclei (gametes) as the case may be pass. As soon as the nuclei pass into the cytoplasmic portion of the pollen tube, callose plugs are left in the empty portion behind them. The growth of the pollen tube is stimulated by the sugary substances secreted by the stigma.

After the pollen tube has emerged from the pollen grain, it grows through the stigmatic papillae and then pass into the tissues of the style. Styles may be hollow or solid-the centre of the latter type is generally filled with transmitting tissue made up of 2-3 layers of glandular cells, while the former type has a wide canal full of mucilage. In hollow styles, the pollen tube grows along the surface of the cells lining the stylar canal and in solid styles the tube grows through the transmitting tissue. After reaching at the top of the ovary, the pollen tube with its nuclei (two male gametes and the tube nucleus) may enter the ovule either through the micropyle or in a round about way through the chalaza. The former is the usual condition and is known as porogamy ; the later condition is known as
chalazogamy or basigamy* which is found in species of Casuarina (Casuarinaceae), Betula (Betulaceae), Juglans (Juglandaceae), etc. Sometimes e.g. in species of Alchemilla (Rosaceae) cucurbita (Cucurbitaceae), Circaeaster (Chloranthaccae) etc. the pollen tube enters the ovule through the funiculus or the integuments-this condition is known as mesogamy*. On penetrating the nucellus of the ovule either through the micropyle or through the chalaza, or . by piercing the integuments or funiculus as the case may be, the pollen tube ultimately penetrates the wall of the embryo sac. After penetrating the wall of the embryo sac the pollen tube may either pass between the oosphere (egg) and one synergid, or between the embryo sac wall and a synergid or directly into a  synergid. Usually only one synergid is destroyed by the impact of the pollen tube and the other remains intact for a brief period. In some cases, synergids degenerate even before the entry of the pollen tube-these indicate that synergids are not essential for fertilization.

Ultimately the tip of the pollen tube bursts and both the male gametes are set free One of the male gametes of the pollen tube fuses within the egg cell (oosphere) ; the egg cell is fertilised to form a diploid zygote or oospore (2n) ; another male gamete fuses With the diploid secondaty or fusion nucleus i.e. definitive nucleus-the result of such fusion is  the formation of primary triploid (3n) endosperm nucleus, this nucleus acts as the first nucleus of the endosperm. Thus in angiosperms, fertilization occurs twice-in the first one of the male gametes fuses with the egg cell and in the other, another male gamete fuses with the diploid secondary or fusion nucleus-this phenomenon is known as double fertilization. Double fertilization phenomenon was first observed by S.G. Nawaschin in 1898 in Lilium martagon and Fritillaria tenella (Liliaceae). The fusion of two polar nuclei (n) and one of the two male gametes is the triple fusion and the resultant nucleus is triploid endosperm nucleus (3n). Diploid (2n) zygote or oospore forms the first cell i.e. mother cell of the embryo.