Reproduction in flowering plants

Pollination is the transfer of pollen grains from the anther to the stigma. In self-pollination the pollen goes to the stigma of the same flower or of another flower of the same plant. In cross-pollination it goes from the anther of one plant to the stigma of another plant of the same species. Cross-pollination may be helped by flowers that are unisexual, by one sex maturing before the other, or by anthers placed lower than the stigma. It needs a carrier to move the pollen from flower to flower: air, insects, water or humans. When a pollen grain falls on the stigma it germinates. The tube nucleus forms the pollen tube, which grows through the stigma and down the style until it reaches the micropyle of the ovule. The tube nucleus then degenerates, and the generative nucleus divides once by mitosis into two male nuclei. One male nucleus (N) fuses with the egg nucleus (N) to give the zygote (2N), which divides to give the embryo. The other male nucleus (N) fuses with the two polar nuclei, which are both haploid, to give the endosperm nucleus, which is triploid (3N). This second fusion is the triple fusion, and the two fusions together are called double fertilization. The endosperm nucleus divides to make the endosperm tissue, which feeds the early embryo. In some seeds the tissue stays outside the embryo and fills part of the seed.