Sexual Reproduction in Flowering Plants

Morphology of Flowering Plants already covers the flower's own structure, the four whorls and their parts. This note covers what those parts actually do: how pollen reaches the stigma, and the genuinely unusual double fertilization event that follows.

Pollination: self versus cross

Pollination, the transfer of pollen from an anther to a stigma, comes in two basic forms. Self-pollination occurs when pollen from a flower's own anther lands on the stigma of that same flower, or another flower on the same plant, keeping the resulting offspring's genetics close to the parent's own. Cross-pollination occurs when pollen travels from one plant's flower to a stigma on a genetically distinct individual of the same species, typically carried by a pollinating agent (wind, water, or an animal such as an insect or bird), and it produces offspring with meaningfully greater genetic variability than self-pollination does, since the resulting seed combines hereditary material from two distinct parent plants rather than effectively one, a genuine adaptive advantage in a changing environment.

Double fertilization: the feature unique to flowering plants

Once a pollen grain reaches a compatible stigma and grows a pollen tube down to the ovule, it delivers not one but two sperm cells, and both are actually used, in a process called double fertilization, a defining, unique feature of angiosperms (flowering plants) not found in gymnosperms or any other plant group. The first fertilization is the one that matches the general pattern found across sexually reproducing organisms: one sperm cell fuses with the egg cell to form a diploid zygote, which will go on to develop into the plant embryo. The second fertilization, called triple fusion, is the genuinely distinctive part: the second sperm cell fuses not with a second egg cell, but with two polar nuclei sitting together in the ovule's central cell, and because this fusion combines three haploid nuclei together (the two polar nuclei plus the sperm nucleus), it produces a triploid nucleus, the primary endosperm nucleus, which develops into the endosperm, a nutrient-rich tissue whose entire biological purpose is to nourish the developing embryo as it grows, functioning as the seed's own dedicated food store built specifically for that job rather than serving any reproductive role itself.

Quick revision points

• Self-pollination: pollen moves within the same flower or plant, keeps offspring genetically close to the parent. Cross-pollination: pollen moves between different plants of the same species (via wind, water, or animal agents), producing greater genetic variability in offspring.
• Double fertilization is unique to angiosperms: a pollen tube delivers two sperm cells, and both are used.
• First fertilization: one sperm fuses with the egg cell, forming a diploid zygote (develops into the embryo).
• Second fertilization (triple fusion): the second sperm fuses with two polar nuclei in the central cell, producing a triploid nucleus that develops into the endosperm, the nutrient-rich tissue that feeds the growing embryo rather than becoming part of it.