Fertilization in plants occurs when haploid gametes meet to create a diploid zygote, which develops into an embryo. In gymnosperms (conifers) and angiosperms (flowering plants), the meeting of the gametes occurs in the following way: male gametes are enclosed in pollen grains and are carried by wind or insects to the female reproductive organs. The final product of fertilization--the embryo--is encased in a seed. For this reason, these two types of tracheophytes are termed seed plants. (See Plant Structures and Functions, The Seed.)
The female gametophyte contains several archegonia, where the egg cells originate and develop. The gametophyte itself is surrounded by layers of sporangia and integument; all of these elements comprise an ovule, which is found on the surface of a female cone. Fertilization occurs when pollen grains (male gametophytes) are carried by the wind to the open end of an ovule, which contains the eggs, or female gametophyte. There, the pollen grain develops an outgrowth called a pollen tube, which eventually penetrates to the egg cell within one of the archegonia. The sperm cells within the pollen tube then vie to fertilize the egg. Once fertilization has occurred, the embryo develops within the female gametophyte, and the ovule becomes the seed, complete with a food source (the gametophyte tissue) and a seed coat (the integument). This embryo, which will eventually become a new sporophyte, consists of two embryonic leaves, the epicotyl and hypocotyl.
The female reproductive organ of angiosperms is the pistil, located in the middle of the flower. As in gymnosperms, the male gametophyte is the pollen grain. In order for fertilization to occur in most flowering plants, insects or other animals must transport the pollen to the pistil. A major distinguishing feature of angiosperms is the practice of double fertilization. An angiosperm ovule contains an egg cell and a diploid fusion nucleus, which is created through the joining of two polar nuclei within the ovule. When a pollen grain comes into contact with the stigma, or top of the pistil, it sends a pollen tube down into the ovary at the pistil's base. As the pollen tube penetrates the ovule, it releases two sperm cells. One fuses with the egg to create a diploid zygote, while the other joins with the fusion nucleus to form a triploid nucleus. This triploid nucleus turns into an endosperm, which nourishes the developing embryo (filling the role of gametophyte tissue in the gymnosperm seed). As in gymnosperms, the ovule becomes a seed, encasing the embryo and endosperm in a seed coat. But unlike gymnosperms, in angiosperms the ovary containing the ovules develops into a fruit after fertilization. The fruit gives the embryos the double benefit of added protection against desiccation and increased dispersal, since it is eaten by far-ranging animals who then excrete the seeds. (For a full discussion of the parts of the flower contributing to reproduction, see Plant Structures, Flowers .)
In order for fertilization to occur, angiosperms either self-pollinate, in which a particular plant fertilizes itself, or cross-pollinate, in which one plant is fertilized by another of the same species. Cross-pollination generally produces far more vigorous plants, and is encouraged through differential development of the male and female gametophytes on a flower, or through the positioning of these gametophytes so that self- pollination is difficult.