Ovule

Ovule, in botany, is the name applied to immature seeds, which are produced within the ovary of a flower. In flowering plants, the development of the ovule is generally as follows.

At the site of the future seed, an outgrowth, the nucellus or megasporangium, develops; this becomes covered by two integuments that grow up from its base, leaving an opening at the top called the micropyle. Within the nucellus is the megaspore mother cell. It divides into two and then into four; one of these megaspores then typically divides into eight nuclei to become the embryo sac of the female gametophyte. It is in this sac that the plant embryo will develop.

The young male plants, or male gametophytes, are popularly referred to as pollen grains; these are contained in modified leaves called stamens. When a pollen grain is placed on the stifma, it sends out a tube that grows down to the ovary and eventually enters the ovule. Two sperms are then discharged into the embryo sac; one of these fuses with the egg nucleus at the micropylar end of the embryo sac, and this fertilized egg then develops into the embryo of the seed. The other sperm fuses with two nuclei near the middle of the embryo sac; the resulting triple-fusion nucleus develops into the endosperm, which usually remains as the food storage tissue of the seed.

See Fertilization; Flower; Pollination.

Gamete

Gamete, sexual reproductive cell that fuses with another sexual cell in the process of fertilization. The cell resulting from the union of two gametes is called a zygote; the zygote usually undergoes a series of cell divisions until it develops into a complete organism.

Gametes, also called germ cells, vary widely in structure. The simplest sexual organisms are isogamous, that is, they produce a single kind of gamete. The identical gametes unite in pairs to produce zygotes. Although all isogametes are apparently alike in structure, they are thought to be different in physiological constitution, because gametes from the same individual do not successfully unite. The simplest isogametes, those of lower fungi such as molds, are small cells that grow on the ends of body filaments and become detached when mature. Other lower organisms, such as lower algae and protozoa, have gametes, which are formed by division of the protoplasm of single cells.

All higher plants are heterogamous, that is, they produce two kinds of gametes. The female gamete is called the egg; the male gamete is called the sperm. The organ of gamete production in plants is called a gametangium.

All animals and animal-like lower organisms that reproduce sexually, except a few protozoans, are also heterogamous. The male gametes are called spermatozoa; female gametes, ova or eggs. The gamete-producing organs of animals are called gonads. The formation of gametes in the gonads of animals is called gametogenesis. By this process the number of chromosomes in the sex cells is reduced in number from diploid to haploid, which is half the number of chromosomes in the normal body cells of the species. The diploid number of human chromosomes, for example, is 46. When a human sex cell divides to form two gametes, each gamete receives only half, or 23, of the normal complement of chromosomes. This type of cell division is called meiosis. The normal total of chromosomes is restored in fertilization when two gametes fuse, each contributing half of the chromosomes required by the zygote.

Fertilization

Fertilization is the process in which gametes—a male's sperm and a female's egg or ovum—fuse together, producing a single cell that develops into an adult organism. Fertilization occurs in both plants and animals that reproduce sexually—that is, when a male and a female are needed to produce an offspring (see Reproduction).

Fertilization is a precise period in the reproductive process. It begins when the sperm contacts the outer surface of the egg and it ends when the sperm's nucleus fuses with the egg's nucleus. Fertilization is not instantaneous—it may take 30 minutes in sea urchins and up to several hours in mammals. After nuclear fusion, the fertilized egg is called a zygote. When the zygote divides to a two-cell stage, it is called an embryo.

Fertilization is necessary to produce a single cell that contains a full complement of genes. When a cell undergoes meiosis, gametes are formed—a sperm cell or an egg cell. Each gamete contains only half the genetic material of the original cell. During sperm and egg fusion in fertilization, the full amount of genetic material is restored: half contributed by the male parent and half contributed by the female.

For information on plant fertilization see the articles on Seed, Pollination, and Plant Propagation.

Pollen grains

A pollen grain contains a sperm cell that fertilizes an egg. If fertilization is successful, a seed is produced. The pollen grains of each species display unique sculpting of the pollen wall, and fossilized pollen serves to identify ancient species.

Pollen grains are microscopic in size, ranging in diameter from less than 0.01mm (about 0.0000004 in) to a little over 0.5 mm (about 0.00002 in). Millions of pollen grains waft along in the clouds of pollen seen in the spring, often causing the sneezing and watery eyes associated with pollen allergies. The outer covering of pollen grains, called the pollen wall, may be intricately sculpted with designs that in some instances can be used to distinguish between plant species. A chemical in the wall called sporopollenin makes the wall resistant to decay.

Pollination

Pollination is a transfer of pollen grains from the male structure of a plant to the female structure of a plant. The pollen grains contain cells that will develop into male sex cells, or sperm. The female structure of a plant contains the female sex cells, or eggs. Pollination prepares the plant for fertilization, the union of the male and female sex cells. Virtually all grains, fruits, vegetables, wildflowers, and trees must be pollinated and fertilized to produce seed or fruit, and pollination is vital for the production of critically important agricultural crops, including corn, wheat, rice, apples, oranges, tomatoes, and squash.

Flowering plants use wind, insects, bats, mammals, and birds to transfer pollen from the stamen, or male portion of the flower, to the stigma, or female portion of the flower. Many plants have evolved closely with certain animals to ensure successful transfer of pollen. For example, many species of rain forest plants can only be pollinated by one particular species of insect, bird, or bat.