The plant life cycle relies on a complex reproductive strategy known as the “alternation of generations.” Unlike animals, where the body is entirely diploid, plants have two separate, multicellular life stages. Understanding what produces a plant’s egg and sperm requires recognizing these two distinct generations that alternate in a continuous cycle. The answer lies in a dual-stage process where one stage focuses on gamete production and the other focuses on spore production.
The Two Phases of Plant Life
Plant reproduction involves two distinct, multicellular phases: the sporophyte and the gametophyte. The sporophyte generation is diploid, meaning its cells contain two sets of chromosomes, one from each parent. This is the familiar, usually larger, plant body, such as a tree, a shrub, or the leafy frond of a fern. The sporophyte’s primary purpose is to produce spores.
The gametophyte generation is haploid, containing only a single set of chromosomes in its cells. This generation develops from a spore and is responsible for sexual reproduction, specifically the production of egg and sperm cells, which are called gametes. While the sporophyte is dominant and long-lived in most familiar plants like flowering plants, the gametophyte is often a tiny, short-lived, and sometimes dependent structure.
The Gamete Producers
The direct answer to what produces egg and sperm cells in plants is the gametophyte. Because the gametophyte is already a haploid organism, it produces its reproductive cells, or gametes, through simple cell division called mitosis. Mitosis ensures that the resulting egg and sperm cells also remain haploid, maintaining the single set of chromosomes. This is a significant difference from animal reproduction, where haploid gametes are formed from a diploid body through meiosis.
In non-flowering plants like mosses and ferns, the gametes are produced in specialized, multicellular structures called gametangia. The female gametangium, which produces the egg cell, is called the archegonium, while the male gametangium, which produces the sperm, is called the antheridium.
Gamete Production in Flowering Plants
For flowering plants, the male gametophyte is highly reduced and exists as the pollen grain, which contains the sperm cells. The female gametophyte, known as the embryo sac, is even more reduced and remains embedded within the ovule of the flower, where it houses the egg cell. In flowering plants, the male gametophyte, or pollen, consists of just a few cells, including the two sperm cells necessary for fertilization.
The Spore Producers
The sporophyte is the generation that initiates the entire cycle by producing spores, which are the starting points for the gametophyte generation. Spores are haploid cells that are capable of growing into a new organism without needing to fuse with another cell. The sporophyte, being diploid, must use a specialized cell division process called meiosis to reduce the chromosome number by half.
This meiotic division takes place within reproductive structures called sporangia, which are found on the sporophyte body. The cells within the sporangia, known as spore mother cells, divide to produce four haploid spores. Once released, a spore will germinate and undergo mitosis to grow into the multicellular, haploid gametophyte, which will then produce the egg and sperm cells.
In flowering plants, the sporophyte is the entire visible plant, and the sporangia are located within the flower’s anthers and ovules. The anthers contain microsporangia, where microspores are produced, which then develop into the male gametophytes (pollen). The ovules contain megasporangia, which produce megaspores that develop into the female gametophyte (embryo sac).
Bringing the Cycle Together
The life cycle is completed when the haploid gametes (egg and sperm) fuse together in a process known as fertilization. This fusion immediately restores the diploid state, creating a single-celled, diploid zygote. The zygote contains a full set of chromosomes, half from the egg and half from the sperm.
The zygote then begins to divide repeatedly through mitosis, developing into a multicellular embryo. This embryo is the first stage of the new sporophyte generation, which will grow into the mature plant body. Once the new sporophyte reaches maturity, it will produce spores, restarting the alternation of generations cycle.