The life cycle of plants, algae, and some fungi involves the alternation of generations, switching between two distinct, multicellular life stages that differ in chromosome count. A gametophyte is definitively a haploid organism, meaning its cells contain a single set of chromosomes (N). The gametophyte functions as the sexual phase, setting the stage for the next generation of the plant.
Defining the Gametophyte’s Ploidy
The term “haploid” (N) refers to a cell or organism containing one complete set of chromosomes, contrasting with the diploid (2N) state, which possesses two sets. The gametophyte is a multicellular structure existing entirely in the haploid state. This means every cell within the gametophyte body carries only one copy of each chromosome.
The primary function of the gametophyte is the production of gametes (sperm and egg). Because the gametophyte is already haploid, it produces these sex cells through mitosis. Mitosis is a cell division process that maintains the haploid chromosome number.
The prominence of the gametophyte varies across plant groups. In non-vascular plants, such as mosses and liverworts, the gametophyte is the dominant and most visible stage. For example, the familiar green structure of a moss is the gametophyte, which is nutritionally independent.
The Sporophyte Counterpart
The sporophyte is the other generation in the plant life cycle. It is a multicellular, diploid (2N) organism, meaning its cells contain two sets of chromosomes. This doubled set results from the fusion of two haploid gametes.
The sporophyte functions by producing spores. Unlike the gametophyte, the sporophyte uses meiosis, a specialized cell division that reduces the chromosome number by half. This process converts diploid sporophyte cells into haploid (N) spores.
In most terrestrial plants, the sporophyte has become the dominant form. In vascular plants, such as trees and flowers, the familiar plant structure is the diploid sporophyte. In these groups, the gametophyte is often microscopic and short-lived, existing only as a few cells contained within structures like pollen and ovules.
The Process of Alternation of Generations
The alternation of generations is the necessary transition between the haploid gametophyte and the diploid sporophyte. This continuous cycle connects the two ploidy states through specific cellular events.
The cycle begins when the diploid sporophyte undergoes meiosis to generate haploid spores (N). These spores germinate and develop into the multicellular gametophyte entirely through mitotic cell division.
The mature gametophyte produces gametes (sperm and egg) through mitosis. The transition back to the diploid phase occurs during fertilization, which is the fusion of two haploid gametes (N + N) to create a single-celled diploid zygote (2N).
The zygote is the first cell of the new sporophyte generation. It divides repeatedly through mitosis, growing into the mature, multicellular sporophyte plant.