The life cycles of plants and algae are defined by the transition between two distinct genetic phases: the haploid stage (n) and the diploid stage (2n). A haploid cell contains a single set of chromosomes, while a diploid cell possesses two sets. The shift between these stages involves two key biological processes: fertilization, where two haploid gametes fuse to create a diploid zygote, and meiosis, which reduces the chromosome number back to the haploid state. Biologists classify plant and algal life histories into three major patterns—Haplontic, Diplontic, and Haplodiplontic—based on which phase forms the multicellular body and when meiosis occurs.
The Haplontic Life Cycle (Gametophyte Dominant)
The Haplontic life cycle is characterized by the dominance of the haploid phase, known as the gametophyte, which constitutes the main, multicellular body. The diploid phase is extremely brief and is represented solely by the unicellular zygote. This pattern is found primarily in many green algae, such as Spirogyra, and most fungi.
The multicellular gametophyte produces haploid gametes through mitosis. Following fertilization, the resulting diploid zygote is formed. This zygote does not divide to grow into a multicellular diploid organism; instead, it immediately undergoes zygotic meiosis.
This meiotic division restores the haploid number of chromosomes and produces haploid spores. These spores then germinate and divide repeatedly by mitosis to develop into the next generation of the multicellular, free-living gametophyte.
The Diplontic Life Cycle (Sporophyte Dominant)
The Diplontic life cycle is dominated by the diploid phase, which forms the large, multicellular body known as the sporophyte. This pattern is characteristic of all animals. In the plant kingdom, it is observed in a few groups of organisms, such as the brown algae genus Fucus.
The sporophyte is the only multicellular stage, and it produces haploid gametes directly through a process called gametic meiosis. The haploid phase is dramatically reduced, existing only as the single-celled gametes, such as sperm and egg.
These haploid gametes fuse to form a diploid zygote. The zygote then grows and divides by mitosis to develop into a new multicellular sporophyte. The diploid sporophyte is the persistent, independent body that functions as the main organism.
The Haplodiplontic Cycle (Alternation of Generations)
The Haplodiplontic life cycle, also called the alternation of generations, is the reproductive strategy found in all true land plants. This cycle involves an alternation between two separate, multicellular generations: a diploid sporophyte and a haploid gametophyte. This pattern represents an evolutionary middle ground, where both genetic phases have a multicellular existence.
The diploid sporophyte generation produces haploid spores through meiosis within specialized structures called sporangia. These spores are reproductive cells that are dispersed and then germinate to grow by mitosis. This mitotic division leads to the formation of the multicellular, haploid gametophyte.
The haploid gametophyte then produces gametes, such as eggs and sperm, through mitosis. When two gametes from different individuals or the same individual fuse, they form a diploid zygote. This zygote grows by mitosis into the next large, multicellular diploid sporophyte.
An important feature of this cycle is the shift in which generation is structurally dominant as plants evolved on land. In non-vascular plants like bryophytes (mosses and liverworts), the green, leafy plant we commonly recognize is the haploid gametophyte, making it the dominant generation. The sporophyte is smaller, short-lived, and dependent on the gametophyte for nutrition.
In vascular plants, which include ferns, conifers, and flowering plants, the trend reverses, and the sporophyte becomes the large, dominant structure. The familiar fern frond, the trunk of a pine tree, or the body of a rose bush are all the diploid sporophyte generation. In these advanced plants, the gametophyte generation is highly reduced, often microscopic, and entirely dependent on the sporophyte for its survival.