Ferns are leafy vascular plants. While they have veins that permit the flow of water and nutrients like conifers and flowering plants, their life cycle is very different. Conifers and flowering plants evolved to survive hostile, dry conditions. Ferns require water for sexual reproduction.
Basic Fern Anatomy
Spores are tiny structures that contain the genetic material needed to grow a new fern. They may be green, yellow, black, brown, orange, or red. Spores are encased in structures called sporangia, which sometimes clump together to form a sorus (plural sori). In some ferns, sporangia are protected by membranes called indusia. In other ferns, the sporangia are exposed to air.
Alternation of Generations
The fern life cycle requires two generations of plants to complete itself. This is called alternation of generations.
One generation is diploid, meaning it carries two identical sets of chromosomes in each cell or the full genetic complement (like a human cell). The leafy fern with spores is part of the diploid generation, called the sporophyte.
A fern's spores don't grow into leafy sporophyte. They aren't like seeds of flowering plants. Instead, they produce a haploid generation. In a haploid plant, each cell contains one set of chromosomes or half the genetic complement (like a human sperm or egg cell). This version of the pant looks like a little heart-shaped plantlet. It is called the prothallus or gametophyte.
Details of the Fern Life Cycle
Starting with the "fern" as we recognize it (the sporophyte), the life cycle follows these steps:
The diploid sporophyte produces haploid spores by meiosis, the same process that produces eggs and sperm in animals and flowering plants.
Each spore grows into a photosynthetic prothallus (gametophyte) via mitosis. Because mitosis maintains the number of chromosomes, each cell in the prothallus is haploid. This plantlet is much smaller than sporophyte fern.
Each prothallus produces gametes via mitosis. Meiosis is not needed because the cells are already haploid. Often, a prothallus produces both sperm and eggs on the same plantlet. While the sporophyte consisted of fronds and rhizomes, the gametophyte has leaflets and rhizoids. Within the gametophyte, sperm is produced within a structure called an antheridium. The egg is produced within a similar structure called an archegonium.
When water is present, sperm use their flagella to swim to an egg and fertilize it.
The fertilized egg remains attached to the prothallus. The egg is a diploid zygote formed by the combination of DNA from the egg and sperm. The zygote grows via mitosis into the diploid sporophyte, completing the life cycle.
Before scientists understood genetics, fern reproduction was mystifying. It appeared as though adult ferns arose from spores. In a sense, this is true, but the tiny plantlets that emerge from spores are genetically different from adult ferns.
Note that sperm and egg may be produced on the same gametophyte, so a fern may self-fertilize. Advantages of self-fertilization are that fewer spores are wasted, no external gamete carrier is required, and organisms adapted to their environment can maintain their traits. The advantage of cross-fertilization, when it occurs, is that new traits may be introduced into the species.
Other Ways Ferns Reproduce
The fern "life cycle" refers to sexual reproduction. However, ferns use asexual methods to reproduce, too.
In apogamy, a sporophyte grows into a gametophyte without fertilization occurring. Ferns use this method of reproduction when conditions are too dry to permit fertilization.
Ferns can produce baby ferns at proliferous frond tips. As the baby fern grows, its weight causes the frond to droop toward the ground. Once the baby fern roots itself, it can survive separate from the parent plant. The proliferous baby plant is genetically identical to its parent. Ferns use this as a method of quick reproduction.
The rhizomes (fibrous structures that resemble roots) can spread through soil, sprouting new ferns. Ferns grown from rhizomes are also identical to their parents. This is another method that permits quick reproduction.
Fast Facts
Ferns use both sexual and asexual reproduction methods.
In sexual reproduction, a haploid spore grows into a haploid gametophyte. If there is enough moisture, the gametophyte is fertilized and grows into a diploid sporophyte. The sporophyte produces spores, completing the life cycle.
Asexual methods of reproduction include apogamy, poliferous frond tips, and rhizome spreading.