Back in the Age of the Dinosaurs, feathery ferns and huge club mosses dominated the forest landscape. The evolution of the seed by some fern species, now extinct, was one of the major leaps in evolutionary progress for the plant kingdom, and gymnosperms began to dominate the landscape, in turn evolving into the flowering plants that preside today. However, mosses and ferns of today still use the reproductive mechanisms used by plants first found in that ancient time.
Mosses are tiny plants that are among the oldest plant species found on Earth. Mosses appeared before plants developed vascular tissue, which allows them to conduct nutrients from the leaves and water from the roots throughout the plant. Therefore, mosses must remain small and simple so that all parts of the plant have easy access to nutrients. Mosses reproduce through spores, a process that requires mechanical help and quite a bit of good luck.
All plants demonstrate what is called an alteration of generations: They have both a sporophyte generation, during which all cells have a full arrangement of chromosomes, and a gametophyte generation, in which cells have only half the expected number of chromosomes. Mosses are distinctive among plants because the gametophyte dominates. When you see tiny moss plants hugging close to the roots of trees, you are observing the gametophyte generation.
Mosses begin by developing male and female shoots. They rely on water to carry the sperm from the male shoots to the egg on the female shoot. When fertilization results, the moss produces the sporophyte generation, visible as a stalk with a club-shaped enlargement at the end. Within that node, cells divide until the resulting spores have only half the expected number of chromosomes. When the wind brushes the stalk, it carries the spores to a new location, where new moss plants arise. As Harvard biology professor John W. Kimball points out, this method of reproduction is effective enough that some species of moss are found worldwide.
Ferns represent the next rung on the evolutionary ladder for the plant kingdom. With the development of simple vascular tissue, ferns were able to grow larger and develop root systems. Reproduction didn't change much, however, and ferns reproduce using spores in a fashion similar to that used by mosses.
Unlike mosses, the frilly fronds that you recognize as a fern indicate that the sporophyte generation, with its full complement of chromosomes in each cell, dominates, representing an important evolutionary shift. Ferns produce small dots on the undersides of their leaves, called sori by botanists and fruit dots by many gardeners. Within the fruit dots, cells divide until they have only half the number of chromosomes, and when they dry out, they crack open and scatter spores. As the American Fern Society points out, ferns produce millions of spores because they rely heavily on luck at this point: Only one of millions of spores released typically reaches a site where conditions are right for growing. The successful spore develops into the gametophyte generation, called the prothallus, a tiny heart-shaped plant that develops male and female sex cells. Like mosses, these cells must be carried to each other by water. Once fertilization occurs, the full number of chromosomes is restored, and the cell divides to form a fern plant.
According to Gerhard Leubner, a biology professor at the University of Freiburg, the first modern tree reproduced using spores, not seeds. However, the evolution of seeds was not far off. Plants began to develop advanced spore-producing structures, protective coatings where fertilization occurred and structures were able to receive male sex cells from the wind, not water. These adaptations gave the new forms advantages over the mosses and ferns and, while we can still see examples of those species today, many have become extinct, and seed-bearing plants dominate.