Unlike seeds, spores are an asexual method of reproduction in plants. What this means is that spores contain both the male and female reproductive organs, so plants with spores can then reproduce on their own. Scientists believe that, at one time, all plants were actually spore-bearing. However, as plants evolved and began to adapt to a land environment, plants began to form seeds rather than spores. Whereas spores required water to survive, seeds did not. As a result, although it takes a plant less energy to produce spores, only primitive plants such as mosses, algae and ferns now rely on spores to reproduce.
For these types of plants, the fertilization process occurs after, rather than before, the spores are dispersed. When the air outside becomes dry, pressure builds up inside the plant. The pressure then forces the plant to eject thousands of small cells, known as spores, into the air. These spores are then carried by wind, insects or birds to a final resting place. Fortunately for the spores, they can survive very harsh conditions and, since they contain no food matter in them, most predators will not attempt to eat them. Once the spore lands, if conditions are right, the gametophyte phase begins.
What are the right conditions? To provoke the gametophyte phase, the spore must land in a warm, moist and shaded area. Each spore has a very small chance of landing in such an ideal spot. So, to increase the chances, the plant ejects many spores in the hopes that one will land in a suitable environment--only about 1 percent do survive.
The gametophyte phase begins when the spore splits, forming two genetically identical cells that fuse together to grow into a small, heart-shaped structure called a prothallus. The prothallus germinates, or grows, several root-like hairs called rhizoids in order to secure itself to the ground. After several weeks of growth, both the male and female reproductive organs develop from the prothallus. The female reproductive organ produces small egg cells called ova, whereas the male reproductive organ produces sperm.
Water is required to begin the fertilization process, so the sperm and eggs must wait for rainfall. Once it rains, the sperm swims down the prothallus's long neck to its base, which holds the ova. As with humans, sperm fuses with an egg and produces an embryo. This embryo will grow into a new spore-producing plant. Once fully developed, this new plant will again eject its spores and the entire cycle will begin once again.