Mosses are small plants that grow in cushiony green tufts in moist, shady places. Most people are familiar with the tiny-leafed moss gametophyte, but the sporophyte is not as obvious.
In moss, the gametophyte and sporophyte phases exist as separate organisms. For the rest of the plant world, the sporophyte phase predominates. In flowering plants, the gametophyte phase is found in the reproductive organs of the flower.
The term used to describe the two phases of the moss life cycle is alternation of generations. Alternation of generations means that a multi-cellular haploid phase (gametophyte) alternates with a multi-cellular diploid phase (sporophyte). Diploid means that there are two sets of chromosomes in the cell and haploid means that there is one set.
A mature moss gametophyte produces reproductive structures at the tip of the plant. For some moss species the male and female reproductive structures are on the same plant, while for others they are on separate plants. Through mitosis, the female gametophyte produces haploid female gametes, or eggs, in the archegonium. The male gametophyte produces haploid male gametes, or sperm, in the antheridium. Sexual reproduction in moss requires water, and the sperm have two flagella so they can swim.
When it rains, sperm are released from the antheridium and splashed onto the female gametophyte. The sperm swim to the archegonium. Once near the archegonium, one sperm swims down the neck to fertilize the egg. This union produces a diploid zygote, the first cell of the sporophyte phase. The zygote divides and grows into an embryo. While the embryo grows, the surrounding archegonium grows too. As the embryo develops into a sporophyte, the archegonium tissue separates leaving a cap-like structure at the tip of the sporophyte called the calyptra. The calyptra protects the sporophyte until it is mature. The sporophyte remains attached to the gametophyte and is dependent on it for nutrients and water.
The mature sporophyte consists of a sporangium (capsule) and stalk. Inside the sporangium, the diploid cells undergo meiosis, producing thousands of haploid spores. Eventually, the calyptra drops off, revealing a ring of teeth-like structures at the tip of the sporangium. These teeth are sensitive to humidity. They alternate between opening and closing, depending on whether the atmosphere is dry or wet. This alternating movement, called hygroscopic movement, disperses the spores.
Moisture needs to be present before a spore can germinate. When conditions are right, the spore divides through mitosis to produce a long, multi-branched, photosynthetic strand called the protonema. Over time, the protonema develops buds, the buds develop into a leafy gametophyte, and the moss life cycle continues.
