During a heavy dew or rain, the male sperm get splashed onto the female gametophyte. They swim to the female eggs. When the male and female sex cells unite, a diploid zygote forms. The zygote divides by mitosis to form an embryo, which in turn develops into a sporophyte, and the cycle begins again. In plants, this continual cycle, which alternates between the spore-producing phase and sex cell-producing phase, is called alternation of generations. In bryophytes, the most visible plant is the green, leaflike gametophyte. Liverworts have a life cycle similar to that of the mosses.
Asexual Reproduction.
In addition to the alternation of generations, both mosses and liverworts reproduce asexually. New moss plants can develop when a small piece of the parent plant breaks off. Liverworts develop small balls of cells within cuplike structures on the surface of the leaflike body. These balls of cells can be carried by water to new areas where they grow into new plants.
Alternate Generations.
All plants have a life cycle in which the sporophyte and gametophyte alternate. In other words, all plants go through a diploid and a haploid stage. In nonvascular plants like mosses and liverworts, the sporophytes depend on the gametophytes for water and nutrients. In the more complex vascular plants, like tulips and oak trees, the sporophyte doesn't depend on the gametophyte for these things.
Importance of Mosses and Liverworts.
Mosses and liverworts are important in the ecology of many areas. Although mosses require moist conditions to grow and reproduce, many of them can withstand long periods of dryness. They are often among the first plants to grow in new environments, such as lava fields, or disturbed environments, such as forests destroyed by fire.
When a volcano erupts, the lava covers the land and destroys the plants living there. After the lava cools, the spores of mosses and liverworts are carried to the new rocks by the wind and begin to grow wherever there is enough water.
