Mosses and liverworts belong to the bryophyte division, an ancient grouping of plants believed to have evolved from green algae. Bryophytes are the earliest land plants and share their small size, lack of vascular tissue, reproduction with spores and preference for a moist environment. Key differences, however, suggest that mosses and liverworts aren't as closely related as originally thought.
Because they lack vascular tissue to transport water and nutrients, mosses and liverworts must both remain tiny. However, differences in appearance make it possible to distinguish between the two. Mosses lack roots, but reddish brown filaments called rhizoids anchor mosses to the surface on which they grow. Tiny, pointed leaves develop, each with a midvein that helps move water from one part of the leaf to another. Leaves grow in a spiral pattern.
Liverworts, on the other hand, have lobed leaves, only one cell thick. Leaves tend to grow in rows, have a leatherlike appearance and lack the midvein that distinguishes mosses. Liverworts also lack roots, and the rhizoids that adhere them to surfaces consist of only a single cell, not a long filament. Liverworts tend to synthesize volatile oils, which gives them a spicy aroma.
Bryophytes generally prefer moist habitats, since they absorb water through their leaves, not roots. However, liverworts tend to grow on flat, moist ground and can even grow across a surface of water, while mosses can cling to the sides of trees and other nonflat surfaces.
All bryophytes reproduce with spores, not seeds. The moss or liverwort produces sperm and eggs, which when fused, develop into a spore-producing structure called the sporophyte. The spores released by the sporophyte have the potential to develop into new moss or liverwort plants. In the case of mosses, the sporophyte grows as a stalk, quickly breaking free of the plant that produced it. Spores at the tip of the stalk gradually release in the wind.
Liverwort sporophytes, on the other hand, develop entirely inside the plant until the spores are ready for release. Rapid growth of the sporophyte pushes it free of the plant and exposes its spores to the wind. Unlike mosses, the spores of liverworts release entirely within just a few minutes.
Until recently, mosses and liverworts were thought to be very closely related. Recent research on mitochondrial DNA, however, has show that these plants may not be closely related at all. This DNA evidence suggests that liverworts were the first land plant to evolve; indeed, the oldest plant spores -- 475 million years old -- are believed to have come from a liverwort. Mosses have characteristics shared by neither liverworts nor vascular plants, suggesting that most plants on Earth today evolved from the liverwort.
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