How Do Tiny Moss Plants Manage to Stand Upright? TEM Provides a Clue

The minuscule but evolutionarily and environmentally important mosses are early land plants. The majority of moss plants have erect growth forms; however, how these tiny plants manage to stand upright has not been fully explored. The information on the type and location of tissues proposed to be responsible for bearing the load of the highly hydrated body parts comes mainly from microscopy studies. Based on light microscopy examination of stem sections, it has been proposed that thick-walled outer tissues are the likely candidates for this function. Recent studies employing transmission electron microscopy have shown that thickened cell walls consist of multiple lamellae, with alternating thick and thin lamellae differing in the orientation of cellulose microfibrils. This cell wall design optimizes mechanical properties by strengthening and supporting tissues in higher land plants and points to early evolution of this type of cell wall design, which likely occurred in mosses.