Fig. 3.3-3. Transverse section of stem of sedge (Scirpus). The large white areas are intercellular spaces which make this stem light-weight (and thus more buoyant) and permit a more rapid diffusion of oxygen downward to submerged portions of the stem. The intercellular spaces are narrow tubes separated from each other by plates of parenchyma and fiber cells which causes the stem to actually be quite strong despite being lightly constructed.