This illustration shows the structure of the thallus of Ricciocarpus: A, section of the thallus, showing the apical cell, x, forming cells that by further division develop into plates of cells separated by air spaces, j. At the left the plates thus formed are seen curving over the apical cell. B, an olderp ortion of the upper part of the thallus. The air spaces, j, are greatly enlarged and the upper cells of the vertical plates have divided, arching over the air spaces but leaving small openings which permit the entrance of air for photosynthesis.
Carlton C. Curtis, Nature and Development of Plants (New York: Henry Holt Company) 273