The James M. DuPont Meteorite Collection -of plants developing as layers or cushions and is differentiated from other families in that the trichomes do not form spores, heterocysts, or hairs. The cylindri- cal trichomes consist of 1 row of cells in branched or unbranched filaments; the broken ends or hormogonia regenerate in a mode characteristic for the various taxa. In many species, a terminal cell develops a thickened outer membrane. Cell division occurs throughout the entire trichome and at rela- tively the same time. Reproduction is by fragmentation. The current divi- sion of the genera is based largely upon the structure of the sheath (Gomont, 1892) and is in need of further study for clarification. The sheaths of Oscilla- tor ia, Arthrospira,