Clonal analysis of variegated leaves of the C4 grass, Stenotaphrum secundatum, indicates that invasions among meristematic layers occur during the organogenetic stage of leaf development, resulting in long, broad white and green stripes. These layer invasions cease prior to the second phase of leaf development when delimitation of leaf regions occurs. Vein precursors also arise during this second phase, so that procambial strand formation is superimposed on the lineage makeup of earlier-formed tissue. Anatomical evidence indicates that procambium arises through formative divisions within ground tissue of leaf primordia and that each strand is derived from a variable number (1-4) of ground meristem precursors. If a developing vein straddles the boundary between previously-formed green and white sectors, then the mature vein is half green and half white, reflecting its mixed cell lineage. In Stenotaphrum, 24.8% of the sectors observed were bounded by such "half veins". However, analysis of planes of cell division in developing veins indicates, that once, formed, procambial strands are discrete lineage units that extend longitudinally by proliferative division. Thus, lineage restriction may play an important role in the third stage of leaf developemt, differentiation of tissues and cells, which also includes the maintenance of cell identity.

Key words: C4 photosynthesis, cell lineage, clonal analysis, leaf development, Stenotaphrum secundatum, vein formation