Cell lineage of vein formation in variegated leaves of the C-4 grass Stenotaphrum secundatum

Clonal analysis of variegated leaves of the C-4 grass, Stenotaphrum secunda tum, indicates that invasions among meristematic layers occur during the or ganogenetic stage of leaf development. resulting in long, broad white and g reen stripes. These layer invasions cease prior to the second phase of leaf development when delimitation of leaf re ions occurs. Vein precursors most ly arise during the second phase, so that procambial strand formation is su perimposed on the lineage makeup of earlier formed tissue. Anatomical evide nce indicates that procambium arises through formative divisions within gro und tissue of leaf primordia and that each strand is derived from a variabl e number (one-four) of ground meristem precursors. If a developing vein str addles the boundary between previously-formed green and white sectors, then the mature vein is half green and half white, reflecting its mixed cell li neage. In Stenotaphrum, 24.8% of the sectors observed were bounded by such 'half veins'. The temporal relationship of layer invasion and tissue system delimitation in this species supports the view that positional signals are more important than lineage history in the determination of tissue type. H owever, analysis of planes of cell division in developing veins indicates, that, once formed. procambial strands are discrete lineage units that exten d longitudinally by proliferative divisions. Thus, lineage restrictions may play an important role in the third stage of leaf development, differentia tion of tissues and cells, which also includes the maintenance of cell iden tity. (C) 2000 Annals of Botany Company.