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Abstract
We mapped the fate of cells in the shoot meristem of the dry-seed embryo of sunflower,
Helianthus annuus L. cv. Peredovic, using irradiation-induced somatic sectors. We
analyzed 249 chlorophyll-deficient or glabrous (hairless) sectors generated in 236
plants. Most sectors observed in the inflorescence extended into vegetative nodes.
Thus cell lineages that ultimately gave rise to reproductive structures also contributed
to vegetative structures. No single sector extended the entire length of the shoot.
Thus the shoot is not derived from one or a few apical initials. Rather, the position,
vertical extent, and width of the sectors at different levels of the shoot suggest
that the shoot is derived from three to four circumferential populations of cells
in each of three cell layers of the embryo meristem. Sectors had no common boundaries
even in plants with two or three independent sectors, but varied in extent and overlapped
along the length of the shoot. Thus individual cells in a single circumferential population
behaved independently to contribute lineages of different vertical extents to the
growing shoot. The predicted number of circumferential populations of cells as well
as the apparent cell number in each population was consistent with the actual number
of cells in the embryo meristem observed in histological sections.