Procambial Initiation for the Vascular System in the Shoot ofAlstroemeria(Alstroemeriaceae, Liliales)

Abstract

Mapping every leaf trace in serial transections of shoot apices reveals a three‐dimensional pattern of procambial development that was not appreciated in previous studies of Alstroemeria. Leaf traces of Alstroemeria originate as isolated procambia, in a sequence of about seven cycles, along the subjacent overlapping leaf insertions that comprise the axis. In each insertion an outer, then an inner, circle of procambial clumps is initiated. More procambial sites are interpolated among the older of both circles until a maximum of about 17 sites occurs in an insertion. Those sites in subjacent insertions become axially linked into strands if they arise near each other. That the inner circle is initiated later than the outer circle and that the inner circle soon produces large procambial sites cause the lower ends of the procambial strands to be centripetally directed. Their fused lower ends are recognizable as the junctions of the median and major lateral leaf traces near the center of the axis. Procambial strands become particular leaf traces when their upper blind ends are linked to (captured by) isolated procambia at the top of the leaf insertion. Major lateral traces have a shorter axial course through the subjacent insertions than the median traces because the upper ends of the future major laterals are initiated and captured in the later cycles of an insertion, whereas the upper ends of the future median traces are initiated and captured in the early cycles. Similarities and differences among angiosperms with the Alstroemeria pattern of leaf trace initiation are ontogenetic characters that enhance comparative morphology.