Initiation of the Vascular System and the Transition to Flowering in Early Tassels ofZea maysLand Race Chapalote (Poaceae)

Serial transections of young tassels of (Zea mays land race) chapalote revealed relationships between the vascular system in its procambial state and the lateral primordia along the axis. A lateral tassel primordium usually consists of an indefinite rim with a prolongation that will become a tassel branch or spikelet pair. A lateral tassel primordium usually develops via modifications of the vegetative leaf primordium in which the leaf apex is enhanced but the leaf base and the bud it produces are suppressed. The clearest sign of the transition from the vegetative state to the tassel is the scale leaf, which is intermediate in form between a vegetative leaf and a lateral tassel primordium. Procambial traces differentiate in isolation in the tassel axis in response to the lateral tassel primordia. Adjacent procambial traces then link axially into sympodia to initiate the three-dimensional vascular system of the tassel axis. Older sympodia occur near the center of the axis interior to more recently initiated procambial traces. Procambial continuity does not occur between the tassel axis and the lateral primordia until isolated traces in the lateral primordia link with the sympodia in the tassel axis. The transition from distichy to polystichy by the lateral tassel primordia occurs as the narrowing of the leaf base makes space available on the tassel axis for lateral primordia out of the vegetative distichous plane.