Procambial Initiation for the Vascular System in the Shoot of Tradescantia zebrina (Commelinaceae)

The complete procambial system was recorded through the apical five disks of insertion of three serially transectioned shoot tips of Tradescantia zebrina (Commelinaceae). The system consists of the interconnected procambial median, major lateral (two pairs), first intermediate (one pair), and later intermediate leaf traces. Leaf traces are initiated by each disk of insertion as isolated procambial segments during four overlapping centripetal and centrifugal waves. The gaps among the originally discrete procambia close rapidly by further procambial differentiation, which links the traces bidirectionally to the older procambial system and to the leaf, the capture process. The procambial sites where the major leaf traces linked with the rest of the system, after the fourth wave, proliferate into a horizontal complex recognizable as the inner portion of the nodal plexus. As the third wave ends and during the fourth wave, a primary thickening meristem (PTM) develops subperipherally in the disk. The PTM contributes to the enlargement of the disk and generates procambial segments for some of the intermediate leaf traces. After the fourth wave, no more procambia are generated by the disk for leaf traces, and the PTM differentiates into a band of tangentially elongate cells. These cells channel horizontally the further procambial differentiation that connects the centrifugally directed basal segments of the leaf traces into a complex recognizable as the outer portion of the nodal plexus. Procambia related to the axillary bud do not appear until the fifth disk after the fourth wave.