Procambial Initiation for the Vascular System in the Spike of Wheat

Analysis of serial sections of procambial traces in spike primordia of Triticum reveals the early development of the vascular system of the rachis. Alternating pairs of traces form in isolation in the rachis in relation to the lower ridge leaf of each double ridge. Vertical merger of these procambial strands creates the lateral trace system consisting of two vertical traces in the rachis. The central trace system differentiates between but on both sides of the lateral traces, forming an ellipse in transection. Like the lateral traces, the central traces form in isolated pairs in the rachis in relation to their lower ridge leaves. Vertical mergers of the central trace procambia create the vertical lateral trace system along both sides of the rachis. As the lateral and central traces are differentiating and merging, isolated traces differentiating basipetally from the glume insertions begin to link the spikelet with the rachis. Glume traces tend to merge with the earliest available traces in the rachis. This pattern of merger frequently causes the distal spikelets to become linked to the lateral trace system and the proximal spikelets to become linked to the central trace system. In Triticum and other pooid grasses, the lower ridge leaves initiate the vascular system for the rachis as do the vegetative leaves for the culm. The vascular system of the inflorescences of panicoid grasses may have different sites promoting procambial differentiation than do the inflorescences of pooid grasses.