Early developmental stages of M. perniciosa basidiomata. A. Globose hyphal aggregate (initial primordium) under a superficial layer of mycelial mat (bar = 0.25 mm). B. Schematic drawing of the area marked in A showing the grouping of protective hyphae (*) laterally involving another more compact group (#). At the top another group of converging hyphae grows downwards (black squares) (bar = 0.12 mm). C. Tissue section showing an emerging undifferentiated "initial" (bar = 0.25 mm). D. Schematic drawing of C showing the expansion of marked hyphae presented in Figure 2B. The arrows indicate the same previous protective layer but the compact bulb has already overlapped it (bar = 0.25 mm). E. Another "initial" in a more advanced developmental state (bar = 0.25 mm). F. Schematic drawing of E showing protective hyphae placed in parallel positions and the laterally expanding bulb hyphae (arrows) (bar = 0.25 mm).