External ultrastructure of fruit body initiation in Morchella.

The external morphological changes occurring during initiation and early stages of fruit body development of a Morchella sp., before the development of asci, were examined by scanning electron microscopy. Four stages of primordial development were distinguished. First, disk-shaped knots of 0.5-1.5 mm were observed on the surface of the substrate. Next, the knot inflated and a primordial stem emerged from its centre. Afterward, the stem lengthened, oriented upward, and two types of hyphal elements developed: long, straight and smooth basal hairy hyphae and short stem hyphae, some of which were inflated and projected out of a cohesive layer of tightly packed hyphal elements. Finally, when the stem was 2-3 mm long, pre-apothecia emerged in the apical end, with ridges and pits having distinguished types of paraphyses. Extracelluar mucilage covered the ridge layer and helped give the tissue its shape and rigidity.