blastidium (noun, pl. blastidia) – a globular propagule containing mycobiont and photobiont, always produced by "budding" and frequently formed in series of two or more, with each new blastidium produced from the tip of the previous one. Blastidia are very similar to marginal soredia and often confused with these. They are, however, anatomically more clearly delimited than the loosely associated soredia and generally covered by a more or less indistinct layer of "cortical" hyphae. This is:part of upper thallus surface. Kind of:upper surface structure.