Definition of mesosporium in English:

mesosporium

Pronunciation /ˌmɛsə(ʊ)ˈspɔːrɪəm/

noun

Botany Mycology
  • The middle layer of the wall of any of several types of three-layered spores, especially of the ascospore of a fungus.

Origin

Late 19th century; earliest use found in Mordecai Cooke (1825–1914), naturalist and mycologist. From meso- + -sporium (in e.g. archesporium).