Botany the outer coat of a spore or oosphere in fungi or lichens. [Compare scientific Latin exosporium (1809 or earlier), French exospore (1817 or earlier).]
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1859 Todd's Cycl. Anat. & Physiol. V. 246/1 The sporangium..burst by the swelling of the exospore.
1882 S. H. Vines tr. J. von Sachs Text-bk. Bot. (ed. 2) 325 The exospore is usually smooth and often variously coloured.