sorus
Concise Oxford English Dictionary © 2008 Oxford University Press:
sorus /ˈsɔːrəs/
▶noun (pl. sori /-rʌɪ/) Botany a cluster of spore-producing receptacles on the underside of a fern frond.
– origin C19: mod. L., from Gk sōros ‘heap’.
'sorus' also found in these Oxford entries:

